Identification of dipeptidyl peptidase IV as a protein shared by the plasma membrane of hepatocytes and liver biomatrix

The histotypic organization of liver parenchyma involves specific intercellular contacts and interaction of hepatocytes with supporting biomatrix. Evidence from this laboratory identified a peptide (Hep105, apparent M_r 105 000) that is shared by the plasma membrane of rat hepatocytes and rat liver biomatrix. This report identifies Hep105 as a peptide component of dipeptidyl peptidase IV (DPPIV; EC 3.4.14.-). A monoclonal antibody (MAb 236.3) was shown to immunoprecipitate DPPIV from non-ionic detergent extracts of surface-labeled ¹²⁵I hepatocytes. The immunoprecipitate contained two ¹²⁵I-labeled peptides: Hep105 and a peptide of apparent M_r 150 000 (Hep150). Proteolysis of ¹²⁵I-labeled Hep105 and Hep150 by Staphylococcus aureus V8 protease yielded essentially identical patterns of ¹²⁵I-labeled peptide degradation products, indicating that Hep105 and Hep150 are structurally related. Only Hep150 exhibited DPPIV activity on transblot analysis, an observation that is consistent with the interpretation that it is the monomeric form of the enzyme. Heating (100 °C, 5 min) of purified Hep150 in the presence of sodium dodecylsulfate (SDS) resulted in its conversion to Hep105 and the disappearance of any demonstrable enzymatic activity. ³H-labeled diisopropyl fluorophosphate was incorporated into Hep105, indicating that Hep105 contains the active site for DPPIV. Purified rat liver biomatrix was shown to possess significant DPPIV activity. Taken together, these data indicate that Hep105 s a peptide component of DPPIV.