25-Hydroxycholecalciferol, 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. 

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). 

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. 

Formation of 1,25-diOH D3 Vitamins D2 and D3 are not biologically active, but are converted in vivo to the active form of the D vitamin by two sequential hydroxylation reactions (Figure 28.23). The first hydroxylation occurs at the 25-position, and is catalyzed by a specific hydroxylase in the liver. The product of the reaction, 25-hydroxycholecalciferol (25-OH D3), is the predominant form of vitamin D in the plasma and the major storage form of the vitamin. 25-OH D3 is further hydroxylated at the one position by a specific 25-hydroxycholecalciferol 1 -hydroxylase found primarily in the kidney, resulting in the formation of 1,25-dihydroxycholecalciferol j (1,25-diOH D3). [Note This hydroxylase, as well as the iver 25-hydroxylase, employ cytochrome P450, molecular oxygen, and NADPH.]... 

The combination of rifampicin and isoniazid reduces serum concentrations of 25-hydroxy cholecalciferol. Rifampicin acts by induction of an enzyme that promotes conversion of 25-hydroxycholecalciferol to an inactive metabolite, and isoniazid acts by inhibiting 25-hydroxyla-tion and 1-hydroxylation (SEDA-14, 258). Children or pregnant women with tuberculosis have increased calcium requirements independent of rifampicin administration... 

Cholecalciferol is hydroxylated at three positions in the carbon skeleton, 1, 24, and 25. In the liver, cholecalciferol is hydroxylated to 25-hydroxycholecalciferol. Further hydroxylation reactions occur in the kidney, resulting in the formation of three new metabolites. These are 1,25-dihydroxycholecalciferol 24,25-dihydroxycholecalciferol and 1,24,25-trihydroxycholecalciferol. 1,25-Dihydroxy- and 1,24,25-trihydroxycholecalciferol are active hormones involved in calcium uptake from the intestine. 

Both dietary and endogenously synthesized vitamin D undergo 25-hy-droxylation in the liver to yield calcidiol (25-hydroxycholecalciferol), which is the main circulating form of the vitamin. This undergoes 1 -hydroxylation in the kidney to produce the active hormone calcitriol (1,25-dihydroxy-cholecalciferol) or 24-hydroxylation in the kidney and other tissues to yield 24-hydroxycalcidiol (24,25-dihydroxycholecalciferol). 

Cholecalciferol 25-hydroxylase is not restricted to the liver kidneys, skin, and gut microsomes also have a cytochrome P450 -dependent enzyme that catalyzes the 25-hydroxylation of cholecalciferol and la-hydroxycholecalciferol, hut not ergocalciferol. Although there is some evidence that calcitriol can reduce the activity of calciferol 25-hydroxylase, it is not known whether this is physiologically important the major factor controlling 25-hydroxylation is the rate of uptake of cholecalciferol into the liver. It is the fate of calcidiol in the kidneys that provides the most important regulation of vitamin D metabolism (Wikvall, 2001). 

Three conditions associated with defective 1-hydroxylation of calcidiol can aU be treated by the administration of either calcitriol itself or la-hydroxycholecalciferol, which is a substrate for 25-hydroxylation in the liver forming calcitriol ... 

Disposition in the Body. Well absorbed after oral administration and subject to enterohepatic circulation decreased absorption may occur in subjects with impaired liver and biliary function. Metabolised by hydroxylation to active metabolites. The major metabolite is 25-hydroxycholecalciferol which is formed in the liver. This is further metabolised by la- or 24-hydroxylation in the kidneys. Most of a dose is excreted in the bile and eliminated in the faeces about 25% of a dose is excreted as conjugates. Unchanged cholecalciferol does not appear to be excreted in the urine. 

It has been suggested that vitamin D3 (378) is metabolized into a more polar substance before stimulating calcium transport to the intestine. The principal metabolite from the blood, produced by the liver, has been found to be 25-hydroxycholecalciferol (379), whereas the trihydroxy-derivative (380) is the principal metabolite from the intestine. Autoxidation of cholesterol via hydroperoxide intermediates afforded a variety of hydroxylated cholesterol derivatives and products of side-chain degradation. ... 

