20beta-Hydroxysteroid oxidoreductase. Kinetics and binding of corticosteroids and corticosteroid-21-aldehydes

Corticosteroid-21-aldehydes were reduced only at C-20 by 20 beta-hydroxysteroid dehydrogenase (EC 1.1.1.53) of Streptomyces hydrogenans, and the reduction occurred by transfer of hydrogen from the B-side of NADH. A kinetic investigation of cortisol, cortisone, cortexolone, and the 21-aldehydes of each indicated: (a) the magnitude of the Michaelis constant for any substrate was independent of the second substrate concentration; (b) the 21-aldehydes had larger Michaelis constants (5- to 8-fold) and larger maximum velocities (16- to 40-fold) than the steroids from which they were synthesized; (c) the Michaelis constant for NADH, 29 muM, was independent of the steroid substrate. With cortisol and cortisol-21-aldehyde, product inhibition patterns showed only slope effects with steroid product and NAD+, suggesting a "random" mechanism. Inhibition studies with the "poor" substrate cortisol indicated that cortisol and cortisol-21-aldehyde were reduced at the same site. The inhibition constant (180 muM) agreed with the Michaelis constant of cortisol (140 muM). The steroid product, 20beta-hydroxyprogesterone, gives noncompetitive inhibition patterns with respect to NADH and cortisol-21-aldehyde, indicating a separate binding site exists on the enzyme for this inhibitor. The intrinsic protein fluorescence of 20beta-hydroxysteroid dehydrogenase was quenched by NADH (56%) with a dissociation constant of 16 muM. NAD" quenched the protein fluorescence somewhat less (31%) with a dissociation constant of 104 muM. The fluorescence of 2-p-toluidine-6-naphthalene sulfonate is enhanced in the presence of enzyme, and there is a blue shift in the emission wavelength maximum. The enzyme-enhanced 2-p-toluidine-6-naphthalene sulfonate fluorescence is quenched by NAD+ (32%) with a dissociation constant of 128 muM. Corticosteroids and their corresponding 21-aldehydes completely quench the enhanced 2-p-toluidine-6-naphthalene sulfonate fluorescence and this feature can be used to determine enzyme-steroid dissociation constants. Corticosteroid-21-aldehydes and NAD+ dissociation constants determined in this manner agree with values obtained in kinetic measurements. The dissociation constants determined for cortisol, cortisone, cortexolone, progesterone, and 20beta-hydroxyprogesterone were at least 1 order of magnitude greater than the corresponding kinetic constants, and these findings suggest the presence of a kinetically insignificant binding site.