Estimation of kinetic parameters of androgen-synthesizing enzyme activities in superfused Leydig cells from rat testes: Difference between endogenous and exogenous substrates. Issue 6 (June 1984)

• Record Type: Journal Article
• Authors: Kühn-Velten, Nikolaus; Wolff, Joachim; Staib, Wolfgang
• Is Part Of: Bioscience reports. Volume 4:Issue 6(1984)
• Page Start: 483
• Page End: 488
• Publication Date: 1984-06
• Subjects: Molecular biology -- Periodicals; Cytology -- Periodicals; 572.8
• DOI: 10.1007/BF01122223
• Languages: English
• ISSNs: 0144-8463

Abstract

Abstract : 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.