Document:

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                                                                   EXHIIBT 10(d)

                               AMENDMENT AGREEMENT

         This AMENDMENT AGREEMENT, dated as of May 12, 2003 (this "Agreement"),
is made and entered into by and among CMS Gas Transmission Company (the
"Seller"), AIG Highstar Capital, L.P. ("Highstar"), AIG Highstar II Funding
Corp. ("Funding," and together with Highstar, the "Highstar Parties"), Southern
Union Company ("Southern Union"), and Southern Union Panhandle Corp. ("Buyer",
and together with Seller, the Highstar Parties and Southern Union, the "Purchase
Agreement Parties").

         WHEREAS, the Purchase Agreement Parties are parties to that certain
Stock Purchase Agreement, dated as of December 21, 2002 (the "Purchase
Agreement"); and

         WHEREAS, the Purchase Agreement Parties desire to amend the Purchase
Agreement so that the Highstar Parties are no longer parties to the Purchase
Agreement;

         NOW, THEREFORE, for and in consideration of the foregoing and the
representations, warranties, covenants and agreements set forth herein, the
Purchase Agreement Parties, intending to be legally bound, hereby agree as
follows (capitalized terms used in this Agreement but not otherwise defined
shall have the meanings ascribed to such terms in the Purchase Agreement):

     1.  Effective immediately, the Purchase Agreement is hereby amended
         pursuant to Section 9.20 of the Purchase Agreement such that the
         Highstar Parties are no longer parties to the Purchase Agreement and
         such that the Highstar Parties' participation in the sale of Panhandle
         is terminated. As further set forth in paragraphs 4 and 5 below, the
         Highstar Parties shall have no election, right, option, claim, or other
         privilege or any obligation or liability arising under, in connection
         with or relating to the Purchase Agreement or the transactions
         contemplated by the Purchase Agreement except for those elections,
         rights, options, claims or other privileges or any obligations or
         liabilities expressly arising hereunder. In addition, the Highstar
         Parties, Southern Union and Buyer agree that upon execution of this
         Agreement, that certain letter agreement, dated as of December 20,
         2002, by and among the Highstar Parties, Southern Union and Buyer
         regarding the formation, capitalization and operation of the Buyer as
         well as all other agreements, arrangements and commitments entered into
         in connection therewith (together, the "Buyer Formation Agreements")
         are hereby terminated. The Highstar Parties acknowledge that (i)
         simultaneously with the execution of this Agreement, Seller, Buyer and
         Southern Union intend to enter into an amended purchase agreement
         relating to the sale of Panhandle to Buyer, pursuant to terms that will
         differ from those set forth in the Purchase Agreement (such new
         agreement, the "Amended Purchase Agreement"), (ii) as soon as
         reasonably practicable thereafter, Seller, Southern Union and Buyer
         intend to consummate the transactions contemplated by any Amended
         Purchase Agreement (the "Amended Transactions"), and (iii) the Highstar
         Parties shall have no

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         election, right, option, claim, or other privilege or any obligation or
         liability arising under, in connection with, or relating to, the
         Amended Purchase Agreement or the Amended Transactions. In addition,
         the Highstar Parties and SU are simultaneously entering into a letter
         agreement regarding the termination of certain other arrangements (the
         "Highstar/SU Letter Agreement"). Notwithstanding anything to the
         contrary contained herein, from and after the date of this Agreement,
         in the event that any of the Highstar Parties suffer or incur any
         Damages as a result of a Third Party Claim, the Highstar Parties shall
         continue to be entitled to any and all rights to indemnification
         provided under Article VIII of the Purchase Agreement with respect to
         such Third Party Claim to the same extent as the Highstar Parties would
         have been entitled to indemnification under Article VIII of the
         Purchase Agreement for any such Third Party Claim had this Agreement
         not been executed and, solely for purposes of determining the right of
         the Highstar Parties to indemnification under this Agreement and
         Article VIII of the Purchase Agreement, the Highstar Parties shall be
         treated as if the Highstar Parties had completed the transactions
         contemplated by the Purchase Agreement. Southern Union shall indemnify
         CMS for fifty percent (50%) of any amount paid by CMS or its Affiliates
         to the Highstar Parties pursuant to the preceding sentence. In
         consideration for the termination by the Highstar Parties of their
         participation in the sale of Panhandle, from and after the date hereof,
         Southern Union shall assume any and all obligations of the Highstar
         Parties to indemnify CMS or any other Person under Articles VII or VIII
         of the Purchase Agreement, and Southern Union hereby assumes all
         obligations of the Sponsors to pay those expenses of the Sponsors and
         the Buyer with respect to the transactions contemplated by the Purchase
         Agreement otherwise allocable to the Sponsors under the Purchase
         Agreement and the Buyer Formation Documents.

