Kinetic studies on rabbit liver glucocorticoid 5alpha-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 11beta-hydroxysteroid dehydrogenase type 1 (11beta-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 (5alpha-reductase, EC 1.3.99.5). The interrelations to liver 11beta-HSD1 will be discussed. The kinetic properties of 5alpha-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 5alpha-reduced 11-dehydrocorticosterone and corticosterone metabolites. We found that both 11-dehydrocorticosterone (Km 4.2 x 10(-6) mol/l, Vmax 2,600 pmol x min(-1) x mg(-1)) and corticosterone (Km 0.5 x 10(-6) mol/l, Vmax 38 pmol x min(-1) x mg(-1)) exhibit a high affinity to 5alpha-reductase. With respect to cosubstrate-, pH-dependence and finasteride inhibition, it is likely that 11-dehydrocorticosterone metabolism is primarily controlled by isoenzyme 5alpha-reductase type 1. This study shows that the deactivation of GCS especially of 11-dehydro-glucocorticoids via 5alpha-reductase is an important metabolic pathway in the liver. The metabolic activation of GCS by 11beta-HSD could possibly lead to an excess of GCS in the hepatocytes. Due to 5alpha-reductase activity this excess can be limited - on the level of CORT as well as of 11-DHC.
Kuhlmann
K
K
Department of Obstetrics and Gynecology, Klinikum Benjamin Franklin, Free University of Berlin, Germany. kuhlmank@zedat-fu-berlin.de
Bühler
H
H
Ragosch
V
V
Halis
G
G
Weitzel
H K
HK
Hundertmark
S
S
D042842:11-beta-Hydroxysteroid Dehydrogenases; D013741:3-Oxo-5-alpha-Steroid 4-Dehydrogenase / Q000378:metabolism*; D058891:5-alpha Reductase Inhibitors; D000818:Animals; D001378:Azasteroids / Q000494:pharmacology; D003345:Corticosterone / Q000031:analogs & derivatives / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018120:Finasteride / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D006913:Hydroxysteroid Dehydrogenases / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D011817:Rabbits
C090665:4,7beta-dimethyl-4-azacholestan-3-one; D058891:5-alpha Reductase Inhibitors; D001378:Azasteroids; D004791:Enzyme Inhibitors; D018120:Finasteride; D006913:Hydroxysteroid Dehydrogenases; D042842:11-beta-Hydroxysteroid Dehydrogenases; D013741:3-Oxo-5-alpha-Steroid 4-Dehydrogenase; C003552:11-dehydrocorticosterone; D003345:Corticosterone