Uremia and insulin resistance: N-carbamoyl-asparagine decreases insulin-sensitive glucose uptake in rat adipocytes

Abstract

Background. In uremia, urea-derived cyanate reacts with amino groups irreversibly forming carbamoyl amino acids (C-AA) and carbamoyl proteins. Carbamoylated molecules can affect binding and trafficking and alter metabolic pathways. The C-AA role in insulin-sensitive glucose transport has not been explored and may contribute to insulin resistance in uremia. Methods. Insulin-stimulated glucose uptake by cultured rat adipocytes was measured using both 3-minute and 3-second assays. Adipocytes were incubated for 24 hours in medium containing 0.5 μmol/mL of 15 different C-AA. 125I-insulin binding studies were done. C-asparagine in plasma from 10 uremic patients on continuous ambulatory peritoneal dialysis (CAPD) was measured using high-performance liquid chromatography (HPLC). Results. Insulin-sensitive glucose uptake was reduced 34% by N-carbamoyl-L-asparagine, (N-C-Asn), in a dose-dependent manner with a half-maximally effective concentration of 0.15 μmol/mL. Fourteen other N-carbamoyl-amino acids as well as 0.5 μmol/mL of asparagine did not affect insulin sensitive glucose uptake. N-C-Asn, L-asparagine, and the other N-carbamoyl amino acids (0.5 μmol/mL) had no effect on basal glucose uptake. These data suggest that that N-C-Asn affects the insulin sensitive glucose transporter system. 125I-insulin binding studies demonstrated that N-C-Asn did not alter insulin binding. Glucose uptake measured using a 3-second assay showed that the glucose affinity of the transporter and glucose phosphorylation were not affected. In uremic patients managed by CAPD, the mean free N-C-Asn plasma level was 1.33 μmol/mL. Conclusion. These data suggest that N-C-Asn concentration may contribute to the insulin resistance seen in uremia.