Pregnenolone transformation

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 ... 

Deletion of the 17a-hydroxy group similarly leads to an effective topical antiinflammatory agent. Treatment of 16a-methylpregnenolone (244) (obtained by conjugate addition of an organometallic to pregnenolone) sequentially with bromine and acetate ion affords the 21 acetate, 245 (see, for example, the transformation of 196 to 198). In an interesting variation on the method for the introduction of a fluorine atom at 6, the... 

Since the predatory water beetles cannot biosynthesise the steroid skeleton de novo, steroidal precursors must be obtained from exogenous sources. Bacillus-strains, isolated from the foregut of the water beetle Agabus affinis, were tested for their ability to transform steroids [101]. After incubation with androst-4-en-3,17-dione two Bacillus strains produced 13 different transformation products. Hydroxylation took place at C6, C7, Cll and C14 resulting in the formation of 6fi-, 7a-, 1 la-, and 14a-hydroxyandrost-4-en-3,17-diones. After incubation with pregnenolone the two Bacillus strains produced a variety of steroids among which 7a-hydroxypregnenolone was the major product [102]. 

An important transformation in steroid biochemistry is the conversion of pregnenolone into progesterone. Progesterone is a female sex hormone, a progestogen, but this reaction is also involved in the production of corticosteroids such as hydrocortisone and aldosterone. The reaction also occurs in plants, and features in the formation of cardioactive glycosides, such as digitoxin in foxglove. 

Several compounds have been developed that inhibit the 17-hydroxylation of progesterone or pregnenolone, thereby preventing the action of the side chain-splitting enzyme and the further transformation of these steroid precursors to active androgens. A few of these compounds have been tested clinically but have been too toxic for prolonged use. As noted in Chapter 39, abiraterone, a newer 17-hydroxylase inhibitor, may prove to be clinically successful. 

A reaction sequence has been published for the transformation of pregnenolone acetate into 18-acetoxy-3/3 -hydroxy- 14a -carda-5,20(22)-dienolide... 

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. 

Pinacolone (rert-butyl methyl ketone) is transformed into pivalic acid (trimethylacetic acid) in 71-74% yield [737], Mesityl oxide is converted into 3,3-dimethylacrylic acid in 49-53% yield [703], Isopropenyl methyl ketone gives methacrylic acid in 41% yield [697], and pregnenolone acetate furnishes 3-acetoxyetienic acid in 91-95% yield [117J] (equations 421-424). 

Conversion of cholesterol to pregnenolone is the rale-limiting step in steroid hormone biosynthesis. It is not ihccnry-malic transformation itself that is rate limiting, however, the translocation of cholesterol to the inner milochondrui membrane of steroid-synihesi/.ing cells is rate limiting. A key protein involved in the translocation is the 5Kruidugcmc... 

Cleavage of the side chain of cholesterol produces pregnenolone (step a in Fig. 46.3), which can then be transformed into progesterone or, via several biosynthetic... 

CYP17A catalyzes both the 17a-hydroxylation of progesterone and subsequent cleavage of the C17-C20 bond to give androstenedione (Fig. 4.49). A similar transformation sequence accoimts for the conversion of pregnenolone to dehydroepiandrosterone. The favored mechanism for this reaction involves formation of the hydroperoxy hemiacetal with the ferric hydroper-oxy anion of the enzyme, followed by 0-0 bond homolysis, C-C bond homolysis, and finally recombination of the compormd II equivalent with... 

The conversion of cholesterol to corticosteroids was further confirmed by in vitro experiments. The first reaction in this biochemical transformation is NADP and ATP dependent. It results in the splitting of the cholesterol side chain to yield pregnenolone and isoca-proic acid (see Fig. 8-11). Pregnenolone is a 21-carbon compound with a jS-carboxyl group in position 3 and an unsaturated bond between carbons 5 and 6. The exact mechanism of the splitting reaction is not clear, although it would appear that 20a-hydroxycholesterol and 22a-hydroxycholesterol can serve as intermediates. Their role has been questioned [192]. 

Finally, a precise clinical use of pregnenolone sulfate can be considered since this compound seems to be active in depressing uterine contractility, without being previously transformed into progesterone (Scom-megna et al, 1970). With respect to possible hormonal control of cancer, Libby and Dao (1970) have indicated that certain breast tumors, which do not regress after adrenalectomy, lack sulfate-activating enzymes. 

Coutts IGC, Hamblin MR, Tinley EJ (1979) The enzymatic oxidation of phenolic tetrahydroiso-quinoline-1-carboxylic acids. J Chem Soc Perkin Trans 1 2744-2750 Davis VE, Cashaw JL, McMurtrey KD, Ruchirawat S, Nimit Y (1982) Metabolism of tetrahydro-isoquinolines and related alkaloids. In Bloom F, Barchas J, Sandler M, Usdin E (eds) Beta-carbolines and tetrahydroisoquinolines. Liss, New York, p 99 Furuya T, Nakano M, Yoshikawa T (1978) Biotransformation of (RS)-reticuline and morphinan alkaloids by cell cultures of Papaver somniferum. Phytochemistry 17 891-893 Gates M (1953) Conversion of codeinone to codeine. J Am Chem Soc 75 4340-4341 Graves JMH, Smith WK (1967) Transformation of pregnenolone and progesterone by cultured plant cells. Nature (London) 214 124 8-1249... 

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