[Radioimmunologic proinsulin determination in the serum using an "insulin-catabolizing protease" from the rat liver]

Proinsulin determinations in human serum are of particular interest, because proinsulin represents only a fraction of the biological activity of insulin. Proinsulin like components are determined by means of an "Insulin Degrading Protease" (ISP) which degrades insulin into non-radioimmunoassayable fission products. The radioimmunoassay before and after incubation with this enzyme provides values for the total-insulin and the proinsulin. The preparation of the ISP is done by homogenization and ultracentrifugation of fresh liver tissue followed by dialysation and dryfreezing. After further concentration by adsorption to a calcium-phosphate-gel the ISP degrades pure porcine insulin within 20' down to a rest of 9%, but only 7% of pure porcine proinsulin is altered. The proinsulin values provided this way are reproducible and exact enough for the clinical use. They correspond largely to those methods using chromatographic columns. In 13 persons the proinsulin fraction of the total insulin after stimulation with glucose and tolbutamid has been registered. The proinsulin shows in the oral glucose tolerance test compared to insulin a lower and delayed increase. After tolbutamid only minor changes of proinsulin values have been seen. As long as it is difficult to prepare a proinsulin specific antibody for a direct proinsulin radioimmunoassay, the ISP-method is even qualified for extensive proinsulin determinations in human serum.