Insulin-like insulinase-resistant material, distinguishable from normal insulin, in juvenile diabetes.

This study compares some properties of the immunoreactive insulin-like material extracted from the urine of children with overt diabetes with that from normal children. Insulin-like species were fractionated by gel filtration and by isoelectric focusing and were tested for sensitivity to an insulin-specific degradative enzyme. Insulin concentration was measured by radioimmunoassay. The major insulin-like component from the urine of ten normal children and fifteen untreated juvenile diabetics and from the urine of four and the serum of one latent diabetics behaved (on gel filtration) as normal insulin, was sensitive to insulinase, and (in all cases studied) had an identical isoelectric point (resolution 0.1 pH units). A proportion of the immunoreactivity extracted from urine (0-4 per cent from normal children, 5-30 per cent from twelve of the thirteen nonobese untreated diabetic children) eluted from the gel filtration column before insulin. This material from diabetic urine was of two size classes, "proinsulin-like" and "mid-insulin," both resistant to degradation by insulinase. Insulinase-resistant immunoreactivity from one patient was analyzed by isoelectric focusing. Urine samples from two obese children with overt diabetes and four children with latent diabetes contained normal proportions (less than 4 per cent) of immunoreactive species larger than insulin. The possible nature and significance of the present insulinase-resistant species are briefly considered.

The excretion of proinsulin and insulin in urine.

Proinsulin-like components (PLC) and insulin have been measured in 24 hr urine samples from 8 healthy subjects. The mean excretion of PLC was 45.8 ng and that of insulin 314 ng; the PLC: insulin ratio was 0.14. Urinary PLC was increased 3.5 fold in a patient with a pancreatic islet cell tumor and the PLC: insulin ratio was 0.35. The urinary PLC: insulin ratio is lower than that of serum, presumably because of the relatively lower urinary clearance of the larger molecular weight PLC.

Metabolism of proinsulin, insulin, and C-peptide in the rat.

The renal extraction and excretion of bovine proinsulin, insulin, and C-peptide and the contribution of the kidney to their total metabolic clearance rate (MCR) were studied in the rat. Metabolic clearance rates were measured by the constant infusion technique and plasma and urine concentrations of each polypeptide were determined by radioimmunoassay. The MCR of insulin (16.4+/-0.4 ml/min) was significantly greater than that of either proinsulin (6.7+/-0.3 ml/min) or C-peptide (4.6+/-0.2 ml/min). Metabolic clearance rates were independent of plasma levels over a range of steady-state plasma concentrations varying from 1 to 15 ng/ml.In contrast to the differences in their metabolic clearance rates, the renal disposition of the three polypeptides was similar, being characterized by high extraction and very low urinary clearance. The renal arteriovenous difference of proinsulin, insulin, and C-peptide averaged 36, 40, and 44%, respectively, and was linearly related to their arterial concentration between 2 and 25 ng/ml. When glomerular filtration was markedly reduced or stopped by ureteral obstruction, the renal extraction of proinsulin, insulin, and C-peptide was invariably greater than the simultaneously measured extraction of inulin, indicating that these polypeptides are removed from the renal circulation by both glomerular filtration and direct uptake from peritubular capillary blood. The fractional urinary clearance of each polypeptide never exceeded 0.6%, indicating that more than 99% of the amount filtered was sequestered in the kidney. The renal removal of proinsulin and C-peptide from the circulation accounts for 55 and 69% of their metabolic clerance rates, while the renal contribution to the peripheral metabolism of insulin was smaller, averaging 33%. This difference is due to the fact that insulin, but not the other two polypeptides, is metabolized to a significant extent by the liver. These results define the renal handling of proinsulin, insulin, and C-peptide in the rat and indicate that in this species the kidney represents a major site for insulin metabolism and is the main organ responsible for the degradation of proinsulin and C-peptide.