Abstract
The cholesterol side chain cleavage enzyme in rat testis has been localized in the mitochondrial fraction of the testis homogenate. The enzyme system has an absolute requirement for a pyridine nucleotide, TPNH being the most effective with DPNH, and TPN also effective but to a lesser degree. Cleavage enzyme activity can be enhanced by repeated washing of the mitochondrial fraction with 0.25 M sucrose, which procedure presumably removes inhibiting endogenous cholesterol. This is supported by the observation that this enzyme system is extremely sensitive to the concentration of exogenously added unlabelled cholesterol. EPNH and DPN Inhibit the effectiveness of TPNH probably by inhibiting the enzymatic oxidation of pregnenolone to progesterone and the accumulated pregnenolone then inhibiting the cleavage of its immediate precursor, 20α,22α-dihydroxycholesterol. The DPN also inhibits perhaps by first being converted to DPNH through enzymatic transhydrogenation with TPMH.
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