Evidence for Separate Handling In Vivo of Different Regions of the Insulin Molecule Using A14- and B1-labeled Insulin Tracers

Abstract

To compare the metabolic characteristics and degradation of insulin tracers labeled unselectively, selectively at the A14 position (A14-monoiodoinsulin), and selectively at the B1 position (B1-monoiodoinsulin), we have followed the time course of disappearance of intact (immunoprecipitable [IP] and trichloroacetic acid [TCA] precipitable) iodoinsulin after bolus injection into greyhounds. We have used noncompartmental analysis to determine metabolic clearance rate (MCR) and apparent distribution space (DS). We have also measured the appearance of non-IP- and non-TCA-precipitable fragments, and have developed a mathematical model using compartmental analysis to explain the observed differences. B1-Monoiodoinsulin has a significantly higher MCR (16.3 ml/min/kg) than both A14-monoiodoinsulin (10.6 ml/min/kg) and unfractionated tracers (7.6 ml/min/kg) as determined by immunoprecipitation, and reaches the values observed for native insulin in greyhounds. MCR values obtained by TCA precipitation are approximately one-half of those obtained by IP for all 3 tracers. The concentration of non-IP fragments is significantly lower with B1-monoiodoinsulin than with the other tracers. Compartmental analysis suggests this to be due to greater intracellular retention of the B1 moiety during the experimental period. We conclude that: (1) by the criterion of MCR, B1-monoiodoinsulin seems to behave more like native insulin than other preparations tested; (2) the reduced MCR of A14-monoiodoinsulin raises doubts about its validity as a tracer for insulin; (3) a high-molecular-weight product of insulin degradation, which includes both the B1 and the A14–A19 regions of the molecule, is released into the circulation; and (4) smaller fragments containing A14–A19 reappear in the circulation more rapidly than fragments containing B1. Thus, the use of specifically labeled insulin tracers allows investigation of the mechanisms of intracellular processing in Vivo.