Document:

<PAGE>   1
                                                                   EXHIBIT 4.22

                                FIRST SUPPLEMENT

                                       to

                                PLEDGE AGREEMENT

                                      among

                        AMERICAN MUTUAL HOLDING COMPANY,

                              GOLDMAN, SACHS & CO.,
                             as Call Option Holder,

                            THE CHASE MANHATTAN BANK,
                               as Collateral Agent

                                       and

                           FIRST UNION NATIONAL BANK,
                                  as Unit Agent

                            Dated September 20, 2000

<PAGE>   2

                          FIRST SUPPLEMENTAL AGREEMENT

     This First Supplemental Agreement (this "First Supplemental Agreement"),
dated as of September 20, 2000, is among AMERICAN MUTUAL HOLDING COMPANY, an
Iowa mutual insurance holding company ("AMHC"), as successor to AMERUS LIFE
HOLDINGS, INC., an Iowa insurance holding company (the "Company"), GOLDMAN,
SACHS & CO., as Call Option Holder (the "Call Option Holder"), THE CHASE
MANHATTAN BANK, as Collateral Agent (the "Collateral Agent"), and FIRST UNION
NATIONAL BANK, as Unit Agent (the "Unit Agent").

     WHEREAS, the Company and the Unit Agent entered into a Master Unit
Agreement, dated as of July 27, 1998 (the "Master Unit Agreement"), in which the
obligations of the Company under the Units are provided;

     WHEREAS, the Company, Call Option Holder, Collateral Agent and Unit Agent
entered into a Pledge Agreement, dated as of July 27, 1998 (the "Pledge
Agreement"), as provided by the Master Unit Agreement, whereby the QUIPS",
Junior Subordinated Debentures or Treasury Securities that from time to time
underlie the Units are pledged to the Collateral Agent to secure the obligations
of the Holders of Units under the Purchase Contracts and Call Options that
underlie such Units;

     WHEREAS, AMHC has executed a Plan of Conversion, dated as of December 17,
1999, pursuant to which AMHC will convert from a mutual insurance holding
company into a stock holding company;

     WHEREAS, the Company and AMHC have entered into an Agreement and Plan of
Merger, dated December 17, 1999, pursuant to which the Company will merge with
and into AMHC, with AMHC continuing as the surviving company (the "Merger"); and

     WHEREAS, AMHC wishes to assume the obligations of the Company under the
Pledge Agreement, pursuant to the requirements of Article Nine of the Master
Unit Agreement, as authorized by Article Eight of the Master Unit Agreement and
without the consent of the Holders.

     NOW THEREFORE, in consideration of the foregoing, the parties agree as
follows:

Section 1.      Capitalized terms not otherwise defined herein shall have
                the meanings set forth in the Master Unit Agreement and the
                Pledge Agreement.

Section 2.      AMHC hereby agrees to assume the obligations of the
                Company, as its successor, under the Pledge Agreement and to
                be bound by the provisions of such agreement.

                                       1

<PAGE>   3

Section 3.      This First Supplemental Agreement shall become effective as
                of the Effective Time (as defined in the Agreement and Plan of
                Merger referred to above) of the Merger.

Section 4.      This First Supplemental Agreement may be executed in two or
                more counterparts, each of which will be deemed to be an
                original, but all of which together will constitute one and
                the same agreement.

Section 5.      This First Supplemental Agreement will be governed by and
                interpreted in accordance with the internal laws of the State
                of New York, without giving effect to principles of conflicts
                of laws.

                IN WITNESS WHEREOF, the parties hereto have executed and
delivered this Supplemental Agreement as of the date set forth above.

                                    AMERICAN MUTUAL HOLDING COMPANY

                                    By:  /s/ Roger K. Brooks
                                         --------------------------------------
                                         Name:  Roger K. Brooks
                                         Title: Chairman, President and
                                                Chief Executive Officer

                                    GOLDMAN, SACHS & CO.,
                                    as Call Option Holder

                                    By:  /s/ Paul S. Efron
                                         --------------------------------------
                                         Name:  Paul S. Efron
                                         Title: Managing Director

                                    THE CHASE MANHATTAN BANK,
                                    as Collateral Agent and Securities
                                    Intermediary

                                    By:  /s/ L.O. Brien
                                         --------------------------------------
                                         Name:  L.O. Brien
                                         Title: Vice President

                                       2

<PAGE>   4

                                    FIRST UNION NATIONAL BANK,
                                    as Unit Agent and as attorney-in-fact of
                                    the Holders from time to time of the Units

                                    By:  /s/ Shawn K. Bednasek
                                         --------------------------------------
                                    Name:  Shawn K. Bednasek
                                    Title:    Vice President

                                       3<PAGE>   1
                                                                   EXHIBIT 4.23

                                FIRST SUPPLEMENT

                                       to

                                    INDENTURE

                                      among

                        AMERICAN MUTUAL HOLDING COMPANY,

                           AMERUS LIFE HOLDINGS, INC.

