Proinsulin is a biosynthetic precursor of insulin which reacts with insulin antibody and has been found in the peripheral circulation. The present studies were undertaken to determine the extent of its intrinsic biologic activity as compared with insulin and to provide evidence regarding the possibility and anatomic location of its conversion to insulin or biologically active related products in peripheral tissues.
The effects of single component porcine insulin and proinsulin on the conversion of glucose-l-C-14 to C-14-O2 and inhibition of the theophylline-stimulated lipolysis were studied in rat adipose tissue cells and pieces. Kunitz pancreatic trypsin inhibitor (KPTI) was used to block tissue conversion of proinsulin to insulin. Proinsulin was twentythree to twenty-seven times less effective than insulin when assayed on isolated fat cells. These results were not changed significantly by KPTI. Whereas insulin appeared less effective on fat pieces than on cells—presumably due to difficulty with diffusion into the more intact tissue—proinsulin, an even larger molecule, was as effective in fat pieces as fat cells. Thus proinsulin was only nine to eighteen times less effective than insulin when assayed on fat pieces. With KPTI, fat pieces became markedly insensitive but still responsive to proinsulin.
These results suggest that the proinsulin molecule has biologic activity not necessitating conversion to insulin for expression. Intact tissues contain trypsin-like activity capable of converting proinsulin to insulin. This activity must be located between the vascular endothelium and before access to, but not including, the cell membrane.