Effects of low molecular weight peptides and divalent cations on degradation and binding of angiotensin II

The effects of peptide inhibitors (bestatin and amastatin) and divalent cations (Ca2+ and Co2+) on the velocity of Asp1 liberation from angiotensin II (A-II) by human placental membrane fractions and binding of 125I A-II to human placental membranes were tested at 22 degrees C and 4 degrees C. Asp1 liberation was measured by high performance liquid chromatography. As expected, the degradation and binding of A-II were temperature sensitive, with both being at 4 degrees C than at 22 degrees C. While amastatin (10(-4) M) and bestatin 10(-6) M) significantly reduced the velocity of Asp1 liberation from A-II to about 45%, amastatin (10(-4) M) and bestatin (10(-4) M) increased 125I A-II binding to 125% and 130%, respectively. Ca2+ (10 mM) and Co2+ (10 mM) activated the velocity of Asp1 liberation from A-II to 140% and 120%, respectively at 22 degrees C. Ca2+ (10(-1) M) and Co2+ (10 mM) also enhanced 125I A-II binding about 130%. Previously we showed that the A-II degrading activity found in human placental membrane fractions is mainly due to aminopeptidases A and M. Since amastatin and bestatin are the specific inhibitors for aminopeptidases A and M, and since Ca2+ and Co2+ are the activators for aminopeptidase A and aminopeptidase M, respectively, it is conceivable that the enzymes regulate the levels of A-II and, therefore, that they may play an important role in the binding of A-II to human placental membrane fractions.