     2.  The Highstar Parties do hereby unequivocally release and discharge
         Seller, its parents, subsidiaries and affiliates, and any of their
         respective officers, directors, agents, managers, employees,
         representatives, legal and financial advisors, principals or partners,
         and any heirs, executors, administrators, successors or assigns of any
         said persons or entities (the "Seller Releasees"), from any and all
         actions, causes of action, choses in action, cases, claims, suits,
         debts, dues, sums of money, accounts, reckonings, bonds, bills,
         specialties, covenants, contracts, controversies, agreements, promises,
         variances, trespasses, injuries, harms, damages, judgments, remedies,
         extents, executions, demands, liens and liabilities whatsoever, in law,
         equity or otherwise (together, "Actions"), in any way arising under, in
         connection with or relating to the Purchase Agreement or the
         transactions contemplated thereby, or any action or failure to act
         under the Purchase Agreement or in connection therewith or in
         connection with the events leading to the removal of the Highstar
         Parties as parties to the Purchase Agreement, which have been asserted
         against the Seller Releasees or which, whether currently known or
         unknown, the Highstar Parties, or any successors or assigns, ever could
         assert, or ever do assert, against the Seller Releasees, relating to
         any claims, or any transactions or occurrences from any time through
         the date hereof in connection with the foregoing; provided, however,
         the Seller Releasees

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         are not released from any Actions which may arise (i) under this
         Agreement or (ii) under the Confidentiality Agreement.

     3.  The Highstar Parties do hereby unequivocally release and discharge
         Southern Union and Buyer, their parents, subsidiaries and affiliates,
         and any of their respective officers, directors, agents, managers,
         employees, representatives, legal and financial advisors, principals or
         partners, and any heirs, executors, administrators, successors or
         assigns of any said persons or entities (the "Southern Union
         Releasees"), from any and all Actions in any way arising under, in
         connection with or relating to the Purchase Agreement and/or the Buyer
         Formation Agreements or the transactions contemplated thereby
         (including under that certain letter agreement, dated as of December
         20, 2002, by and among the Highstar Parties and Southern Union), or any
         action or failure to act under the Purchase Agreement and/or the Buyer
         Formation Agreements or in connection therewith or in connection with
         the events leading to the removal of the Highstar Parties as parties to
         the Purchase Agreement, which have been asserted against the Southern
         Union Releasees or which, whether currently known or unknown, the
         Highstar Parties, or any successors or assigns, ever could assert, or
         ever do assert, in any capacity, against the Southern Union Releasees,
         relating to any claims, or any transactions or occurrences from any
         time through the date hereof in connection with the foregoing;
         provided, however, the Southern Union Releasees are not released from
         any Actions which may arise (i) under this Agreement, (ii) the
         Highstar/SU Agreement, or (iii) under the Confidentiality Agreement.

