A microcompartmentation analysis of intermediate leakage response to substrate excess in a membrane‐bound bifunctional enzyme: Local control of hydroxyprogesterone channeling efficiency during cytochrome P450XVII‐catalysed androgen biosynthesis

Abstract

Evidence is presented for the first time that the cytochrome P450XVII-catalysed androgen formation from progesterone (P) in rat testicular microsomal membranes represents a metabolic sequence that exhibits the ability of intrinsic regulation of intermediate transfer and product formation efficiency. Exposure of this system, which catalyses a hydroxylation and oxidative cleavage reaction sequence, to increasing P concentration results in a decreased specific retention of the putative intermediate, 17 alpha-hydroxyprogesterone (HP) in the membrane compartment, and in a decreased HP conversion to androgens in favour of increasing HP transfer into the extramembrane space. This behaviour results in a decreased ratio of product vs. intermediate formation rates, which is interpreted as a partial "uncoupling" of the normal hydroxylation and cleavage reaction sequence catalysed by P450XVII. A similar pattern can likewise be observed in isolated testicular Leydig cells after exposure to increasing P concentrations under more physiological continuous-flow conditions. Further calculations indirectly indicate that the specific retention of HP in the membrane compartment can partially be attributed to its specific association with the P450XVII during catalysis. The results strongly suggest the existence of a local "channel" that becomes more leaky and therefore less effective if loaded with high influx rates. This pattern may be related to significant but incomplete competition of exogenously entering P and endogenously formed and transiently bound HP for oxygen attack at the P450XVII active site.

Varicocele: An Endocrinological Perspective

“…Scholler et al found a significantly more elevated basal concentrations of 17-OH-progesterone in varicocele-bearing patients, confirming the suggested deficiency of testicular C-17,20-lyase, due to the severe dependence of this cytochrome P450-XVII-dependent enzyme on the scrotal temperature ( 42 , 43 ).…”

Simultaneous identification and quantification of synthetic cannabinoids (cannabimimetics) in serum, hair, and urine by rapid and sensitive HPLC tandem mass spectrometry screenings: overview and experience from routine testing

“…In contrast to general opinion, metabolites of CB receptor ligands are not necessarily inactive. Even in THC-COOH, one of the final end products of THC obtained after consecutive [41] cytochrome P450 (CYP)-catalyzed hydroxylations, analgesic properties remain whereas the psychoactive effect is lost [42] . The endocannabinoid, anandamide, is hydroxylated by CYP3A4 and CYP4F2 in human liver and brain.…”