Measurement of serum C-peptide immunoreactivity by radioimmunoassay in insulin-dependent diabetics.
A radioimmunoassay for C-peptide utilizing synthetic C-peptide as an antigen and tyrosylated synthetic C-peptide for iodination was evaluated for its clinical use. Mean fasting C-peptide levels in 24 normal subjects was 2.6 +/- 0.8 ng/ml. During the oral glucose tolerance test, baseline C-peptide in five normal subjects was 1.5 +/- 0.8 ng/ml, and at 60 min was 5.6 +/- 1.6 ng/ml. For two insulin-dependent diabetic patients, diagnoses of factitious hypoglycemia were documented on the basis of simultaneous free insulin and C-peptide determinations. Sera from 24 insulin-dependent diabetics were analyzed for free and total immunoreactive C-peptide and insulin levels. For 20% of juvenile and 64% of maturity-onset diabetics, the presence of proinsulin-like material bound to insulin antibodies was demonstrated by measurement in unextracted serum. This accounted for 20% to 100% of total C-peptide immunoreactivity in these patients. Simple polyethylene glycol precipitation of immune complexes and the measurement of free immunoreactive C-peptide in the supernatant demonstrated subnormal levels (less than 0.5 ng/ml) in all juvenile diabetics and normal levels (1.8 +/- 1.3 ng/ml) in 70% of maturity-onset diabetics.
D000293:Adolescent; D000328:Adult; D000936:Antigen-Antibody Complex; D002096:C-Peptide / Q000097:blood / Q000276:immunology*; D003920:Diabetes Mellitus / Q000097:blood / Q000276:immunology*; D003922:Diabetes Mellitus, Type 1 / Q000097:blood / Q000276:immunology; D005260:Female; D005951:Glucose Tolerance Test; D006801:Humans; D007003:Hypoglycemia / Q000175:diagnosis; D007328:Insulin / Q000097:blood; D007330:Insulin Antibodies; D010455:Peptides / Q000276:immunology*; D011863:Radioimmunoassay
D000936:Antigen-Antibody Complex; D002096:C-Peptide; D007328:Insulin; D007330:Insulin Antibodies; D010455:Peptides