Vitamin 25- hydroxylation

Metabolites of vitamin D, eg, cholecalciferol (CC), are essential in maintaining the appropriate blood level of Ca ". The active metabolite, 1,25-dihydroxycholecalciferol (1,25-DHCC), is synthesized in two steps. In the fiver, CC is hydroxylated to 25-hydroxycholecalciferol (25-HCC) which, in combination with a globulin carrier, is transported to the kidney where it is converted to 1,25-DHCC. This step, which requites 1-hydroxylase formation, induced by PTH, may be the controlling step in regulating Ca " concentration. The sites of action of 1,25-DHCC are the bones and the intestine. Formation of 1,25-DHCC is limited by an inactivation process, ie, conversion of 25-HCC to 24,25-DHCC, catalyzed by 24-hydroxylase. 

P-Hydroxy steroids which contain the 5,7-diene system and can be activated with uv light to produce vitamin D compounds are called provitamins. The two most important provitamins are ergosterol (1) and 7-dehydrocholesterol (3). They are produced in plants and animals, respectively, and 7-dehydrocholesterol is produced synthetically on a commercial scale. Small amounts of hydroxylated detivatives of the provitamins have been synthesized in efforts to prepare the metaboHtes of vitamin D, but these products do not occur naturally. The provitamins do not possess physiological activities, with the exception that provitamin D is found in the skin of animals and acts as a precursor to vitamin D, and synthetic dihydroxalated... 

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

Although it is being found that vitamin D metaboUtes play a role ia many different biological functions, metaboHsm primarily occurs to maintain the calcium homeostasis of the body. When calcium semm levels fall below the normal range, 1 a,25-dihydroxy-vitainin is made when calcium levels are at or above this level, 24,25-dihydroxycholecalciferol is made, and 1 a-hydroxylase activity is discontiaued. The calcium homeostasis mechanism iavolves a hypocalcemic stimulus, which iaduces the secretion of parathyroid hormone. This causes phosphate diuresis ia the kidney, which stimulates the 1 a-hydroxylase activity and causes the hydroxylation of 25-hydroxy-vitamin D to 1 a,25-dihydroxycholecalciferol. Parathyroid hormone and 1,25-dihydroxycholecalciferol act at the bone site cooperatively to stimulate calcium mobilization from the bone (see Hormones). Calcium blood levels are also iafluenced by the effects of the metaboUte on intestinal absorption and renal resorption. 

Clinical stresses which interfere with vitamin metabohsm, can result in calcium deficiency leading to osteomalacia and osteoporosis (secondary vitamin D deficiency). These stresses include intestinal malabsorption (lack of bile salts) stomach bypass surgery obstmctive jaundice alcoholism Hver or kidney failure decreasing hydroxylation of vitamin to active forms inborn error of metabohsm and use of anticonverdiants that may lead to increased requirement. 

Biotransformations are carried out by either whole cells (microbial, plant, or animal) or by isolated enzymes. Both methods have advantages and disadvantages. In general, multistep transformations, such as hydroxylations of steroids, or the synthesis of amino acids, riboflavin, vitamins, and alkaloids that require the presence of several enzymes and cofactors are carried out by whole cells. Simple one- or two-step transformations, on the other hand, are usually carried out by isolated enzymes. Compared to fermentations, enzymatic reactions have a number of advantages including simple instmmentation reduced side reactions, easy control, and product isolation. 

FIGURE 6.17 Hydroxylation of proUne residnes is catalyzed by prolyl hydroxylase. The reaction requires -ketoglntarate and ascorbic acid (vitamin C). 

The 4-phosphopantetheine group of CoA is also utilized (for essentially the same purposes) in acyl carrier proteins (ACPs) involved in fatty acid biosynthesis (see Chapter 25). In acyl carrier proteins, the 4-phosphopantetheine is covalently linked to a serine hydroxyl group. Pantothenic acid is an essential factor for the metabolism of fat, protein, and carbohydrates in the tricarboxylic acid cycle and other pathways. In view of its universal importance in metabolism, it is surprising that pantothenic acid deficiencies are not a more serious problem in humans, but this vitamin is abundant in almost all foods, so that deficiencies are rarely observed. 

