Thermal degradation of single 7-cholesteryl acetate hydroperoxide

Steroid and sterol 7-hydroxylation: ancient pathways

B-ring hydroxylation is a major metabolic pathway for cholesterols and some steroids. In liver, 7 alpha-hydroxylation of cholesterols, mediated by CYP7A and CYP39A1, is the rate-limiting step of bile acid synthesis and metabolic elimination. In brain and other tissues, both sterols and some steroids including dehydroepiandrosterone (DHEA) are prominently 7 alpha-hydroxylated by CYP7B. The function of extra-hepatic steroid and sterol 7-hydroxylation is unknown. Nevertheless, 7-oxygenated cholesterols are potent regulators of cell proliferation and apoptosis; 7-oxygenated derivatives of DHEA, pregnenolone, and androstenediol can have major effects in the brain and in the immune system. The receptor targets involved remain obscure. It is argued that B-ring modification predated steroid evolution: non-enzymatic oxidation of membrane sterols primarily results in 7-oxygenation. Such molecules may have provided early growth and stress signals; a relic may be found in hydroxylation at the symmetrical 11-position of glucocorticoids. Early receptor targets probably included intracellular sterol sites, some modern steroids may continue to act at these targets. 7-Hydroxylation of DHEA may reflect conservation of an early signaling pathway.

Mass spectral fragmentations of cholesterol acetate oxidation products

In this work, electron-impact mass spectroscopy (EI-MS) was employed on a wide range of sterol compounds in order to study their behaviour with regard to their functional groups. In particular, some specific mechanisms of fragmentation occurring in these substrates (i.e. retro-Diels-Alder reaction, neutral molecules eliminations, specific hydrogen migrations) were investigated. Loss of the alkyl side chain and of the D ring were observed in all cases. Finally, a classification of sterols on the basis of characteristic mass spectral fragments is suggested, and further applications to substrates with functional groups on positions other than the A and B rings is proposed.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Sterol synthesis. Synthesis of 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluorocholest-5-en-7-one and its effects on HMG-CoA reductase activity in Chinese hamster ovary cells, on ACAT activity in rat jejunal microsomes, and serum cholesterol levels in rats

3 beta-Hydroxycholest-5-en-7-one (I; 7-ketocholesterol) is an oxysterol of continuing interest in biology and medicine. In the present study, we have prepared a side-chain fluorinated analog, 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluorocholest-5-en-7-one (VI), with the anticipation that the F7 substitution would block major metabolism of the 7-ketosterol, and thereby enhance its potential in vivo effects on serum cholesterol levels and other parameters. Chromium trioxide/dimethyl pyrazole oxidation of the acetate derivative of the previously described 25,26,26,26,27,27,27-heptafluorocholest-5-en-3 beta-ol (Swaminathan et al., 1993. J. Lipid Res. 34, 1805-1823) followed by mild alkaline hydrolysis gave VI. The effects of VI on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity in Chinese hamster ovary (CHO-K1) cells, on acyl coenzyme A-cholesterol acyltransferase (ACAT) activity in rat jejunal microsomes, and on serum cholesterol levels and other parameters in male Sprague-Dawley rats were determined and compared with those obtained with I and with another alpha, beta-unsaturated ketosterol, i.e. 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (II). I and VI showed essentially the same potency, considerably less than that of II, in lowering the levels of HMG-CoA reductase activity in CHO-K1 cells. Whereas addition of II to rat jejunal microsomes inhibited ACAT activity (IC50 approximately 3 microM), I and VI had no effect under the conditions studied (from 1 to 16 microM). Dietary administration of I, at levels of 0.1 and 0.15%, had no effect on food consumption, gain in body weight, or serum cholesterol levels. At 0.2%, I caused a modest decrease in body weight gain and a slight decrease in serum cholesterol levels (relative to ad libitum but not pair-fed control animals). The F7-7-ketosterol VI, at 0.26% in diet (the molar equivalent of 0.2% I), had no effect on food consumption, body weight, or serum cholesterol levels. Administration of I (0.1, 0.15 or 0.2% in diet) caused increases in the weight of small intestine. In contrast, no effect of VI (0.26% in diet) on small intestinal weight was observed.