Radioimmunoassay of Human Proinsulin C-peptide Using Synthetic Human Connecting Peptide

Abstract

Radioimmunoassay of human C-peptide was established using antiserum to synthetic human connecting peptide and synthetic tyrosylated human connecting peptide. Synthetic human connecting peptide and natural human C-peptide showed the same degree of cross-reactivity with the synthetic human connecting peptide antiserum. Natural human proinsulin reacted approximately one fourth as well as the human connecting peptide or natural human C-peptide when expressed on an equimolar basis.
Synthetic porcine and bovine connecting peptide, human insulin and natural porcine proinsulin did not react with the human connecting peptide antiserum. Synthetic human connecting peptide did not cross with insulin antiserum. The senstitivity of the radioimmunoassay was 0.05ng/tube.
The assay system was available to determine C-peptide immunoreactivity in plasma, because proinsulin and proinsulin-like components had been reported to be very low in plasma. The fasting level of C-peptide immunoreactivity in plasma of normal subjects assayed by this system was 0.88±0.21ng/ml.