Steroid hydroxylation hormones

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 ... 

Sinee steroid hormones can only be obtained in small quantities direetly from mammals, attempts were made to synthesize them fiom plant sterols whieh ean be obtained eheaply and economically in large quantities. However, all adrenoeortieal steroids are eharaeterized by the presence of an oxygen at position 11 in the steroid nueleus. Thus, although it is easy to hydroxylate a steroidal eompound it is extremely diffieult to obtain site-specific hydroxylation, so that many of the routes used for synthesizing the desired steroid are lengthy, complex and eonsequently expensive. This problem was... 

With the exception of two dehydrogenases, all of the steroidogenic enzymes belong to the cytochrome P-450 (abbreviated as CYP) family of enzymes. The CYP enzymes are often involved with redox or hydroxylation reactions, and are also found in the liver where they are key players in biotransformation reactions (see Section 6.4). Different members of the CYP family are therefore involved with both synthesis in adrenal and gonads and hepatic inactivation of steroid hormones. 

The steroid hormones are mainly inactivated in the liver, where they are either reduced or further hydroxylated and then conjugated with glucuronic acid or sulfate for excretion (see p. 316). The reduction reactions attack 0X0 groups and the double bond in ring A. A combination of several inactivation reactions gives rise to many different steroid metabolites that have lost most of their hormonal activity. Finally, they are excreted with the urine and also partly via the bile. Evidence of steroids and steroid metabolites in the urine is used to investigate the hormone metabolism. 

Approximately 50% of a dose of mestranol is de-methylated to form ethinyl estradiol. Ethinyl estradiol also can be deethinylated. Subsequently, the metabolism of these two synthetic estrogens proceeds by means of the same pathways as the natural steroid hormones. The principal metabolites of mestranol and ethinyl estradiol are hydroxylated derivatives that are conjugated with either glucuronic acid or sulfate. The synthetic steroid estrogens, in contrast to the natural estrogens, are excreted primarily in the feces. 

Many steroids have an alcoholic hydroxyl attached to the ring system, and are known as sterols. The most common sterol is cholesterol, which occurs in most animal tissues. There are many different steroid hormones, and cholesterol is the precursor for all of them. Cholesterol is also the precursor of vitamin D. 

Cytochromes P450 form a very large group of heme enzymes that catalyze the hydroxylation of a variety of substrates. They are important in drug metabolism, in cholesterol and steroid hormone biosynthesis, and in numerous other pathways. They have been found to participate in reactions other than hydroxyla-tions. 

In some instances, combinations of Cig and silica columns are also used for better purification of the crude extracts (431, 445). A combination of Cg, silica, and amino solid-phase extraction columns has been successfully employed to fractionate anabolic and catabolic steroid hormone residues from meat in polar and nonpolar neutral and phenolic compounds, and to purify further each fraction effectively (452). Another combination of two solid-phase extraction columns, one using a graphitized carbon black sorbent and the other Amberlite resin in the hydroxyl form, allowed neutral anabolics to be isolated and separated from acidic anabolics and their metabolites (453). A combination of basic alumina column placed in tandem with an ion-exchange column has also been applied for the purification of the crude extracts in the determination of diethylstilbestrol and zeranol (427), and estradiol and zeranol in tissues (450). 

Mitochondrial system The function of the mitochondrial cyto chrome P450 monooxygenase system is to participate in the hydroxylation of steroids, a process that makes these hydropho bic compounds more water soluble. For example, in the steroid hormone-producing tissues, such as the placenta, ovaries, testes, and adrenal cortex, it is used to hydroxylate intermediates in the conversion of cholesterol to steroid hormones. The liver uses this system in bile acid synthesis (see p. 222), and the kidney uses it to hydroxylate vitamin 25-hydroxycholecalciferol (vitamin D, see p. 384) to its biologically active 1,25-hydroxylated form. 

Steroid hormones are generally converted into inactive metaboic excretion products in the liver. Reactions include reduction of unsaturated bonds and the introduction of additional hydroxyl groups. The) resulting structures are made more soluble by conjugation with curonic acid or sulfate (from PAPS, see p. 160). Approximate ) twenty to thirty percent of these metabolites are secreted into the bile and then excreted in the feces, whereas the remainder ae t released into the blood and filtered from the plasma in the kidney, passing into the urine. These conjugated metabolites are fairb1 water-soluble and do not need protein carriers. 

In addition to these major processes, many other chemical events also occur. Mitochondria concentrate Ca2+ ions and control the entrance and exit of Na+, K+, dicarboxylates, amino acids, ADP, P and ATP, and many other substances.16 Thus, they exert regulatory functions both on catabolic and biosynthetic sequences. The glycine decarboxylase system (Fig. 15-20) is found in the mitochondrial matrix and is especially active in plant mitochondria (Fig. 23-37). Several cytochrome P450-dependent hydroxylation reactions, important to the biosynthesis and catabolism of steroid hormones and... 

Cytochromes P450 are monooxygenases whose cosubstrates, often NADH or NADPH, deliver electrons to the active center heme via a separate flavoprotein and often via an iron-sulfur protein as well 476a b A typical reaction (Eq. 18-55) is the 11 (3-hydroxylation of a steroid, an essential step in the biosynthesis of steroid hormones (Fig. 22-11). The hydroxyl group is introduced without inversion of configuration. The same enzyme converts unsaturated derivatives to epoxides (Eq. 18-56), while other cytochromes P450... 

Several sex-dependent differences have been observed in the action of cytochrome P450 isoenzymes on steroid hormones.290 2903 Thus, androstenedione is hydroxylated by rat liver enzymes specific for the 60, 7a, 16a, and 160 positions.291 Of these the 16 hydroxylase is synthesized only in males, and synthesis of the 6 hydroxylase is also largely suppressed in females. 

The fundamental role of ascorbic acid in metabolic processes is not well understood. There is some evidence that it may be involved in metabolic hydroxylation reactions of tyrosine, proline, and some steroid hormones, and in the cleavage-oxidation of homogentisic acid. Its function in these metabolic processes appears to be related to the ability of vitamin C to act as a reducing agent. 

Vitamin D3 is a precursor of the hormone 1,25-dihy-droxyvitamin D3. Vitamin D3 is essential for normal calcium and phosphorus metabolism. It is formed from 7-dehydrocholesterol by ultraviolet photolysis in the skin. Insufficient exposure to sunlight and absence of vitamin D3 in the diet leads to rickets, a condition characterized by weak, malformed bones. Vitamin D3 is inactive, but it is converted into an active compound by two hydroxylation reactions that occur in different organs. The first hydroxylation occurs in the liver, which produces 25-hydroxyvita-min D3, abbreviated 25(OH)D3 the second hydroxylation occurs in the kidney and gives rise to the active product 1,25-dihydroxy vitamin D3 24,25 (OH)2D3 (fig. 24.13). The hydroxylation at position 1 that occurs in the kidney is stimulated by parathyroid hormone (PTH), which is secreted from the parathyroid gland in response to low circulating levels of calcium. In the presence of adequate calcium, 25(OH)D3 is converted into an inactive metabolite, 24,25 (OH)2D3. The active derivative of vitamin D3 is considered a hormone because it is transported from the kidneys to target cells, where it binds to nuclear receptors that are analogous to those of typical steroid hormones. l,25(OH)2D3 stimulates calcium transport by intestinal cells and increases calcium uptake by osteoblasts (precursors of bone cells). 

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