125I-Angiotensin 1-7 binds to a different site than angiotensin 1-7 in tissue membrane preparations

PURPOSE

To study the receptor for Angiotensin (Ang) 1-7 using a radioligand (¹²⁵I-Ang 1-7)-binding assay. For more than a decade, Mas has been viewed as the receptor for Ang 1-7; however, Ang 1-7 binding has not been pharmacologically characterized in tissue membrane preparations.

METHODS

Radioligand-binding assays were carried out using tissue membrane preparations using radioiodinated Angiotensin 1-7 (¹²⁵I-Ang 1-7) to characterize its binding site. Non-radioactive ¹²⁷I-Ang 1-7 was used to test if the addition of an iodine to the tyrosine⁴ moiety of Ang 1-7 changes the ability of Ang 1-7 to competitively inhibit ¹²⁵I-Ang 1-7 binding.

RESULTS

¹²⁵I-Ang 1-7 binds saturably, with moderately high affinity (10-20 nM) to a binding site in rat liver membranes that is displaceable by ¹²⁷I-Ang 1-7 at nanomolar concentrations (IC₅₀ = 62 nM) while Ang 1-7 displaces at micromolar concentrations (IC₅₀ = 80 µM) at ~22 °C. This binding was also displaceable by inhibitors of metalloproteases at room temperature. This suggests that ¹²⁵I-Ang 1-7 binds to MMPs and/or ADAMs as well as other liver membrane elements at ~ 22 °C. However, when ¹²⁵I-Ang 1-7-binding assays were run at 0-4 °C, the same MMP inhibitors did not effectively compete for ¹²⁵I-Ang 1-7.

CONCLUSIONS

The addition of an iodine molecule to the tyrosine in position 4 of Ang 1-7 drastically changes the binding characteristics of this peptide making it unsuitable for characterization of Ang 1-7 receptors.

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Abstract P2027: Preliminary Characterization Of The 125 I-angiotensin 1-7 Binding Site In Rat Liver

Angiotensin 1-7 (Ang 1-7) is a product of the renin-angiotensin system (RAS) that can reduce blood pressure and fibrosis. For more than a decade Mas has been viewed as the receptor for Ang 1-7, however, there are lingering doubts regarding the nature of the interaction between the heptapeptide and Mas. To date, no one has pharmacologically characterized Ang 1-7 binding in tissue membrane preparations. In this study we show that 125 I-Ang 1-7 binds to a site for which Ang 1-7 has low (μmolar) affinity, suggesting that the addition of an iodine molecule to the tyrosine in position 4 of Ang 1-7 drastically changes the characteristics of this peptide making it unsuitable for characterization of Ang 1-7 receptors. 125 I-Ang 1-7 binds with moderately high affinity (10-20 nM) and saturability to a binding site in rat liver membranes that is displaceable by 127 I-Ang 1-7 (iodoAng 1-7) at nanomolar concentrations. This binding is also displaceable by inhibitors of metalloproteases. To test the hypothesis that 125 I-Ang 1-7 binds to a peptidase, a series of peptidase inhibitors were evaluated for their ability to inhibit 125 I-Ang 1-7 binding. The non-selective matrix metalloprotease (MMP) and A Disintegrin And Metalloprotease (ADAM) inhibitors marimastat, batimastat, and GM-6001, which inhibit MMP1,2,3,7,8,13,14 and ADAM 9,17, partially inhibited 125 I-Ang 1-7 binding with nanomolar affinities, similar to their IC 50 values for MMPs, suggesting that 125 I-Ang 1-7 binds to MMPs and/or ADAMs as well as other tissue elements. Further testing with the specific MMP-2 (ARP100), MMP-9 and ADAM-17 inhibitors indicate that 125 I-Ang 1-7 is not binding to these peptidases. Continuing studies are underway to establish conditions under which the 125 I-Ang 1-7 ligand is not metabolized by peptidases, while retaining the ability to bind to peptidases is underway to enable the identification of the specific peptidase(s) or other binding moieties to which 125 I-Ang 1-7 binds to determine their potential therapeutic importance as a druggable targets.