Cholesterol, side chain cleavage

Figure 42-3. Cholesterol side-chain cleavage and basic steroid hormone structures. The basic sterol rings are identified by the letters A-D. The carbon atoms are numbered 1-21 starting with the A ring. Note that the estrane group has 18 carbons (Cl 8), etc.

Payne AH. Hormonal regulation of cytochrome P450 enzymes, cholesterol side-chain cleavage and 17 alpha-hydroxylase/C 17-20 lyase in Leydig cells. Biol Reprod 1990 42(3) 399-404. 

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

Cholesterol is transported into the mitochondria of steroidogenic tissue, where side chain cleavage is carried out. In common with other mixed-function oxidase systems, the cholesterol side chain cleavage requires reduced nicotinamide-adenine dinucleotide phosphate... 

Mitotane (Lysodren) produces selective atrophy of the zona fasciculata and zona reticularis, which results in a decrease in the secretion of 17-hydroxy corticosteroids. Direct inhibition of cholesterol side-chain cleavage and 11 (3/18-hydroxylase activities has also been demonstrated. Mitotane is capable of inducing remission of Cushing s disease, but only after several weeks of therapy and at the price of severe gastrointestinal distress. Moreover, more than half of patients relapse following cessation of therapy. Other side effects include lethargy,... 

Administration of 2,3,7,8-TCDD to rodents was also shown to reduce blood corticosterone levels (Balk and Piper 1984 DiBartolomeis et al. 1987 Mebus and Piper 1986). This effect has been attributed to decreased corticosterone synthesis by decreasing cholesterol side-chain cleavage in the adrenal gland. More recent studies suggested that 2,3,7,8-TCDD may interfere with secretion or synthesis of appropriate, bioactive ACTH from the anterior pituitary gland, which could compromise adrenal steroidogenenesis (Bestervelt et al. 1993). 

Ketoconazole [kee toe KON a zole], a substituted imidazole, is one of a family of azoles useful in treating systemic mycoses. In addition to its antifungal activity, ketoconazole also inhibits gonadal and adrenal steroid synthesis in humans by blocking C17-20 lyase, Up-hydroxylase, and cholesterol side-chain cleavage thus, it suppresses testosterone and cortisol synthesis. 

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... 

The rate-limiting step in progestin biosynthesis is the side-chain cleavage of cholesterol to pregnenolone taking place in the mitochondria. At least three steps in the cholesterol side-chain cleavage (SSC) reaction may be under hormonal regulation [72,73]. FSH stimulates the synthesis of the SSC cytochrome P-450 which results in the stimulation of SSC activity [74]. 

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).

P450 XIA1 see Noninducible Trimethylsilylethyl pregn-5-enediol Cholesterol side-chain cleavage... 

Figure 9.85 Normal phase HPLC profiles of the reaction product of the cholesterol side chain cleavage system. Peaks were identified on the basis of their retention times. (i4) Without cholesterol oxidase treatment. Cholesterol (100 nmol) was incubated with cytochrome P450scc (70 pmol) in the presence of adrenodoxin, adrenodoxin reductase, and an NADPH-generating system. Monitoring was at 214 nm. Peaks 1, cholesterol 2, pregnenolone 3, deoxycorticosterone acetate (internal standard) (B) The reaction mixture of (A) was further incubated with cholesterol oxidase at 37°C for 10 minutes. Monitoring was at 240 nm. Peaks 1, cholestenone 2, progesterone 3, deoxycorticosterone acetate (internal standard). (From Sugano et al., 1989.)...

NADH dehydrogenase and, 189 ubiquinone reductase and, 178,182,183 Cholesterol, side chain cleavage, 83, 84-85 Choline, oxidation to betaine, 260 Choline dehydrogenase electron transport system and, 261-263 properties, 260-201 Chromatium sulfate reduction by, 281 transhydrogenase of, 54 function of, 80 molecular properties, 58, 69 purification, 55, 56 Chromium... 

Cytochrome F-450 (cholesterol side chain cleavage, rat adrenal mitochondria) 28.6... 

The resolution of two high spin forms in adrenal mitochondria has been exploited in studies of the stimulation of steroid biosynthesis by ACTH and the effects of stress. Mitochondria from stressed rats showed increased amounts of high spin P-450, most notably in that corresponding to cholesterol side chain cleavage and similar but not identical effects were observed when hypophysectomised rats were treated with ACTH [118]. This supports the thesis that ACTH stimulation increases the availability of cholesterol for side chain cleavage. In adrenals, ESR can also be used to monitor the presence of reducing equivalents to P-450 as, in this case, the P-450 is linked to NADPH via an iron-sulphur protein, adrenal ferredoxin. This shows typical signals on reduction at g = 2.01, 1.94 [119]. A similar iron-sulphur protein (g = 2.03, 1.94) occurs in mammalian testes connected with a P-450 system [120]. 

Cholesterol side-chain cleavage (SCC, desmolase) 17(X-Hydroxylase/17,20 lyase 2THydroxyIasc p-Hydroxylase... 

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