     4.  Seller does hereby unequivocally release and discharge the Highstar
         Parties, their parents, subsidiaries and affiliates, and any of their
         respective officers, directors, agents, managers, employees,
         representatives, legal and financial advisors, principals or partners,
         and any heirs, executors, administrators, successors or assigns of any
         said persons or entities (the "Highstar Releasees"), from any and all
         Actions in any way arising under, in connection with or relating to the
         Purchase Agreement or the transactions contemplated thereby, or any
         action or failure to act under the Purchase Agreement or in connection
         therewith, or in connection with the events leading to the removal of
         the Highstar Parties as parties to the Purchase Agreement, which have
         been asserted against the Highstar Releasees or which, whether
         currently known or unknown, Seller, Southern Union or Buyer, or any
         successors or assigns, ever could assert, or ever do assert, in any
         capacity, against the Highstar Releasees, relating to any claims, or
         any transactions or occurrences from any time through the date hereof
         in connection with the foregoing; provided, however; the Highstar
         Releasees are not released from any Action which may arise (i) under
         this Agreement or (ii) under the Confidentiality Agreement.

     5.  Southern Union and Buyer do hereby unequivocally release and discharge
         the Highstar Releasees from any and all Actions arising under, in
         connection with or relating to the Purchase Agreement and/or the Buyer
         Formation Agreements or

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         the transactions contemplated thereby (including under that certain
         letter agreement, dated as of December 20, 2002, by and among the
         Highstar Parties and Southern Union), or any action or failure to act
         under the Purchase Agreement and/or the Buyer Formation Agreements or
         in connection therewith, or in connection with the events leading to
         the removal of the Highstar Parties as parties to the Purchase
         Agreement, which have been asserted against the Highstar Releasees or
         which, whether currently known or unknown, Seller, Southern Union or
         Buyer, or any successors or assigns, ever could assert, or ever do
         assert, in any capacity, against the Highstar Releasees, relating to
         any claims, or any transactions or occurrences from any time through
         the date hereof in connection with the foregoing; provided, however;
         the Highstar Releasees are not released from any Action which may arise
         (i) under this Agreement, (ii) the Highstar/SU Agreement, or (iii)
         under the Confidentiality Agreement.

     6.  The Highstar Parties agree that, if requested by the Federal Trade
         Commission ("FTC") as a condition to the expiration or termination of
         the waiting period under the HSR Act to permit Buyer's acquisition of
         Panhandle or by the Missouri Attorney General as a condition to not
         opposing Buyer's acquisition of Panhandle, the Highstar Parties and any
         applicable subsidiary will negotiate in good faith, with the FTC, the
         Missouri Attorney General, and the other parties to this Agreement, as
         applicable, mutually acceptable terms and conditions of consent
         order(s) governing the future interactions of the Highstar Parties and
         their subsidiaries with Southern Union and its affiliates (including,
         but not limited to, provisions that the Highstar Parties and their
         subsidiaries will not acquire any interest in Buyer or Panhandle and/or
         will not enter into any management agreement with Southern Union
         relating to the Central Pipeline (as hereinafter defined), without
         prior FTC or Missouri Attorney General approval), and the Highstar
         Parties will sign such a consent order if the terms and conditions of
         such consent order are reasonably acceptable to the Highstar Parties.
         The "Central Pipeline" means the Central Pipeline acquired by Highstar,
         through AIG Highstar Capital, L.P. and Southern Star Central Corp.,
         from The Williams Companies, that distributes natural gas from
         producing locations in Kansas, Oklahoma, Texas, Wyoming and Colorado to
         consuming areas in the Midwest. In no event shall the Highstar Parties
         be required to enter into any such consent with the FTC that would
         prohibit the Highstar Parties or any applicable subsidiary from
         acquiring Energy Worx, Inc. or from retaining management of Energy
         Worx, Inc. to manage the Central Pipeline.

     7.  The parties hereby agree that the Confidentiality Agreement, which for
         purposes of this Agreement, shall be deemed to also include (a) the
         letter agreement, dated as of November 1, 2002, by and between the
         Highstar Parties and Southern Union regarding the disclosure of
         Confidential Information to Southern Union, and (b) the letter
         agreement, dated as of December 6, 2002, among the Purchase Agreement
         Parties, pursuant to which Southern Union agreed to be subject to the
         Confidentiality Agreement, shall continue in full force and effect
         pursuant to its terms. Seller hereby demands that the Highstar Parties
         return or destroy the

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         Information (as defined in the Confidentiality Agreement) and all
         copies thereof, pursuant to and otherwise subject to Section 4 of the
         Confidentiality Agreement.