CYP27A1 catalyzes the side chain oxidation (27-hydroxylation) in bile acid biosynthesis. Because bile acid synthesis is the only elimination pathway for cholesterol, mutations in the CYP27A1 gene lead to abnormal deposition of cholesterol and cholestanol in various tissues. This sterol storage disorder is known as cerebrotendinous xanthomatosis. CYP27B1 is the 1-alpha hydroxylase of vitamin D3 that converts it to the active vitamin form. The function of CYP27C1 is not yet known. 

Vitamin D3 (VD3) and retinoids synergistically inhibit the growth and progression of squamous cell carcinomas and actinic keratoses in chronically sun exposed skin. One reason for this synergism may be the direct influence of VD3 on the isomerization and the metabolism of RA. Here, VD3 inhibits the isomerization of 13-cis-RA to the more receptor active all-trans and 9-cis-isomers. Moreover, the VD3 derivative secocholestra-trien-l,3,24-triol (tacalcitol), used for the treatment of severe keratinizing disorders inhibits 4-hydroxylation of all-ri ans-RA. 

Peptidyl hydroxyprohne and hydroxylysine are formed by hydroxylation of peptidyl proline or lysine in reactions catalyzed by mixed-function oxidases that require vitamin C as cofactor. The nutritional disease scurvy reflects impaired hydroxylation due to a deficiency of vitamin C. 

H)2-D3 is a weak agonist and must be modified by hydroxylation at position Cj for full biologic activity. This is accomplished in mitochondria of the renal proximal convoluted tubule by a three-component monooxygenase reaction that requires NADPFl, Mg, molecular oxygen, and at least three enzymes (1) a flavoprotein, renal ferredoxin reductase (2) an iron sulfur protein, renal ferredoxin and (3) cytochrome P450. This system produces l,25(OH)2-D3, which is the most potent namrally occurring metabolite of vitamin D. 

Figure 42-9. Formation and hydroxylation of vitamin D3.25-Hydroxylation takes place in the liver, and the other hydroxylations occur in the kidneys. 25,26(OH)2-D3 and 1,25,26(OH)3-D3 are probably formed as well. The formulas of 7-dehydrocholesterol, vitamin D3, and 1,25(OH)2-D3 are also shown. (Modified and reproduced, with...

A number of iron-containing, ascorbate-requiring hydroxylases share a common reaction mechanism in which hydroxylation of the substrate is linked to decarboxylation of a-ketoglutarate (Figure 28-11). Many of these enzymes are involved in the modification of precursor proteins. Proline and lysine hydroxylases are required for the postsynthetic modification of procollagen to collagen, and prohne hydroxylase is also required in formation of osteocalcin and the Clq component of complement. Aspartate P-hydroxylase is required for the postsynthetic modification of the precursor of protein C, the vitamin K-dependent protease which hydrolyzes activated factor V in the blood clotting cascade. TrimethyUysine and y-butyrobetaine hydroxylases are required for the synthesis of carnitine. 

VALGiMiGLi L, BANKS J T, INGOLD K u and LuszTYK J (1995) Kiuetic solveut effects on hydroxylic hydrogen atom abstractions are independent of the nature of the abstrarting radical. Two extreme tests using vitamin E and phenol, J Am Chem Soc, 117, 9966-71. 

The heart has a relatively low catalase activity, which, together with the superoxide dismutase (SOD) system, acts to remove hydrogen peroxide and superoxide radicals. In addition, in man, dietary vitamin C plays an important role in the reduction of vitamin E, an intrinsic antioxidant component of biological membranes (Chen and Thacker, 1986 Niki, 1987). Both vitamins C and E can also react directly with hydroxyl and superoxide radicals (HalliwcU and Gutteridge, 1989 Meister, 1992). 

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