P-450scc

Fig. 3. Gonadal steroid biosynthetic pathway and the catalytic enzymes 1) cytochrome P-450scc 2) -hydroxysteroid dehydrogenase 3) 17a-hydroxylase (P-450scc17) 4) 17,20-desmolase or 17,20-lyase 5) 17(3-hydroxysteroid dehydrogenase 6) 5a-reductase and 7) P-450 aromatase.

Vijayakumar S, Salerno JC. Molecular modeling of the 3-D structure of cytochrome P-450scc. Biochim Biophys Acta 1992 1160 281-286. 

In 1979, Simpson [24] postulated that the outer mitochondrial membrane was the site of action of a labile protein factor, necessary to facilitate the transport of cholesterol, and Privalle et al. [19] provided evidence to support the notion that transference of cholesterol from the outer to the inner membrane required an agent that is cycloheximide dependent. When rats were ether-stressed in vivo and cholesterol SCC was deliberately inhibited, cholesterol accumulated in the adrenal mitochondria, most (90%) of this being associated with the inner membrane cytochrome P-450scc. After administration of aminoglutethimide to rats to block SCC, there was a two-fold increase in inner membrane cholesterol, while cycloheximide abolished this increase. Thus, it appears that cholesterol accumulates in the inner mitochondrial membrane as a result of stress and that transference from outer to inner membrane requires a protein factor. 

Pederson and co-workers (see Ref. 25) have isolated a peptide of Mr 2200, from ACTH-stimulated rat adrenals, which contained 15% of basic aminoacid residues. The polar side-chain groups were thought to alter membrane structure so that transference of cholesterol towards the cyt P-450scc on the inner membrane would be favoured. 

Finally, in this section, mention must be made of the specificity of the SCC reaction. C22 to C27 sterols, all with saturated side-chains, underwent cleavage in rat and bovine adrenals and porcine testis, at much the same rate as cholesterol itself however, sterols with polar side-chains, e.g. 24-, 25- or 26-hydroxycholesterol, were cleaved at higher rates [46]. The 5-cholesten-3/3-ol structure seems to be a necessary requirement for the substrate for SCC [47], but the more polar sterols may enter mitochondria more readily than cholesterol itself and bind to P-450scc which, in... 

The short-term action of ACTH on the adrenocortical cell is the stimulation of the conversion of cholesterol to glucocorticoid, mineralocorticoid or androgen-precursor steroids (Fig. 2). The conversion of cholesterol to the end-product steroids involves two mitochondrial cytochrome P-450 enzymes, cytochrome P-450scc (cho-... 

Fig. 2. Pathway of biosynthesis of the glucocorticoid, cortisol, in the adrenal cortex. Cholesterol, from stores in cholesteryl esters or from other sources (see text) is converted via mitochondrial cytochrome P-450SCC (cholesterol side-chain cleavage enzyme) to pregnenolone, which then is successively converted by the microsomal enzymes cytochrome P-450,7 (17a-hydroxylase), 3 j8-hydroxysteroid dehydrogenase/ isomerase and cytochrome P-450c2, (21-hydroxylase) to 11-deoxycortisol, followed by conversion by the mitochondrial cytochrome P-450ll(3 (11/3-hydroxylase) to cortisol. The short-term action of ACTH in stimulation of steroidogenesis is to increase the availability of cholesterol for conversion by cytochrome P-450scc. From Ref. 9.

The rate of total steroidogenesis is determined by the rate of supply of cholesterol to cytochrome P-450scc. The rate of flux through this step determines the rate of synthesis of the sum of the steroid products, but does not determine the rate of synthesis of any individual steroid. The pattern of steroidogenesis, i.e. which steroids are produced and in what ratio, is determined by the relative activities of the enzymes of the steroidogenic pathway beyond the formation of pregnenolone. 

Roskelley CD, Auersperg N (1990) Density separation of rat adrenocortical cells morphology, steroidogenesis, and P-450scc expression in primary culture. Vitro Cell Dev Biol 26(5) 493-501... 

