Estimation of kinetic parameters of androgen-synthesizing enzyme activities in superfused Leydig cells from rat testes: difference between endogenous and exogenous substrates.
Kinetic parameters of 3 beta-hydroxysteroid dehydrogenase/isomerase, steroid-17 alpha-monooxygenase, and steroid-17,20-lyase activities were estimated under steady-state conditions. Purified Leydig cells from rat testes were superfused with pregnenolone, progesterone, or 17 alpha-hydroxyprogesterone. The Km values for both the monooxygenase- and the lyase-catalyzed reactions were by factors of five to ten higher if analyzed with the exogenously added substrate (0.98 and 0.65 microM, respectively) than if calculated from endogenous substrate derived from a precursor (0.10 and 0.13 microM, respectively). This discrepancy may be explained by different substrate partition between the intra- and extracellular spaces and by different substrate concentration at the active site of the respective enzyme, depending on whether the actual substrate is of exogenous or endogenous source.
D015096:3-Hydroxysteroid Dehydrogenases / Q000378:metabolism*; D000446:Aldehyde-Lyases / Q000378:metabolism*; D000818:Animals; D006908:Hydroxyprogesterones / Q000378:metabolism; D007700:Kinetics; D007985:Leydig Cells / Q000201:enzymology*; D008297:Male; D011284:Pregnenolone / Q000378:metabolism; D011374:Progesterone / Q000378:metabolism; D051381:Rats; D013254:Steroid 17-alpha-Hydroxylase / Q000378:metabolism*; D013250:Steroid Hydroxylases / Q000378:metabolism*; D013379:Substrate Specificity
D006908:Hydroxyprogesterones; D011374:Progesterone; D011284:Pregnenolone; D015096:3-Hydroxysteroid Dehydrogenases; D013250:Steroid Hydroxylases; D013254:Steroid 17-alpha-Hydroxylase; D000446:Aldehyde-Lyases