EVIDENCE FOR ISOFORMS OF 11-BETA-HYDROXYSTEROID DEHYDROGENASE IN THE LIVER AND KIDNEY OF THE GUINEA-PIG

In the human and in rodents like the rat and mouse, the liver enzyme 1 1 beta-hydroxysteroid dehydrogenase type I (11 beta-HSD-I) is a functi onal oxidoreductase preferring NADP(+)/NADPH as cosubstrate, while the renal isoenzyme (11 beta-HSD-II) prefers NAD(+) as cosubstrate, and s eems to be a pure oxidase and protects the tubular mineralocorticoid ( MC) receptor from occupancy by cortisol and corticosterone. We studied the enzyme kinetics of 11 beta-HSDs in kidney and liver microsomes of the guinea pig, a species whose zoological classification is still a matter of debate. With a fixed concentration of 10(-6) mol/l cortisol, liver and kidney microsomes preferred NAD(+) to NADP(+) (10(-3) mol/l ) for the conversion to cortisone. Kidney microsomes converted cortiso l to cortisone with K-m values of 0.64 mu mol/l and 9.8 mu mol/l with NAD(+) and NADP(+) as cosubstrates respectively. The reduction of cort isone to cortisol was slow with kidney microsomes, but could be marked ly enhanced by adding an NADH/NADPH regenerating system: with NADPH as preferred cosubstrate, the approximate K-m was 7.2 mu mol/l. This ind icated the existence of both isoenzymes in the guinea pig kidney. Live r microsomes oxidized cortisol to cortisone with similar K-m and V-max values for NAD(+) to NADP(+) as cosubstrates (K-m of 4.3 mu mol/l and 5.0 mu mol/l respectively). The NAD(+) preference for the oxidation o f 10(-6) mol/l cortisol described above may be due to a second, NAD(+) -preferring 11 beta-HSD with a K-m of 1.4 mu mol/l. In contrast to the kidney, liver microsomes actively converted cortisone to cortisol wit h a preference for NADPH (K-m; 1.2 mu mol/l; V-max: 467 nmol/min per m g protein). Thus, the main liver enzyme is similar to the oxidoreducta se of other species (11 beta-HSD-I) and is also present in the kidney, while the main kidney enzyme is clearly NAD(+)-preferring. This kidne y enzyme (analogous to 11 beta-HSD-II of other species) seems to be su itable for the protection of the MC receptor from the high free plasma cortisol levels of the guinea pig.