Kinetic parameters of 3β-hydroxysteroid dehydrogenase/isomerase, steroid-17α-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 17α-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 μM, respectively) than if calculated from endogenous substrate derived from a precursor (0.10 and 0.13 μM, respectively). This discrepancy may be explained by different substrate partition between the intra- and extraceIJular 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.
Estimation of kinetic parameters of androgen-synthesizing enzyme activities in superfused Leydig cells from rat testes: Difference between endogenous and exogenous substrates
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Nikolaus Kühn-Velten, Joachim Wolff, Wolfgang Staib; Estimation of kinetic parameters of androgen-synthesizing enzyme activities in superfused Leydig cells from rat testes: Difference between endogenous and exogenous substrates. Biosci Rep 1 June 1984; 4 (6): 483–488. doi: https://doi.org/10.1007/BF01122223
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