Presence and possible role of a renal brush-border Gly-Pro-X-releasing exopeptidase

¹ Section for Clinical Research and Molecular Medicine, Medical Department A, University of Bergen, N-5016 Haukeland Sykehus, Bergen, Norway; and Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, South Carolina 29425

Differential pelleting of a rat renal cortical homogenate clearly demonstrated the microsomal localization of an N-terminal exopeptidase of the tripeptidyl peptidase (TPP) class that typically requires a free N-terminus to catalyze the release of collagen-related (Gly-Pro-X) "triplets" at pH 7.0 (TPP 7). Once fractionated by differential pelleting, microsomal populations of different size were subfractionated by equilibrium banding in sucrose gradients for the purpose of comparing the distribution profiles and the isopycnic banding densities of TPP 7 to those for known marker enzymes. This analytical approach permitted the localization of these enzymes to specific membrane domains in the renal cortex and provided evidence for the brush-border location of TPP 7. Notably, dipeptidyl peptidase IV (DPP IV), an established plasma membrane exopeptidase with a prolyl-bond specificity, gave banding densities and distributions that were consistent with the presence of both TPP 7 and DPP IV in the same membrane compartment. Because triplets of the Gly-Pro-X type released by TPP 7 would be ideal substrates for DPP IV, a coupled TPP 7-DPP IV exopeptidase mechanism at the luminal surface (brush border) of proximal tubule cells could therefore make a major contribution to the renal degradation and reabsorption of filtered collagen fragments.

Submitted on February 11, 1987
Accepted on May 13, 1987