title: A mathematical analysis of the substrate effect observed in 3beta-hydroxysteroid dehydrogenase reactions of rat testicular microsomes.
abstract: The substrate effect in enzyme reactions has been explained mostly in terms of an additional substrate binding site on the enzyme other than the catalytic site. A rate equation for the reaction is introduced according to the steady state mechanism as follows: v = (Ps3+Qs2+Rs)/(s3+Ls2+Ms+N), were the six parameters, L,M,N,P,Q, and R, can be determined by the least-squares method from the experimental points. The v vs. s curve has an asymptote parallel to the s abscissa, and can be classified into one of four types. The type A curve has an intersection with the asymptote and an apparent maximum velocity; the curve descends toward the asymptote. Type B has no intersection and no stationary point; the curve ascends toward the asymptote. Type C has two intersections and two stationary points, an apparent maximum velocity and a minimum velocity; the curve ascends toward the asymptote. Type D has no intersection and two stationary points; the curve ascends toward the asymptote. The equation was applied to the 3beta-hydroxysteroid dehydrogenase [EC 1.1.1.145] reaction of rat testicular microsomes. The conversion of 3beta-hydroxyandrost-5-ene-17-one was represented by type C, with an apparent maximum velocity of 0.338 nmole/min/mg protein at 0.912 muM of the substrate concentration, minimum velocity of 0.108 nmole/min at 16.6 muM, and saturating velocity of 0.169 nmole/min at infinite concentration of the substrate. The converson of 3beta-hydroxypregn-5-ene-20-one was of type B, having two inflexion points, 0.320 nmole/min at 2.735 muM and 0.814 nmole/min at 12.39 muM, and a saturating velocity of 3.80 nmoles/min at infinite concentration of the substrate.
mesh_terms: D000818:Animals; D001665:Binding Sites; D003687:Dehydroepiandrosterone / Q000378:metabolism; D006913:Hydroxysteroid Dehydrogenases / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008861:Microsomes / Q000201:enzymology; D008956:Models, Chemical; D011284:Pregnenolone / Q000378:metabolism; D011375:Progesterone Reductase / Q000378:metabolism; D011485:Protein Binding; D051381:Rats; D013737:Testis / Q000201:enzymology*
chemical_list: D003687:Dehydroepiandrosterone; D011284:Pregnenolone; D006913:Hydroxysteroid Dehydrogenases; D011375:Progesterone Reductase