Pregnenolone, synthesis

Pregnenolone is transported from the mitochondria to the ER, where a hydroxyl oxidation and migration of the double bond yield progesterone. Pregnenolone synthesis in the adrenal cortex is activated by adrenocorticotropic hormone (ACTH), a peptide of 39 amino acid residues secreted by the anterior pituitary gland. 

Guameii P, Papadoupoulas V, Pan B, et al Regulation of pregnenolone synthesis in C6-2B glioma cells by 4 -chlorodiazepam. Proc Natl Acad Sci USA 89 5118-5122, 1992... 

In the early 1930 s, when the prime research aim was the commercial synthesis of the sex hormones (whose structures had just been elucidated), the principal raw material available was cholesterol extracted from the spinal cord or brain of cattle or from sheep wool grease. This sterol (as its 3-acetate 5,6-dibromide) was subjected to a rather drastic chromic acid oxidation, which produced a variety of acidic, ketonic and hydroxylated products derived mainly by attack on the alkyl side-chain. The principal ketonic material, 3j -hydroxyandrost-5-en-17-one, was obtained in yields of only about 7% another useful ketone, 3 -hydroxypregn-5-en-20-one (pregnenolone) was obtained in much lower yield. The chief acidic product was 3j -hydroxy-androst-5-ene-17j -carboxylic acid. All three of these materials were then further converted by various chemical transformations into steroid hormones and synthetic analogs ... 

Ozonization of A -steroids usually gives complex mixtures (however, see ref. 48). Ozonolysis became a practical step in the general synthesis of B-norsteroids with the discovery that added methanol" (or formaldehyde ) improves yields significantly. Thus, Tanabe and Morisawa prepared 5/ -hydroxy-6/ -formyl-B-norsteroids (74) from cholesterol acetate, dehydroepiandrosterone acetate and pregnenolone acetate in overall yields of 64-74% by the reaction sequence represented below. 

Testosterone synthesis from pregnenolone can occur along two distinct metabolic pathways (Fig. 63.1). The names given to these two routes of metabolism refer to the position in the steroid molecule where an unsaturated bond is maintained. Thus, in the delta-4 pathway an unsaturated position is between C4 and C5 of ring A, whereas in the delta-5 pathway, the unsaturated position is between C5 and C6 of ring B. In the human testis, the delta-5 pathway is the predominant (but not exclusive) one used for the biosynthesis of testosterone. 

A. In the Leydig cell the rate-limiting step in testosterone synthesis is the enzymatic cleavage of side chains from cholesterol to form pregnenolone. 

The synthesis of adrenal steroids is illustrated in Fig. 5.3.1. Cortisol, corticosterone, and aldosterone are formed by sequential hydroxylations and oxidoreductions from pregnenolone and progesterone. 17a-Hydroxypregnenolone (17HP) is a branchpoint constituent because it can be converted to cortisol or adrenal androgens. All of the components of this pathway can be quantified by MS/MS. The steroids around the periphery are urinary metabolites and these are measured by GC-MS following hydrolysis of conjugates and derivatization. 

FIGURE 21-47 Side-chain cleavage in the synthesis of steroid hormones. Cytochrome P-450 acts as electron carrier in this mixed-function oxidase system that oxidizes adjacent carbons. The process also requires the electron-transferring proteins adrenodoxin and adrenodoxin reductase. This system for cleaving side chains is found in mitochondria of the adrenal cortex, where active steroid production occurs. Pregnenolone is the precursor of all other steroid hormones (see Fig. 21-46). 

The step-by-step synthesis of the steroid hormones pregnenolone and progesterone from cholesterol (C27) was presented in chapter 20 (see fig 20.22). Note that pregneno-lone (C2i) and progesterone (table 20.4) (C2 ) are intermediates in the biosynthesis of all of the major adrenal steroids, including cortisol (C2i), corticosterone (C21), and aldosterone (C21). The same two compounds are intermediates in the synthesis of the gonadal steroid hormones, testosterone (C,9) and 17/3-estradiol (CI8). Because the synthesis of all these hormones follows a common pathway, a defect in the activity or amount of an enzyme along that pathway can lead to both a deficiency in the hormones beyond the affected step and an excess of the hormones, or metabolites, prior to that step. 

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