Matocha, M.F. and M.R. Waterman (1984). Discriminatory processing of the precursor forms of cytochrome P-450scc and adrenodoxin by adrenocortical and heart mitochondria. J. Biol. Chem. 259, 8672-8678. 

Hanukoglu, 1., C.T. Privalle, and C.R. Jefcoate (1981). Mechanisms of ionic activation of adrenal mitochondrial cytochromes P-450scc and P-45011 beta. J. Biol. Chem. 256, 4329—4335. 

Lambeth, J.D, S.E. Kitchen, A.A. Farooqui, R. Tuckey, and H. Kamin (1982). Cytochrome P-450scc-substrate interactions. Studies of binding and catalytic activity using hydroxycholes-terols.Biol. Chem. 257, 1876-1884. 

Tuckey, R.C. and H. Kamin (1983). Kinetics of oxygen and carbon monoxide binding to adrenal cytochrome P-450scc. Effect of cholesterol, intermediates, and phosphatidylcholine vesicles. J. Biol. Chem. 258, 4232 237. 

Nagahisa, A., T. Foo, M. Gut, and W.H. Orme-Johnson (1985). Competitive inhibition of cytochrome P-450scc by (22/f)- and (22S)-22-aminocholesterol Side chain stereochemical requirements for C-22 amine coordination to the active-site heme. J. Biol. Chem. 260, 846-851. 

Maines, M.D., P.M. Sluss, and M. Iscan (1990). cis-Platinum-mediated decrease in serum testosterone is associated with depression of luteinizing hormone receptors and cytochrome P-450scc in rat testis Endocrinology 126, 2398-2406. 

Tsubaki, M., Hiwatashi, A., Ichikawa, Y., and Hori, H. (1987). Electron paramagnetic resonance study of ferrous cytochrome P-450scc-nit oxide complexes Effects of cholesterol and its analogues. Biochemistry 26, 4527- 4534. 

Adamovich TB, Pikuleva lA, Chashchin VF, Us-anov SA (1989) Selective chemical modification of cytochrome P-450SCC lysine residues. Identification of lysines involved in the interaction with ad-renodoxin. Biochim Biophys Acta 996 247-253... 

Hume R, Kelly RW, Taylor PL, Boyd GS (1984) The catalytic cycle of cytochrome P-450scc and intermediates in the conversion of cholesterol to pregnenolone. EurJBiochem 140 583-591... 

Tuckey RC, Kamin H (1982) The oxyferro complex of adrenal cytochrome P-450scc. Effect of cholesterol and intermediates on its stability and optical characteristics. J Biol Chem 257 9309-9314... 

Nagahisa A, Spencer RW, Orme-Johnson WH (1983) Acetylenic mechanism-based inhibitors of cholesterol side chain cleavage by cytochrome P-450SCC. J Biol Chem 258 6721-6723... 

Olakanmi O, Seybert DW (1990) Modified acetylenic steroids as potent mechanism-based inhibitors of cytochrome P-450scc. J Steroid Biochem 36 273-280... 

Krueger RJ, Nagahisa A, Gut M, Wilson SR, Orme-Johnson WH (1985) Elfect of P-450scc inhibitors on corticosterone production by rat adrenal cells. J Biol Chem 260 852-859... 

Nagahisa A, Orme-Johnson WH, Wilson SR (1984) Silicon mediated suicide inhibition an efficient mechanism-based inhibitor of cytochrome P-450scc oxidation of cholesterol. J Am Chem Soc 106 1166-... 

Lambeth JD, Stevens VL (1984) Cytochrome P-450scc enzymology, and the regulation of intra-mitochondrial cholesterol delivery to the enzyme. EndocrRes 10 283-309... 

Vickery LE, Kellis JT (1983) Inhibition of adrenocortical cytochrome P-450scc by (20R)-20-phenyl-5-pregnene-3/ ,20-diol mechanism and implications for the structure of the active site. J Biol Chem 258 3832-3836... 

Tuckey RC, Cameron KJ (1993) Catalytic properties of cytochrome P-450scc purified from the human placenta comparison to bovine cytochrome P-450scc. Arch Biochem Biophys 1163 185-194... 

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