Proximal tubular metalloprotease activity is decreased in the senescent rat kidney

Proximal tubule brush border membrane-enriched fractions (BBM) from young (4 months) and old (20-22 months) male Sprague-Dawley rats were prepared by differential centrifugation and metalloprotease activity was measured using radioiodinated insulin B chain as substrate. Proteolytic activity is expressed as Units (U), where 1 U = 1 microgram insulin B chain degraded per min and the specific activity is the U per mg BBM protein used in the assay. Total proteolytic activity (measured at pH 7) was decreased 2-fold in BBM from old rats (2.47 +/- 0.11 vs 4.71 +/- 0.35 U/mg BBM protein, p less than 0.01). The chelator, 1,10-phenanthroline, completely inhibited the proteolytic activity in both groups, suggesting that the BBM insulin B chain-degrading activity in both old and young rats was entirely due to metalloproteases. In the presence of thiorphan, a specific inhibitor of the metalloprotease endopeptidase 24.11, approximately 60% inhibition of proteolytic activity occurred in both groups. Thus, total metalloprotease and endopeptidase 24.11 activities are markedly diminished in the proximal tubule of the senescent rat kidney.