     8.  Attached hereto as Exhibits A and B are the respective forms of press
         release to be issued by Seller and Southern Union on signing of this
         Agreement, with respect to this Agreement and the removal of the
         Highstar Parties as parties to the Purchase Agreement.

     9.  Each party agrees that it will not, and will cause its respective
         subsidiaries not to, and will use its reasonable best efforts to cause
         its directors, officers and employees not to, make any public
         statements or any statements reasonably calculated to become public
         (orally, in writing, electronically or otherwise), or instigate, assist
         or participate in making any such statement, which would reasonably be
         considered to disparage any other party or its business or operations,
         or their respective businesses and operations, or any other party's
         present and former officers, partners, directors, employees, agents,
         stockholders or representatives, in their capacity as such. Until the
         first anniversary of the date hereof, except as otherwise agreed by the
         parties, Southern Union, Buyer and Seller shall not make any public
         statements or any statements reasonably calculated to become public
         regarding, or respond to inquiries from the media, analysts, investors
         and other third parties, or otherwise comment on, the Highstar Parties
         in connection with the Panhandle transaction and the reasons for the
         removal of the Highstar Parties as parties to the Purchase Agreement,
         except as provided in the form of the press releases attached hereto as
         Exhibit A and Exhibit B or as required by law or a governmental or
         regulatory authority. Until the first anniversary of the date hereof,
         except as otherwise agreed by the parties, the Highstar Parties shall
         not make any public statements or any statements reasonably calculated
         to become public regarding, or respond to inquiries from the media,
         analysts, investors and other third parties, or otherwise comment on,
         Southern Union, Buyer, Seller, Panhandle and the Panhandle
         Subsidiaries, the Panhandle transaction and the reasons for the removal
         of the Highstar Parties as parties to the Purchase Agreement, except as
         provided in the form of press releases attached hereto as Exhibit A and
         Exhibit B or as required by law or a governmental or regulatory
         authority. Notwithstanding the foregoing, each party shall have a fair
         opportunity to make statements in response to statements made by any
         other party or in response to requests of a governmental or regulatory
         authority.

     10. Each party represents to the other parties that: (a) it is duly
         organized and validly existing under the laws of the jurisdiction of
         its incorporation or formation and in good standing; (b) it has the
         power to execute and perform its obligations under this Agreement and
         has taken all necessary action to authorize such execution, delivery
         and performance; (c) such execution, delivery and performance does not
         violate or conflict with any law applicable to it, any provision of its
         charter or bylaws or other similar governing documents or any order or
         judgment of any court or other agency of government applicable to it or
         any of its assets; (d) all

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         governmental and other consents that are required to have been obtained
         by it with respect to this Agreement have been obtained and are in full
         force and effect and all conditions of any such consents have been
         complied with; and (e) its obligations under this Agreement constitute
         its legal, valid and binding obligations, enforceable in accordance
         with the terms hereof.

     11. Any term or condition of this Agreement may be waived at any time by
         the party that is entitled to the benefit thereof, but no such waiver
         shall be effective unless set forth in a written instrument duly
         executed by or on behalf of the party waiving such term or condition.
         No waiver by any party of any term or condition of this Agreement, in
         any one or more instances, shall be deemed to be or construed as a
         waiver of the same or any other term or condition of this Agreement on
         any future occasion. All remedies, either under this Agreement or by
         any laws or otherwise afforded, will be cumulative and not alternative.

     12. This Agreement shall be binding upon and inure solely to the benefit of
         the parties hereto and their respective successors and assigns. Except
         as otherwise expressly contemplated hereby, none of the provisions of
         this Agreement shall be for the benefit of or enforceable by any third
         party, including any creditor of any party or any of their affiliates.
         Except as otherwise expressly contemplated hereby, no such third party
         shall obtain any right under any provision of this Agreement or shall
         by reasons of any such provision make any Claim in respect of any
         Liability (or otherwise) against any party hereto.

