Since circulating proinsulin has been suggested to be important in the pathogenesis of noninsulin-dependent diabetes, and biosynthetic human proinsulin (HPI) may have a therapeutic role in patients with diabetes mellitus, the biological activity of proinsulin metabolites is of potential significance. Moreover, recent studies have suggested that the majority of circulating proinsulin immunoreactivity consists of metabolites. We, therefore, compared the blood glucose-lowering ability and MCR of the two proinsulin metabolites des-(31,32)HPI and des-(64,65)HPI with intact HPI in seven anesthetized dogs after an overnight fast. Intravenous bolus injections of 12.5 micrograms HPI/kg BW and equimolar amounts of des-(31,32)HPI and des-(64,65)HPI were given on three separate occasions. In addition to blood glucose, des-(31,33)HPI, des-(64,65)HPI, and HPI were measured using an insulin RIA and peptide-specific standard curves. Kinetic parameters were derived by fitting two exponentials to the respective decay curves. The MCR of HPI (3.3 +/- 0.1 ml/kg.min) was significantly lower (P less than 0.05) than that of des-(64,65)HPI (6.4 +/- 0.6 ml/kg.min), but was not significantly different from that of des-(31,32)HPI (3.8 +/- 0.4 ml/kg.min). The MCR of biosynthetic insulin (17.2 +/- 1.8 ml/kg.min), as measured in three of the dogs, was higher than that of HPI or the two metabolites. The blood glucose-lowering ability (defined as nadir glucose/fasting glucose, expressed as a percentage) of des-(64,65)HPI (49.3 +/- 5.0%) was significantly greater (P less than 0.05) than that of intact HPI (87 +/- 2.2%), and the glucose-lowering ability of des-(31,32)HPI (75.2 +/- 3.8%) was intermediate. In conclusion, HPI metabolites are more biologically active than intact HPI. The extent of in vivo conversion of proinsulin to metabolites may enhance the biological activity of proinsulin and, thus, have physiological, pathophysiological, and therapeutic significance.