PROINSULIN CLEAVED BY FURIN IS PROCESSED TO CHROMATOGRAPHICALLY MATURE INSULIN BY CARBOXYPEPTIDASES IN NONNEUROENDOCRINE CELLS

Proinsulin is converted to mature insulin by two reactions, cleavage b y the prohormone convertases PC2 and PC3, and removal of basic residue s by carboxypeptidase H. These reactions are performed in the secretor y granules of pancreatic beta cells. When we replaced the processing s ites of proinsulin with furin-cleavable sites, the three nonneuroendoc rine cell lines Hep G2, CHO, and NIH/3T3 produced insulin with the sam e size as synthetic human insulin. Although the three cell lines expre ssed different quantities of carboxypeptidase H mRNA, the cytosol frac tions of the cells exhibited similar levels of carboxypeptidase activi ty, suggesting that additional carboxypeptidases were active. The insu lins resulting from the three cell Lines were eluted as a single peak on a cation-exchange chromatography column, indicating that proinsulin can be maturated to insulin even in nonneuroendocrine cells.