24- Hydroxycholesterol 7-hydroxylation

A1 Liver, other tissues ER 24-Hydroxycholesterol 7-hydroxylation... 

Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues. Blood-borne cholesterol is excluded from the CNS by the blood-brain barrier. Neurons express a form of cytochrome P-450, 46A, that oxidizes cholesterol to 24(S)-hydroxycholesterol [11] and may oxidize it further to 24,25 and 24,27-dihydroxy products [12]. In other tissues hydroxylation of the alkyl side chain of cholesterol at C22 or C27 is known to produce products that diffuse out of cells into the plasma circulation. Although the rate of cholesterol turnover in mature brain is relatively low, 24-hydroxylation may be a principal efflux path to the liver because it is not further oxidized in the CNS [10]. 

Martin C, Bean R, Rose K, Habib F, Seek J. 2001. cyp7bl catalyses the 7alpha-hydroxylation ofdehydroepiandroster-one and 25-hydroxycholesterol in rat prostate. Biochem J... 

An improved route to 2a-hydroxycholesterol has been devised as part of the preparation of 2a-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = R3 = OH).123 Hydroxylation of the A bond of cholesta-l,5-dien-3/3-ol by means of 9-borabicyclo[3,3,l]nonane followed by reaction with alkaline hydrogen peroxide produced the 2-equatorial 2a,3a-diol in 70—80% yield. The conventional four-step sequence, acetylation, bromination, dehydrobromination, and hydrolysis, gave 2a -hydroxycholesta-5,7-dien-3/3-ol which was converted into 2a-hydroxy-vitamin D3. The isomeric 2/3-hydroxy-vitamin D3 has also been reported.124 Reaction of the 1/6,2/3-oxide obtained by peroxidation of the adduct (265) with lithium aluminium hydride results in a mixture of 2/3,3/3-dihydroxycholest-5,7-diene and its 1/3,3/3-dihydroxy-epimer in the ratio 8 1. Irradiation of the former 5,7-diene furnished the expected previtamin, which on equilibration gave 2/3-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = a-OH, R3 = OH). 

Recent findings show that vitamin D3 must be hydroxylated at C-25 by the liver and then at C-la by the kidney to 1 a,25-dihydroxy-vitamin D3 before it can induce calcium transport. It is therefore not surprising to find several reports on the synthesis of both the la,25-dihydroxy-vitamin and la,25-hydroxycholesterol. 

The probable mechanism of the synergetic effect of L-ascorbic acid and substances capable of binding bile acids in the gastrointestinal tract is shown in Scheme 1. An increased intake of L-ascorbic acid accelerates the formation of 7a-hydroxycholesterol and thereby also accelerates the overall rate of cholesterol transformation into bile acids. Bile acids excreted from the liver into the gastrointestinal tract become bound and thus are prevented from affecting 7 -hydroxylation of cholesterol through a feedback mechanism. In this way, a permanent disequilibrium occurs between cholesterol and bile acids with increased cholesterol transformation to bile acids and enhanced irreversible output of the products of this reaction, which results in a decline of cholesterol levels in the blood and tissues. 

The mitochondrial 26-hydroxylase is inhibited by biliary drainage and is not influenced by starvation or treatment with phenobarbital [126]. Gustafsson reported that the mitochondrial 26-hydroxylation of cholesterol, 7a-hydroxycholesterol and 7a-hydroxy-4-cholesten-3-one was stimulated whereas 26-hydroxylation of 5)8-cholestane-3a,7a,12a-triol was inhibited by biliary obstruction [127]. Whether the... 

It was early suggested that a species of cytochrome P-450 could be involved in the 26-hydroxylation [126,130,131]. Bjorkhem and Gustafsson showed that the hydroxylase was a mixed-function oxidase by demonstrating incorporation of into 26-hydroxycholesterol when the reaction was performed in an atmosphere... 

