Prednisolone enhances aminopeptidase turnover in adult rat small intestine.

In adult male rats, fed prednisolone (0.75 mg/kg/day) for 7 days, brush border aminopeptidase activity was increased (P less than 0.001) by 106% compared to pair-fed controls. [14C]Tyrosine was injected intraperitoneally 16 h and [3H]tyrosine 6 h before death. The 3H/14C ratio was 1.79 +/- 0.21 (S.D.) in purified microvillus membranes from treated rats compared to 1.30 +/- 0.16 (P less than 0.01) in controls. Polyacrylamide gel electrophoresis of brush border membranes under denaturing conditions showed that the increased double-isotope ratio in membranes from treated rats was mainly in the high molecular weight protein subunits (greater than 80 kDa). Detergent-solubilized aminopeptidase was purified after in vivo labeling by protein A-Sepharose-antiaminopeptidase affinity chromatography. The 3H/14C ratio in aminopeptidase was 2.42 +/- 0.15 (P less than 0.05) in treated rats compared to 1.63 +/- 0.13 in controls. Over the experimental period steady-state isotope reutilization and protein labeling was demonstrated and there was no isotope metabolism. Total microvillus membrane lipid content was unaffected by prednisolone. We conclude that prednisolone increases brush border aminopeptidase activity by increasing enzyme turnover. Other high molecular weight brush border proteins were similarly affected.