                                       and

                      FIRST UNION NATIONAL BANK, as Trustee

                          Date as of September 20, 2000

<PAGE>   2

                          FIRST SUPPLEMENTAL INDENTURE

     This FIRST SUPPLEMENTAL INDENTURE (this "First Supplemental Indenture") is
made as of the 20th day of September, 2000, among AMERICAN MUTUAL HOLDING
COMPANY, an Iowa mutual insurance holding company ("AMHC"), AMERUS LIFE
HOLDINGS, INC., an Iowa insurance holding company (the "Company"), and FIRST
UNION NATIONAL BANK, as Trustee (the "Trustee").

                                   WITNESSETH:

     WHEREAS, the Company and the Trustee executed and delivered an Indenture,
dated as of June 16, 1998 (the "Indenture"), providing for the issuance by the
Company of its unsecured debentures, notes or other evidences of indebtedness;

     WHEREAS, AMHC has executed a Plan of Conversion, dated as of December 17,
1999, pursuant to which AMHC will convert from a mutual insurance holding
company into a stock holding company;

     WHEREAS, the Company and AMHC have executed and delivered an Agreement and
Plan of Merger, dated December 17, 1999, pursuant to which the Company will
merge with and into AMHC, with AMHC as the surviving company (the "Merger");

     WHEREAS, AMHC desires to assume the obligations of the Company under the
Indenture;

     WHEREAS, this First Supplemental Indenture is being executed pursuant to
the requirements of Article Seven of the Indenture, as authorized by Article
Eight of the Indenture and without the consent of the Holders;

     WHEREAS, the execution and delivery by the Company of this Supplemental
Indenture have been duly authorized by Board Resolution and all other action
required to make this First Supplemental Indenture a valid and binding
instrument has been duly taken and performed; and

     WHEREAS, concurrently with the execution and delivery of this First
Supplemental Indenture, the Company is delivering to the Trustee an Officers'
Certificate, stating that all conditions precedent to the consummation of the
Merger under this Indenture have been met.

     WHEREAS, concurrently with the execution and delivery of this First
Supplemental Indenture, the Company is delivering to the Trustee an Opinion of
Counsel, stating that the Merger and this First Supplemental Indenture comply
with this Indenture and that all conditions precedent to the consummation of the
Merger under this Indenture have been met.

     NOW, THEREFORE, the parties agree as follows:

<PAGE>   3

                                   ARTICLE ONE
                                   ASSUMPTION

     SECTION 1.01. AMHC hereby assumes the due and punctual payment of the
principal, premium, if any, interest, if any, and any Additional Amounts, with
respect to all of the Securities and the performance or observance of every
covenant under this Indenture and the Securities on the part of the Company to
be performed under the Securities, the coupons and this Indenture.

                                   ARTICLE TWO
                                  MISCELLANEOUS

     SECTION 2.01. Definitions. For all purposes of this First Supplemental
Indenture, except as otherwise herein expressly provided, the definitions, terms
and expressions used herein shall have the same meanings as corresponding
definitions, terms and expressions used in the Indenture.

     SECTION 2.02. Continuance of Indenture. This First Supplemental Indenture
supplements the Indenture and shall be a part of and subject to all the terms
thereof. The Indenture, as supplemented and amended by this First Supplemental
Indenture, is in all respects ratified and confirmed, and the Indenture and this
First Supplemental Indenture shall be read, taken and construed as one and the
same instrument.

     SECTION 2.03. Concerning the Trustee. The Trustee accepts the amendment of
the Indenture effected by this First Supplemental Indenture and agrees to
execute the trust created by the Indenture, as hereby amended, but only upon the
terms and conditions set forth in the Indenture, as hereby amended, including
the terms and provisions defining and limiting the liabilities and
responsibilities of the Trustee, which terms and provisions shall in like manner
define and limit its liabilities in the performance of the trust created by the
Indenture, as hereby amended.

     SECTION 2.04. Governing Law. This First Supplemental Indenture shall be
governed by and construed in accordance with the laws of the State of New York
without regard to the principles of conflicts of laws.

     SECTION 2.05. Counterparts. This First Supplemental Indenture may be
executed in any number of counterparts, each of which when so executed shall be
deemed to be an original, and all of such counterparts shall together constitute
one and the same instrument.

<PAGE>   4

     IN WITNESS WHEREOF, the parties hereto have caused this Indenture to be
duly executed, and their respective corporate seals to be hereunto affixed and
attested, all as of the day and year first written above.

                                    AMERICAN MUTUAL HOLDING COMPANY

                                    By:  /s/ Roger K. Brooks
                                         --------------------------------------
                                         Name:  Roger K. Brooks
                                         Title: Chairman, President and
                                                Chief Executive Officer

                                    AMERUS LIFE HOLDINGS, INC.

                                    By:  /s/ Roger K. Brooks
                                         --------------------------------------
                                         Name:  Roger K. Brooks
                                         Title: Chairman, President and
                                                Chief Executive Officer

                                    FIRST UNION NATIONAL BANK

                                    By:  /s/ Shawn K. Bednasek
                                         --------------------------------------
                                         Name:  Shawn K. Bednasek
                                         Title: Vice President

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