New Methods to Non‐invasively Measure the Kinetics of Pancreatic Beta Cell Insulin in Circulation in Vivo Using ² H ₂ O Labeling and Mass Spectrometry

Insulin is a peptide hormone that regulates the metabolism of carbohydrates, proteins and fats. Pancreatic beta cells specifically synthesize and secretes insulin into blood circulation. Type II diabetes is characterized as the inability to respond to and produce insulin. As a progressive disease from a normal to a diabetic state, patients exhibit severe hyperglycemia, beta cell exhaustion and inability to synthesize insulin. There is currently minimal knowledge to measure insulin kinetics and methods to monitor the transition over the course of diabetic disease, however. Our objective is to measure the kinetics of insulin synthesis and secretion from pancreatic beta cells into blood in humans by use of stable isotopic metabolic labeling with heavy water (2H2O) and mass spectrometry. We hypothesize that pancreatic reserve or treatment thereof can be monitored or diagnosed by measuring plasma insulin kinetics as a non-invasive metric of pancreatic insulin. To test our hypothesis, insulin was isolated by immunoprecipitation from plasma in humans and its synthesis rate measured by 2H2O labeling and mass spectrometry. We recently determined by metabolic labeling that insulin in the human body has a half-life of 4.9 days, reflecting the turnover (replacement) of insulin in beta cell secretory granules by newly synthesized molecules. After breakfast and lunch, the contribution from recently synthesized insulin in plasma fell slightly from ~ 60% new at day 6, suggesting secretion from a less labeled reserve or secretory pool. There was no difference in labeled plasma insulin after each meal that suggests a steady state contribution of newly synthesized pancreatic insulin secreted into the plasma. In 2 subjects’ post-bariatric surgery for morbid obesity, insulin in blood was almost 100% replaced by newly synthesized molecules after 6 days consistent with reduced beta cell secretory reserve for insulin. In pre-diabetic fatty liver disease patients, the half life of insulin was determined as 7.2 days suggesting slower synthesis and potential onset of diabetes as compared to normal healthy controls that exhibited a half-life of 4.9 days (*p=0.01). These are the first insulin turnover direct experimental data in humans and the results are very promising, suggesting that pancreatic beta cell reserve, the key factor in progression from insulin resistance to type II diabetes, can be monitored by measuring plasma insulin kinetics in human subjects.