Mitropoulos et al. have measured the rate of excretion and the specific activities of cholic acid and chenodeoxycholic acid in bile fistula rats fed [ H]cholesterol and infused with [ " C]mevalonate or [ C]7a-hydroxycholesterol [255]. It was concluded that newly synthesized hepatic cholesterol was the preferred substrate for the formation of cholic acid. It could not be excluded, however, that part of the chenodeoxycholic acid had been formed from a pool of cholesterol different from that utilized in cholic acid biosynthesis. The mitochondrial pathway, starting with a 26-hydroxylation, could have accounted for a significant fraction of the chenodeo-... 

Another hydroxylation role for vitamin C in the hepatic microsomal fraction is the stepwise conversion of cholesterol to the bile acid, cholic acid, via 7a-hydroxycholesterol, 3a,7a-dihydroxycoprostane and 3a,7a,12o-trihydroxycoprostane. Also, in lipid metabolism, conventional fatty acids with an even number of carbon atoms are a-oxidised by a mono-oxygenase and subsequently decarboxylated to form an odd-numbered carbon derivative and both these steps appear to require ascorbic acid. As the initial a-oxidation is brought about by a... 

The classic reaction of P450 7A1 is cholesterol 7a-hydroxylation, and esterified cholesterol is not a substrate . However, recent experiments have established that the enzyme also catalyzes the 7a-hydroxylation of 24-hydroxycholesterol, with preference for the (5)-isomer . 7a-Hydroxylation (with recombinant human P450 7A1) was observed with 20(5)-hydroxycholesterol, 25-hydroxycholesterol, and... 

The significance of P450 46A1 rests in the fact that the brain is the most cholesterol-rich tissue in the body . LXRs, members of the nuclear hormone receptor family, are activated by 24-hydroxycholesterol. LXR 3 and P450 46 have overlapping expression patterns in brain. The system may be counter-balanced by P450 39, which catalyzes the 7a-hydroxylation of... 

Thus, product 4 may have a dual origin, arising from autoxidation as well as from en2ymatic monooxygenation [16] However, autoxidation always yields a mixture of 3 and 4 [16]. Also 20-hydroxycholesterol can be formed both by autoxidation and by enzymatic means. 20(S)- and 22(R)-hydroxycholesterol are intermediates in the synthesis of steroid hormones [16]. Cholesterol 5a,6a-epoxide 6 is mostly associated with autoxidation, but has also been shown to be enzymatically formed in different systems. It is also formed by the action of agents such as hydroxyl radical or hydrogen peroxide on cholesterol [9]. 

Figure 2.13a Fragmentation of GP-tagged oxysterols illustrated by 24S,25-epoxycholesterol. (a) Major fragments observed in MS spectra recorded on an ion trap, (b) Major fragment observed in MS spectra recorded on an ion trap. MS/MS spectra recorded on tandem quadrupole or Q-TOF instruments are a composite of (a) and (b). (c) The effect of 7a-hydroxylation on fragmentation as illustrated by 7a-hydroxycholesterol. (Continued)...

Several oxysterol classes present in oxLDL appear to be cytotoxic toward fibroblasts, ECs, and vascular smooth muscle cells, especially 7-hydroperoxycholes-terol (7-OOH-chol), 7P- and 7a-hydroxycholesterol (7-OH-chol), 7-ketocholesterol (7-keto-chol), and cholesterol epoxides (epoxy-chol). 7p-OOH-chol, a precursor of hydroxyl- and keto-oxysterols, was reported to be the most toxic. During LDL oxidation 7P-OOH-chol was produced in three to five times higher quantities than 7a-OOH-chol, other oxysterols and even hydroxy-nonenal, which is one of the most abundant lipid oxidation products. Cytotoxicity of oxysterols was connected to increased cellular oxidative stress. Some studies suggest that oxysterols are even involved in oxidative stress induction. Animal models indicate that dietary oxysterols can significantly decrease glutathione levels and increase expression of glutathione peroxidase and superoxide dismutase. In apolipoprotein-deficient mice, the NADPH-oxidase activity was induced by 7-keto-chol, 7p-OH-chol, and Sp,6P-epoxy-chol. The increased activity of NADPH oxidase yields more superoxide anions that amplify oxidative stress. 

---