1554-P: Insights Into Insulin Synthesis and Turnover Using ¹³C₅¹⁵N-Valine to Label C-Peptide in Humans

The insulin secretory response to an intravenous, as opposed to an oral, glucose challenge is characterized by 2 phases the first of which is thought to represent preformed insulin whereas the second represents provisioning from newly synthesized insulin or newly mobilized insulin granules. The first phase is absent in people with type 2 diabetes (T2DM). Whether this reflects defects in insulin synthesis, insulin storage or both remains to be ascertained. To address these questions, we studied 10 subjects with and without T2DM. Subjects were admitted to the Clinical Research Unit at 1700 and consumed a standard meal and fasted overnight. At 2100 5g of arginine was given. An infusion of ¹³C₅¹⁵N-Valine (Prime 1.5mg/Kg body mass and then 1.5mg/Kg/hr) commenced and continued till 0600 the following morning. At 0800 subjects underwent a 75g OGTT followed by 5g I.V. arginine at 1200. Blood was sampled at frequent intervals to measure hormone and substrate concentrations as well as C-peptide labelled with ¹³C₅¹⁵N-Valine. This was purified via repetitive cycling through an affinity resin conjugated with C-peptide antibody in a custom manufactured mass spectrometric immunoassay pipette tip. Peptides were detected in positive mode with a high resolution, Q Exactive Plus Mass Spectrometer optimized by infusion of synthetic peptides. During the overnight fast, average C-peptide enrichment in the peripheral circulation was higher in T2DM (0.3 ± 0.3 vs. 2.3 ± 0.2 %, nondiabetic vs T2DM respectively, p<0.01). After oral glucose, C-peptide enrichment was lower in T2DM (6.7 ± 0.5 vs. 5.0 ± 0.4 %, p=0.05). In contrast after I.V. arginine challenge, enrichment did not differ (5.6 ± 1.2 vs. 3.5 ± 0.4 %, p=0.08). These data imply that insulin turnover is higher in T2DM: a greater proportion of newly synthesized insulin is secreted during fasting. This may explain the depletion of 1st phase insulin and other easily releasable granule pools in T2DM.