Comparison of prohormone-processing activities in islet microsomes and secretory granules: evidence for distinct converting enzymes for separate islet prosomatostatins

Abstract

In previous work we have examined the nature of converting enzymes for proinsulin, proglucagon, and prosomatostatin-I (PSS-I) in secretory granules isolated from anglerfish islets. The purpose of the present study was to extend the examination of precursor conversion to islet microsomes and to compare prohormone processing, including that of PSS-I and prosomatostatin-II (PSS-II), in islet secretory granules and microsomes. Microsomes (rough endoplasmic reticulum [RER] and Golgi complex) and secretory granules were prepared from anglerfish islets by differential and discontinuous density-gradient centrifugation. Microsomes were further fractionated into Golgi- and RER-enriched subfractions. Lysed secretory granule or microsome preparations were incubated in the presence of a mixture of radioactively labeled islet prohormones. Extracts of products generated were subjected to analysis by gel filtration and high-pressure liquid chromatography. Accuracy of product cleavage was monitored by comparing high-pressure liquid chromatography retention times from the radiolabeled in vitro conversion products with the retention times of labeled products from tissue extracts. All converting activity in microsomes was found to be similar to that in granules in that it had a pH optimum near pH 5 and was inhibited by p-chloromercuribenzoate. No significant differences in the converting activity of Golgi complex- and RER-enriched subfractions of microsomes was observed. The proinsulin, proglucagon, and PSS-II converting-enzymes, which were found in islet secretory granules, were also present and membrane-associated in islet microsomes. However, converting activity for PSS-I was displayed only in secretory granules. This suggests that two or more separate enzymes are involved in processing PSS-I and PSS-II, and that these enzymes have either differential distribution or differential activity in RER/Golgi complex and secretory granules. The demonstration of converting enzyme activity in islet microsomes supports the proposal that these enzymes may be synthesized at the RER and are internalized along with the prohormones.