Sterol, steroid

Similarly, the introduction of double bonds, isomerisation or hydrolysis are also frequently encountered reactions in central metabolism. Many of these reactions have their analogues in sterol/steroid interconversions. Below we will confine ourselves to a limited number of examples. 

Many examples of microbial hydroxylation of sterols/steroids have been reported. These hydroxylations usually involve mixed function oxidases which utilise molecular oxygen and cytochrome P-450. The reaction can be represented by ... 

Several tissues of the body are able to synthesize cholesterol de novo (i.e. from its raw materials) the liver is one of these organs. Structurally, cholesterol belongs to the group of compounds called sterols (steroid alcohols) and is derived metabolically from acetate... 

Common to all of the steroids is a molecular core structure consisting of four saturated rings, known as gonane. At the end of the steroid core, many steroids also carry a side chain, as seen in cholestane, the basic component of the sterols (steroid alcohols). 

Lipids are important components of the diet fatty acids are the higher energetic source as they ensure 9kcal/g. Furthermore, some peculiar fatty acids themselves and several components of the unsaponifiable fraction are biologically active molecules, as they can act as vitamins (tocopherols— vitamin E), provitamins (carotenes—vitamin A, cholecalcipherol—vitamin D), vitamin-like (essential fatty acids), and hormones or hormone precursors (sterols—steroidal hormones). 

Ketoconazole is an effective antifungal agent by virtue of its capacity to block sterol/steroid s)mthesis (ergosterol in the case of fungi). In man it inhibits steroid s)mthesis in gonads and adrenal cortex and it has been used in Cushing s syndrome and prostatic cancer. 

Table 9.5 Some reaction types commonly encountered in sterol/steroid interconversions.

Waxes are complex mixtures of nonpolar lipids. They are protective coatings on leaves, stems, and fruits of plants and the skin and fur of animals. Esters composed of long-chain fatty acids and long-chain alcohols are prominent constituents of most waxes. Well-known examples of waxes include camauba wax, produced by the leaves of the Brazilian wax palm, and beeswax. The predominant constituent of camauba wax is the wax ester melissyl cerotate (Figure 11.5). Triacontyl hexadecanoate is one of several important wax esters in beeswax. Waxes also contain hydrocarbons, alcohols, fatty acids, aldehydes, and sterols (steroid alcohols). 

Many reactions involved in this pathway are omitted. The point of origin of the glucuronic acid cycle from UDPG is indicated (see Fig. 3). The metabolism of pyruvate may proceed in any of several different directions, viz., amino acids, fatty acids, sterols, steroids, and the tricarboxylic acid cycle. Specific enzymes catalyze each conversion indicated by arrows. Broken arrows indicate that several steps are involved. 

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