Vitamin 25-Hydroxycholecalciferol

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. 

Fig. 2. Homeostatic control of blood Ca " level where PTH is parathyroid hormone [9002-64-6], CC, cholecalciferol, ie, vitamin D HCC, hydroxycholecalciferol DHCC, dihydroxycholecalciferol CaBP, calcium-binding protein NAD PH, protonated nicotinarnide-adenine dinucleotide...

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). 

The metabohtes of vitamin D are usually more toxic than the vitamin because the feedback mechanisms that regulate vitamin D concentrations are circumvented. 25-Hydroxycholecalciferol has a one-hundredfold increase in toxicity over vitamin D when fed to chicks (220) and 1 a,25-dihydroxy vitamin D is several times more toxic than the 25-hydroxy analogue. Vitamin D2 seems to have less toxicity than vitamin D, a circumstance which is beheved to be caused by the more efficient elimination of 25-hydroxy and the 1 a,25-dihydroxy vitamin D2 from the animals. Estimated safe upper dietary levels are given in Table 11. 

Therapeutic Function Calcium Regulator, Vitamin D Chemical Name 9,10-Secocholesta-5,7,10(19)-triene-1,3-diol Common Name la-Hydroxycholecalciferol 1a-Hydroxyvitamin D3 Structural Formula ... 

Preparation of la-hydroxycholecalciferol a solution of 13.5 mg of 1a,3/3-dihydroxypro-vitamin D3 in 200 ml of ether is allowed to stand still in the dark at room temperature in an argon gas atmosphere for 2 weeks. During this period, the position of the maximum ultraviolet absorption is shifted from 260 m/u to 264 m/u, and the absorption intensity becomes... 

The following are recent reviews on the molecular and physical properties of this liver enzyme which converts cholecalciferol (vitamin D3) to 25-hydroxycholecalciferol. 

Cholecalciferol (D3) and its active form 1,25-di-hydroxycholecalciferol are only to a certain extend vitamins because they can be synthesized by the human body. However deficiencies resulting in rickets in children and osteomalacia in adults do exist. Cholecalciferol can be synthesized by humans in the skin upon exposure to ultraviolet-B (UVB) radiation from sunlight, or it can be obtained from the diet. Plants synthesize ergosterol, which is converted to vitamin D2 (ergocalciferol) by ultraviolet light. Vitamin D2 may be less active in humans. Vitamin D promotes uptake of calcium and phosphate in the intestine and it stimulates osteoclasts to break down hydroxyapatite and release calcium into blood. Vitamin D is discussed in more detail in Chapter 24, Section V.a. 

Vitamin D preparations that are available include er-gocalciferol (also termed calciferol, or vitamin D2), cholecalciferol (vitamin D3), alfa-calcidol (la-hydroxycholecalciferol) and calcitriol (1,25-hydroxycholcalciferol). 

The term vitamin D is used for a range of compounds which possess the property of preventing or curing rickets. They include ergocalciferol (calciferol, vitamin D ), chole-calciferol (vitamin Dg), dihydrotachysterol, alfacalcidol (la-hydroxycholecalciferol) and calcitriol (1,25-dihydroxycholecalciferol). 

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.]... 

Regulation of 25-hydroxycholecalciferol 1-hydroxylase 1,25-diOH D3 is the most potent vitamin D metabolite. Its formation is tightly i regulated by the level of plasma phosphate and calcium ions (Figure 28.24). 25-Hydroxycholecalciferol1 -hydroxylase activity is I increased directly by low plasma phosphate or indirectly by bw I plasma calcium, which triggers the release of parathyroid hormone I... 

Which one of the following statements concerning vitamin D is correct A. Chronic renal failure requires the oral administra tion of 1,25-dihydroxycholecalciferol. B. It is required in the diet of individuals exposed to sunlight. C. 25-Hydroxycholecalciferol is the active form of the vitamin. D. Vitamin D opposes the effect of parathyroid hor mone. E. A deficiency in vitamin D results in an increased secretion of calcitonin. Correct answer = A. Renal failure results in the decreased ability to form the active form of the vitamin, which must be supplied. The vitamin is not required in individuals exposed to sunlight. 1,25-dihydroxycholecalciferol is the active form of the vitamin. Vitamin D and parathyroid hormone both increase serum calcium. A deficiency of vitamin D decreases the secretion of calcitonin. 

Vitamin D-binding protein and its associated vitamin are lost in nephrotic urine. Biochemical abnormalities in nephrotic patients (children and adults) include hypocalcemia, both total (protein-bound) and ionized hypocalciuria, reduced intestinal calcium absorption and negative calcium balance reduced plasma 25-hydroxycholecalciferol and 24,25-dihydroxycholecalciferol and, surprisingly, also 1,25-dihydroxycholecalciferol and blunted response to parathormon (PTH) administration and increased PTH levels. Clinically, both osteomalacia and hyperparathyroidism have been described in nephrotic patients, more commonly in children than in adults, but bone biopsies are commonly normal, and clinically significant bone disease is very rare in nephrotic subjects. There is, however, evidence that patients with renal failure accompanied by nephrotic range proteinuria may be particularly prone to develop renal osteodystrophy. 

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). 

Type 1 vitamin D-resistant rickets is due to a genetic defect in calcidiol 1-hydroxylase, so that litde or no calcitriol is formed. Patients respond well to the administration of la-hydroxycholecalciferol, which is a substrate for 25-hydroxylation in the liver, leading to normal circulating concentrations of calcitriol. 

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. ... 

The product of the reaction 25-hydroxycholecalciferol or 25 -OH D3 is the predominant form of vitamin D in the plasma and is the major form in which vitamin D is stored. 

There are various physiological forms known as vitamins D, namely vitamin D2 (calciferol, ergocaldferol), vitamin D3 (cholecalciferol), phosphate esters of D2, D3, 25-hydroxycholecalciferol, 1,25-dihydroxychole-caldferol, and 5,25-dihydroxycholecalciferol. There are active anologs and related compounds known as vitamins D, namely 22-dihydroergosterol (vitamin D4), 2-dehydrostigmasterol (vitamin Dg), and 7-dehydro-sitosterol (vitamin D5) [3]. 

Vitamin D is readily absorbed from the gastrointestinal tract. Cholecalciferol is metabolized in the liver to 25-hydroxycholecalciferol and then to l-a-25-dihydroxycalciferol in the kidney. This mobilizes stores of calcium from the bone matrix to the plasma. Cholecalciferol is stored in adipose and muscle tissue. The metabolites of vitamin D compounds are excreted primarily in bile and feces. 

A new synthesis of la-hydroxycholecalciferol (la-hydroxy-vitamin D3) from cholesterol employs transformations in rings A and B which differ only in detail from an earlier sequence for introduction of the la-hydroxy-group the la,2a-epoxy-3j8,6j8-diol derivative (382) was the key intermediate, allowing regeneration of... 

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). 

---