Side chain cleavage enzyme

Testiculat androgens are synthesized in the interstitial tissue by the Leydig cells. The immediate precursor of the gonadal steroids, as for the adrenal steroids, is cholesterol. The rate-limiting step, as in the adrenal, is delivery of cholesterol to the inner membrane of the mitochondria by the transport protein StAR. Once in the proper location, cholesterol is acted upon by the side chain cleavage enzyme P450scc. The conversion of cholesterol to pregnenolone is identical in adrenal, ovary, and testis. In the latter two tissues, however, the reaction is promoted by LH rather than ACTH. 

CYPUAl P450scc Cholesterol side-chain cleavage enzyme— Pregnenolone... 

Takase M, Ukena K, Yamazaki T, Kominami S, Tsutsui K. 1999. Pregnenolone, pregnenolone sulfate, and cytochrome P450 side-chain cleavage enzyme in the amphibian brain and their seasonal changes. Endocrinology 140 1936-1944. 

Ukena K, Usui M, Kohchi C, Tsutsui K. 1998. Cytochrome P450 side-chain cleavage enzyme in the cerebellar Purkinje neuron and its neonatal change in rats. Endocrinology 139 137-147. 

Another possibility for the stimulation of the conversion of cholesterol at a fixed substrate concentration is to stimulate the cholesterol side-chain cleavage enzyme activity. Calcium has been shown to do this but very high levels are required [59]. Other factors have been shown to have similar effects but most of the claims made in the past have not been confirmed. This applies to the rate limiting production of NADPH, the effects of specific cytosolic fractions on isolated mitochondria [60] and the stimulatory actions of specific phospholipids [61,62]. 

Fig. 3. Steroidogenic pathway in granulosa cells. A. Lipoprotein in receptors. B. 3-Hydroxy-3-methyl-glutaryl coenzyme A reductase (HMG-CoA reductase). C. Acyl-coenzyme A (cholesterol acyl transferase). D. Cholesterol esterase. E. Cholesterol transport to the mitochondria. F. Cholesterol side-chain cleavage enzymes (phospholipid membrane environment and enzyme levels). G. 3/3-Hydroxysteroid dehydrogenase (3/3-HSD). H. 20a-Hydroxysteroid dehydrogenase (20a-HSD). I. Aromatases.

Induction of cholesterol side-chain cleavage enzymes... 

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.

Fig. 10. Hypothesis for the interaction of the A-kinase (A-K) system activated by ACTH with the C-kinase system (C-K) in the long-term regulation of the enzymes of steroidogenesis throughout the adrenal cortex. The primary determinant of zonation of A-kinase and C-kinase activities, via zonation of cell surface receptors or other mechanisms, is hypothesized to be a gradient (e.g., of steroids) created by the pattern of blood flow in the adrenal cortex. The resultant levels of induction of steroidogenic enzymes are indicated by to show particular elevation and by to show particular lack of induction or suppression of induction. Other enzymes involved in steroidogenesis are shown in parentheses. SCC=cholesterol side-chain cleavage enzyme 3/3=3/3-hydroxysteroid dehydrogenase 17a=17a-hy-droxylase 21 =21-hydroxylase 11/3= 11/3-hydroxylase CMO= corticosterone methyl oxidase activity of 11/3-hydroxylase. Secreted steroids are indicated as B=corticosterone Aldo=aldosterone F=cortisol DHEA(S)= dehydroepiandrosterone (sulfate).

As the SCCE described above (Section 6.3.1.1) maybe part of a protein complex in the mitochondria, more effort was directed to study the possible interaction partners, especially the peripheral-t)q)e benzodiazepine receptor (PBR) (Papadopoulos et al, 1997 Koch, 2002) and the acyl-CoA-binding protein (ACBP Metzner et al, 2000). The ACBPs bind to the peripheral-t)q)e PBR present in the envelope of mitochondria (Gamier et al, 1994). This interaction stimulates the transport of cholesterol into mitochondria (Papadopoulos and Brown, 1995). The cholesterol taken up into the mitochondria is available as a substrate to the side-chain cleavage enzyme which transforms cholesterol into pregnenolone (Papadopoulos et al, 1997). Because of its interaction with PBR, ACBP is also described as diazepam-binding inhibitor or endozepine. Some isoforms of the latter were isolated and characterized from D. lanata (Metzner et al, 2000). Lindemann and Luckner (1997) speculated that cardenolide formation is regulated mainly by the availability of cholesterol and its transport into mitochondria, where the P450scc is assumed to be located. 

Figure 51-6 Biosynthesis of corticosteroids. Roman numerals I (side-chain cleavage enzyme), II (3-p-ol dehydrogenase and/or A" isomerase), III (21-hydroxylase), and IV (lip-hydroxyiase) indicate sites of major blocks that cause adrenogenital syndromes. (Copyright 1959 CIBA Pharmaceutical Co. Division of CIBA-GEIGY Corp. Reproduced, with permission, from The CiBA Collection of Medical Illustrations by Netter FH.AII rights reserved.)...

Concentrations of all classes of steroid hormones are low when a deficiency of the cholesterol side-chain cleavage enzyme is present. This disorder is extremely rare few affected children survive infancy, and sexual differentiation is absent in boys. 

ACTH exerts effects on the zona fasciculata by surface binding, calcium-dependent activation of membrane-bound adenylate cyclase and intracellular cAMP mediation. It increases activities of cholesterol esterase and cholesterol side-chain cleavage enzyme (CYPI 1 A), thereby stimulating production of pregnenolone. Cholesterol side-chain cleavage enzyme is a cytochrome P-450 enzyme present in mitochondria. ACTH promotes the... 

Fig. 8. A schematic diagram showing cellular processes known to require SCP2. The reactions in cholesterol biosynthesis and esterification have been shown for liver. The reactions involving cholesterol transport from cytoplasmic lipid inclusion droplets to mitochondria have been demonstrated in endocrine tissues. Choi and C. cholesterol ACAT, acyl-CoA cholesterol acyl transferase C.E., cholesterol ester SEH, sterol ester hydrolase (hormone-dependent) P-450s,, cytochrome P-450 cholesterol side-chain cleavage enzyme PREG, pregnenolone.

Fig. 16. Mitochondrial import of cholesterol. The StAR protein is the major cholesterol (CHOL) carrier bringing the lipid to import sites. The StAR protein is phosphorylated by cyclic AMP-dependent protein kinase (PKA) that is recruited to the mitochondria by the protein PAP7. PAP7 is a binding partner of the peripheral benzodiazepine receptor/translocator protein (TSPO), which forms a complex with the voltage-dependent anion channel (VDAC) and an adenine nucleotide transporter (ANT) at contact sites between the inner and outer mitochondrial membranes. The multiprotein complex constitutes a cholesterol transporter that moves cholesterol from StAR to the inner mitochondrial membrane where the side-chain cleavage enzyme (CYP-11 Al) converts it to pregnenolone (PREG).

Black, S.M., J.A. Harikrishna, G.D. Szklarz, and W.L. Miller (1994). The mitochondrial environment is required for activity of the cholesterol side-chain cleavage enzyme, eytochrome P450jj,. Proc. Natl. Acad. Sci. USA 91, 7247-7251. 

T. Hara, N. Sato et al. (2002). Compound heterozygous mutations in the cholesterol side-chain cleavage enzyme gene (CYPl 1 A) cause congenital adrenal insufficiency in humans. J. Clin. Endocrinol. Metab. 87, 3808-3813. 

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