<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