A direct assay for proinsulin in plasma and its applications in hypoglycaemia.
A direct radioimmunoassay in unextracted plasma is described. The assay has a sensitivity of 4 pmol/l (2 standard deviation from zero). The proinsulin antiserum was immuno-adsorbed against human C-peptide and insulin coupled to glass beads. Cross-reactivity of the antiserum was assessed and shown to be less than 0.01% with both peptides. In normal healthy fasting subjects the plasma proinsulin level was 6.7 +/- 1.7 pmol/l (n = 17) (mean +/- SD). Fasting proinsulin levels in non-insulin dependent diabetics were significantly elevated compared with non diabetics (14.2 +/- 2 pmol/l (n = 11) vs 6.7 +/- 1.7 (n = 17) P less than 0.005). The insulin/proinsulin ratio was 3.4:1 in the non-insulin dependent diabetic compared with 6:1 in non-diabetics. Samples from 21 insulinoma patients were assayed and mean fasting plasma proinsulin level was 255 pmol/l +/- 479 when the patients were hypoglycaemic. The range in pro-insulin levels was large (30-2300 pmol/l). Mean fasting proinsulin level in three hypoglycaemic subjects due to sulphonylurea overdose was 15.7 +/- 2.3 pmol/l. The molar ratio of proinsulin to insulin was 1:6 in healthy subjects, 1:1 in insulinoma patients and 10:1 in sulphonylurea induced hypoglycaemic patients.
Hampton
S M
SM
Department of Biochemistry, University of Surrey, Guildford, UK.
Beyzavi
K
K
Teale
D
D
Marks
V
V
D002096:C-Peptide / Q000097:blood; D003924:Diabetes Mellitus, Type 2 / Q000097:blood; D006801:Humans; D007003:Hypoglycemia / Q000097:blood*; D007328:Insulin / Q000097:blood; D007340:Insulinoma / Q000097:blood; D010190:Pancreatic Neoplasms / Q000097:blood; D011384:Proinsulin / Q000097:blood*; D011863:Radioimmunoassay; D012680:Sensitivity and Specificity
D002096:C-Peptide; D007328:Insulin; D011384:Proinsulin