Kinetic Studies on Rabbit Liver Glucocorticoid 5α-Reductase

The serum concentration of active glucocorticosteroids depends not only on adrenal synthesis but also on enzymatic activation of 11-dehydro-glucocorticoids in the liver by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). In order to define the respective involvement of other regulative enzymes in the metabolism of 11-dehydro-glucocorticoids in the liver, the objective of this study was to evaluate the kinetic behavior of NADPH:delta 4-3-ketosteroid-5alpha-reductase (5α-reductase, EC 1.3.99.5). The interrelations to liver 11β-HSD1 will be discussed. The kinetic properties of 5α-reductase of the rabbit liver were measured by a radioenzymatic assay and characterized with respect to protein-, substrate-, cosubstrate-, and pH-dependence. Michaelis-Menten enzyme kinetic parameters (Km and Vmax) were obtained for the formation of 5α-reduced 11-dehydrocorticosterone and corticosterone metabolites. We found that both 11-dehydrocorticosterone (Km 4.2 × 10^-6 mol/l, Vmax 2600 pmol × min^-1 × mg^-1) and corticosterone (Km 0.5 × 10^-6 mol/l, Vmax 38 pmol × min^-1 × mg^-1) exhibit a high affinity to 5α-reductase. With respect to cosubstrate-, pH-dependence and finasteride inhibition, it is likely that 11-dehydrocorticosterone metabolism is primarily controlled by isoenzyme 5α-reductase type 1. This study shows that the deactivation of GCS especially of 11-dehydro-glucocorticoids via 5α-reductase is an important metabolic pathway in the liver. The metabolic activation of GCS by 11β-HSD could possibly lead to an excess of GCS in the hepatocytes. Due to 5α-reductase activity this excess can be limited - on the level of CORT as well as of 11-DHC.

Key words

5-Alpha-Reductase - Glucocorticosteroids - EC 1.3.99.5 - Liver - Rabbit