Hydrolysis of a Carboxy-Terminal Fragment of Parathyroid Hormone-Related Protein by Rat Kidney: Evidence for a Crucial Role of Meprin

Abstract

Although parathyroid hormone-related protein (PTHrP) is known to be secreted into the circulation as heterogeneous forms, with its N-terminal and C-terminal fragments as well as the intact form, the fate of these molecules in the plasma has not been fully understood. As for a C-terminal fragment, the kidney seems to be physiologically important in its metabolism, because it is elevated in circulating plasma in patients with chronic renal failure. In this study, we examined the mechanism by which a C-terminal fragment of PTHrP was metabolized by rat kidney and by other rat organs in vitro. When human (h) PTHrP-(109-141) was incubated for 2 h with an extract of rat kidney, it was almost completely hydrolyzed. This hydrolysis was readily blocked by the additions of o-phenanthrolline and dithiothreitol, indicating the participation in this process of a metallo-protease possessing disulfide bonds. This hydrolytic activity showed a meprin-like character, with being sensitive to actinonin but not to phosphoramidon. Furthermore, the participation of meprin itself in this process was directly confirmed by the experiment comparing degradation products of the peptide by the microvillar membranes of rat kidney with those by a purified rat meprin, in which a large part of the metabolites by rat kidney membranes corresponded to those by rat meprin. Among other rat organs examined, the extracts of the small intestine, pancreas, spleen and urinary bladder exerted remarkable hydrolytic activities for hPTHrP-(109 - 141). The study on inhibitor sensitivities of these activities suggested the involvement of a meprin-like metalloprotease in the activity of the small intestine, and of serine protease(s) in those of the pancreas, spleen and urinary bladder. These results indicate that hydrolytic activities for hPTHrP(109 - 141) in rat organs in vitro are confined to several organs, and that meprin is probably of physiological importance in its metabolism by rat kidney.

Key words

Parathyroid Hormone-Related Peptide - Meprin - Kidney - Protease