Metabolism of Vasoactive Peptides by Human Plasma and Purified Human Aminopeptidase M

Authors: İbrahim F. BENTER, Shigehiko MIZUTANI

Abstract: Aminopeptidase M (AmM; EC 3.4.11.2) is a membrane-bound peptidase present on renal brush border and vascular plasma membrane. In the present study, AmM, purified from human placenta hydrolyzed alanyl-, leucyl- and arginyle- /beta- naphtylamides at rates of 3.25±0.04, 1.53±0.04 and 0.68±0.02 µmol/min/mg, respectively, exhibited little or no /alpha-glutamyl-, aspartyl-or glycyl-prolyl-naphthylamidase activities (<0.10 /mumol/min/mg) and was inhibited by o-phenanthroline, amastatin (IC_{150}=30 nM) and bestatin (IC_{150}=4 nM). The alanylnaphtylamidase activity of unfractionated human plasma was found to be identical to purified AmM regarding relative rates of hydrolysis of alanyl-, leucyl- and arginyl- naphthyladies (100:46:20), pH optimum, and inhibition profile. In comperative studies with the purified enzyme, immunoreactive AmM accounted for essentially all of the alanyl-2-naphtylamidase activity of human plasma. N-terminal metabolism of (Met^5) enkephalin by purified AmM was4.30+-0.28 /mumol/min/mg, followed by hepta (5-11) substance P (2.72+-0.13 /mumol/min/mg) somatostatin (1.00+-0.04 /mumol/min/mg) (Asn^1) angiotensin II (0.98+-0.30 /mumol/min/mg), angiotensin III (0.32+-0.06 /mumol/min/mg) and des (Asp^1)-angiotensin I (0.14+-0.03/mumol/min/mg). In contrast, substance P, bradykinin, (Sar^1, Ala^8) angiotensin II and neurokinin analogs containing modified N-termini (e.g. Ac-Arg) were resistant to hydrolysis by AmM. These data support a significant role for vascular and plasma AmM in the metabolism of circulating vasoaktive peptides in the human.

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