The role of dipeptidyl peptidase IV in the cleavage of glucagon family peptides -: In vivo metabolism of pituitary adenylate cyclase-activating polypeptide-(1-38)

Dipeptidyl peptidase IV ( DP- IV) is a cell surface serine dipeptidase that is involved in the regulation of the incretin hormones, glucagon- like peptide ( GLP- 1) and glucose- dependent insulinotropic polypeptide ( GIP). There is accumulating evidence that other members of the glucagon family of peptides are also endogenous substrates for this enzyme. To identify candidate substrates for DP- IV, a mass spectrometry- based protease assay was developed that measures cleavage efficiencies (k(cat)/K-m) of polypeptides in a mixture, using only a few picomoles of each substrate and physiological amounts of enzyme in a single kinetic experiment. Oxyntomodulin and the growth hormone-( 1 - 43) fragment were identified as new candidate in vivo substrates. Pituitary adenylate cyclase- activating polypeptide-( 1 - 38) ( PACAP38), a critical mediator of lipid and carbohydrate metabolism, was also determined to be efficiently processed by DP- IV in vitro. The catabolism of exogenously administered PACAP38 in wild type and DP- IV-deficient C57Bl/ 6 mice was monitored by tandem mass spectrometry. Animals lacking DP- IV exhibited a significantly slower clearance of the circulating peptide with virtually complete suppression of the inactive DP- IV metabolite, PACAP-( 3 - 38). These in vivo results suggest that DP- IV plays a major role in the degradation of circulating PACAP38.