INSULIN STIMULATES CELL-SURFACE AMINOPEPTIDASE ACTIVITY TOWARD VASOPRESSIN IN ADIPOCYTES

We previously discovered that insulin stimulates the marked translocat ion of a novel membrane aminopeptidase, designated vp165 for vesicle p rotein of 165 kDa, to the cell surface in adipocytes. To examine the h ypothesis that this enzyme acts on peptide hormones, we assessed the r elative affinity of the enzyme for 22 peptide hormones by measuring th e inhibitory effect of each on the hydrolysis of a fluorogenic substra te, and we directly assayed the cleavage of four of these. Angiotensin III, angiotensin IV, and Lys-bradykinin bound to the enzyme with half -saturation constants between 20 and 600 nM and were cleaved by vp165. Vasopressin bound with lower affinity but at saturation was cleaved m ore rapidly. Subsequently, the effect of insulin on the rates of cleav age of I-125-labeled vasopressin by intact 3T3-L1 and rat adipocytes w as determined. With both cell types, vasopressin cleavage was stimulat ed approximately threefold. These findings indicate that a physiologic al role for vp165 may be the processing of peptide hormones and that i nsulin could enhance the cleavage of extracellular substrates by elici ting the translocation of vp165 to the cell surface.