Subsequently there was introduced a la-hydroxylated form (la-hydroxycholecalciferol) alfacalcidol (One-Alpha), that requires only hepatic hydroxylation to become the highly active la-25-dihydroxycholecalciferol (calcitriol). Alfacalcidol (and of course calcitriol) is therefore effective in renal failure since the defective renal hydroxylation stage is bypassed. Its extraordinary potency and efficacy is indicated by the usual adult maintenance dose, often only 0.25-1 micrograms/d. 

Vitamin D3, whether of dietary or skin origin, is hydroxylated in the liver to 25-hydroxycholecalciferol (25-HCC). This undergoes another hydroxylation in the kidneys to the very active compound 1,25-dihydroxycholecalciferol (1,25-DHCC). This reaction is catalyzed by la-hydroxylase, a mitochondrial cytochrome P-450 mixed-function oxidase normally found in the kidneys, although some pathological tissues such as sarcoid granulomas may also possess the enzyme (H12). 

Vitamin D can be obtained from some foods, but its major source is through the action of ultraviolet radiation, which converts 7-dehydrocholesterol to cholecalciferol in the skin. Hydroxylation of this compound in the liver produces 25-hydroxycholecalciferol, which is then converted in the kidney to 1,25-hydroxycholecalciferol, the active form of vitamin D. Vitamin D plays a major role in promoting absorption of calcium and maintaining bone mineralization. Recently, research has focused on immunosuppressive effects of vitamin D. The vitamin D receptor has been detected in lymphocytes and the thymus, and vitamin D plays a role in T cell-mediated immune response (Deluca Cantoma, 2001). 

Renal injury associated with impaired hydroxylation of 25-hydroxycholecalciferol or acidosis Inadequate nutritional intake of calcium, vitamin D, or both Hypoparathyroidism Acute pancreatitis... 

Cholecalciferol is hydroxylated at three positions in its carbon skeleton 1, 24, and 25. In the liver, cholecalciferol is hydroxylated to 25-hydroxycholecalciferol. Further reactions occur in the kidney, resulting in... 

Vitamin D is the sunshine vitamin . It was originally discovered as a crude mixture called vitamin Dj (no longer available as a supplement). Ergosterol, the plant equivalent of cholesterol, is converted to vitamin D2 by ultraviolet light. Vitamin D3 (cholecalciferol) is formed in the skin from 7-dehydrocholesterol (an intermediate in the cholesterol biosynthesis pathway) in the presence of ultraviolet light, which opens the B-ring of the steroid nucleus (Fig. 51.3). Cholecalciferol is successively hydroxylated first in the liver forming 25-hydroxycholecalciferol (25-HCC) and then in the kidney to form the most active form 1,25-dihydroxy cholecalciferol (1,25-DHCC), also known as calcitriol. 

Several other types of investigations suggest that 25-hydroxycholecalciferol or one of its derivatives is the active agent. When injected in vitamin-D deficient rats, the hydroxylated compound is four times as active as vitamin D in curing rickets, and it exhibits its effect more rapidly than when vitamin D is injected. Trum-mel and DeLuca demonstrated an effect of the metabolite on calcium transport in vitro [16]. 

Vitamin D2 and D3 are hydroxylated first in the liver to the prohormone 25-hydroxycholecalciferol (calcidiol) and subsequently in the kidney to the vitamin D hormone la,25-dihy-droxycholecalciferol (calcitriol). Calcitriol acts as an inductor of proteins in various organs. It promotes calcium resorption in the intestine and... 

A major role of vitamin D is to maintain the concentrations of calcium and phosphorus in the blood, primarily by enhancing the absorption of dietary calcium from the alimentary tract and regulating the interchange of calcium between blood and bone. It is likely that there are other cellular roles as yet little understood. The active form of vitamin D responsible for the hormone-like functions described above is 1,25-dihydroxycholecalciferol. Cholecalciferol, whether absorbed from the diet or synthesized in the skin, is first hydroxylated in the liver to 25-hydroxycholecalciferol. This is the main form of the hormone circulating in the blood, bound to a sterol binding protein. A further hydroxylation occurs in the kidneys to give 1,25-... 

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