Identification of proinsulin and C-peptide in human serum by a specific immunoassay

An immunoassay for the measurement of human proinsulin and C-peptide is described. By a combination of this system and an insulin immunoassay, proinsulin, insulin, and C-peptide were identified in acid-ethanol extracts of serum after gel filtration. In keeping with the results of biosynthetic studies, insulin and C-peptide were found to be present in equimolar amounts. The absolute values of serum proinsulin determined independently in the two assays corresponded closely.