     13. Except as otherwise expressly provided herein, this Agreement
         constitutes the entire agreement, and supersedes all prior agreements
         and understandings, written or oral, among the parties with respect to
         the subject matter of this Agreement. This Agreement may be altered,
         amended or changed only by a writing making specific reference to this
         Agreement and signed by duly authorized representatives of each party.

     14. This Agreement shall be governed by and construed in accordance with
         the laws of the State of New York (regardless of the laws that might
         otherwise govern under applicable New York principles of conflicts of
         law).

     15. This Agreement may be executed by facsimile signatures by any party and
         such signature shall be deemed binding for all purposes hereof, without
         delivery of an original signature being thereafter required. This
         Agreement may be executed in one or more counterparts, each of which,
         when executed, shall be deemed to be an original and all of which
         together shall constitute one and the same document.

     16. Any disputes arising out of or relating to this Agreement or the
         breach, termination or validity thereof or the parties' performance
         hereunder shall be resolved as provided by Section 9.9 of the Purchase
         Agreement.

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     17. All demands, notices, consents, approvals, reports, requests and other
         communications hereunder must be in writing, will be deemed to have
         been duly given only if delivered personally or by facsimile
         transmission (with confirmation of receipt) or by an
         internationally-recognized express courier service or by mail (first
         class, postage prepaid) to the parties at the addresses or telephone or
         facsimile numbers set forth in Section 9.7 of the Purchase Agreement
         (or to such other address as the addressee shall have last furnished in
         writing in accord with this provision to the addressor) and will be
         deemed effective upon delivery; provided, however, that any
         communication by facsimile shall be confirmed by an
         internationally-recognized express courier service or regular mail.

     18. Each party shall maintain at all times a duly appointed agent in the
         State of New York, which may be changed upon ten (10) Business Days'
         notice to the other party, for the service of any process or summons in
         connection with any issue, litigation, action or proceeding brought in
         any such court. Any such process or summons may also be served on a
         party by mailing a copy of such process or summons to it at its address
         set forth in Section 9.7 of the Purchase Agreement (or to such other
         address as the addressee shall have last furnished in writing in accord
         with this provision to the addressor) and will be deemed effective upon
         delivery; provided, however, that any communication by facsimile shall
         be confirmed by an internationally-recognized express courier service
         or regular mail. Each party hereby irrevocably consents to the
         exclusive personal jurisdiction and venue of any New York State or
         United States Federal court of competent jurisdiction sitting in New
         York County, New York, in any action, Claim or proceeding arising out
         of or in connection with this Agreement and agrees not to commence or
         prosecute any action, Claim or proceeding in any other court. Each of
         the parties hereby expressly and irrevocably waives and agrees not to
         assert the defense of lack of personal jurisdiction, forum non
         conveniens or any similar defense with respect to the maintenance of
         any such action or proceeding in New York County, New York.

     19. THE PARTIES HEREBY KNOWINGLY, VOLUNTARILY AND INTENTIONALLY WAIVE ANY
         RIGHTS THEY MAY HAVE TO A TRIAL BY JURY IN RESPECT OF ANY LITIGATION
         BASED HEREON, OR ARISING OUT OF, UNDER, OR IN CONNECTION WITH, THIS
         AGREEMENT OR ANY OTHER DOCUMENT DELIVERED IN CONNECTION HEREWITH, OR
         ANY COURSE OF CONDUCT, COURSE OF DEALING, STATEMENTS (WHETHER ORAL OR
         WRITTEN) OR ACTIONS OF ANY PARTY. EACH PARTY ACKNOWLEDGES AND AGREES
         THAT IT HAS RECEIVED FULL AND SUFFICIENT CONSIDERATION FOR THIS
         PROVISION (AND EACH OTHER PROVISION OF EACH OTHER DOCUMENT DELIVERED IN
         CONNECTION HEREWITH TO WHICH IT IS A PARTY) AND THAT THIS PROVISION IS
         A MATERIAL INDUCEMENT FOR SUCH PARTY ENTERING INTO THIS AGREEMENT AND
         EACH SUCH OTHER DOCUMENT.

