Determination of insulin to C-peptide ratios as marker for a surreptitious insulin application

The misuse of recombinant human insulin (I) and its chemically modified synthetic analogues were frequently reported in scientific as well as non-scientific literature since more than 10 years. Although the performance enhancing potency is still in discussion, the usage of this prohibited substance seemed to be widespread. Considering the recent development of mass spectrometry-based approaches to determine the chemically modified synthetic insulin analogues in urine and plasma, an unambiguous assay to uncover the misuse of recombinant human insulin is still missing. The protein biosynthesis of insulin in the Langerhans cells of the pancreas is characterized by the stepwise enzymatic cleavage of the single chain precursor molecule proinsulin into insulin and C-peptide (C). Thus, the amount of I and C, secreted by exocytosis from the vesicles into the bloodstream, is equimolar and the ratio of I/C in plasma is described to be constant. Plasma insulin and C-peptide concentrations of 50 healthy volunteers were determined by means of a sensitive ELISA (Enzyme-Linked Immunosorbent Assay). In addition 20 plasma samples provided from type II diabetics and 6 samples from type I diabetics with known amount of applied insulin were analysed. Einleitung (geändert)