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         IN WITNESS WHEREOF, the Purchase Agreement Parties have executed this
agreement as of the date first written above.

                         CMS GAS TRANSMISSION COMPANY

                         By:  /s/ William J. Haener
                            ----------------------------------------------
                              Name: William J. Haener
                              Title: President and CEO

                         SOUTHERN UNION PANHANDLE CORP.

                         By:  /s/ Thomas F. Karam
                            ----------------------------------------------
                              Name: Thomas F. Karam
                              Title: President and Chief Operating Officer

                         SOUTHERN UNION COMPANY

                         By:  /s/ Thomas F. Karam
                            ----------------------------------------------
                              Name: Thomas F. Karam
                              Title: President and Chief Operating Officer

                         AIG HIGHSTAR CAPITAL, L.P.

                         By:  /s/ Christopher H. Lee
                            ----------------------------------------------
                              Name: Christopher H. Lee
                              Title: Managing Partner

                         AIG HIGHSTAR II FUNDING CORP.

                         By:  /s/ Michael Walsh
                            ----------------------------------------------
                              Name: Michael Walsh
                              Title: Treasurer<PAGE>
                                                                   EXHIBIT 10.54

                THIRD AMENDMENT TO COMMERCIAL SUBLEASE AGREEMENT

         This Third Amendment to Commercial Sublease Agreement ("AMENDMENT")
dated March 3, 2003, is made by and between SOUTHWEST MICHIGAN INNOVATION
CENTER, INC. (FORMERLY KNOWN AS SWMF HOLDINGS CORPORATION) ("LANDLORD") and
ESPERION THERAPEUTICS, INC. ("TENANT").

                             PRELIMINARY STATEMENT

         A. Landlord and Tenant entered into a Commercial Sublease dated June
22, 2001 (the "SUBLEASE") covering the following numbered rooms in McCracken
Hall at Western Michigan University: 5010, 5030, 5040, 5041, 5042, 5070, 5090,
5110, 5111, and 5280C.

         B. Pursuant to paragraph 3 of the Sublease, the term of the Sublease is
for one (1) year commencing October 15, 2001 and expiring October 14, 2002.

         C. Pursuant to paragraph 3, Tenant has the option to renew the Sublease
for four (4) additional six (6) month terms, the first of which shall commence
immediately after the end of the original term of the Sublease.

         D. Tenant has given Landlord notice of intent to renew the Sublease for
the first six-month renewal term and the parties are entering into this
Amendment to memorialize such renewal.

                                   AGREEMENT

         1. The Sublease is hereby amended to extend the term of the Sublease
for one (1) additional six (6) month period and the term of the Sublease will
commence on April 15, 2002 and expire on October 14, 2003, unless further
extended in accordance with the terms of paragraph 3 of the Sublease.

         2. In accordance with the Sublease, rent during the additional term
shall be the same as the rent specified in Section 4 of the Sublease, as
amended.

         3. Except as expressly stated otherwise in this Amendment, all rights
and obligations of Tenant and Landlord contained in the Sublease remain in full
force and effect.

         4. This Amendment may be executed in counterparts and by facsimile,
each of which shall constitute an original and all of which shall together
constitute one and the same Amendment.

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         IN WITNESS WHEREOF, the parties have executed this Third Amendment as
of the day and year first above written.

                                  SOUTHWEST MICHIGAN INNOVATION CENTER, INC.

                                  By: /s/ Barry Broome
                                     -------------------------------------
                                     Barry Broome
                                             Its:  Executive Director/CEO

                                  ESPERION THERAPEUTICS, INC.

                                  By: /s/ Timothy M. Mayleben
                                     -------------------------------------
                                     Timothy M. Mayleben
                                             Its:  Chief Operating Officer & CFO

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