1-Hydroxycholecalciferols

Selenium dioxide oxidation of cholecalciferol afforded a mixture from which the la- and 1 -hydroxycholecalciferols were isolated.Autoxidation of pseu-dodiosgenin diacetate (74) gave the alcohol (75) from which the lactone (76)... 

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... 

A solution of 20 mg of 25-hydroxychoiecaiciferoi hydrate, prepared as described above, in 20 mi of methyiene chioride is dried with 200 mg of anhydrous sodium suifate. The solution is fiitered and the fiitrate is evaporated to yield 25-hydroxycholecalciferol essentially anhydrous as an amorphous oil. 

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). 

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.]... 

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. 

KT Koshy, AL VanDerSlik. High-performance liquid chromatographic method for the determination of 25-hydroxycholecalciferol in the bovine liver, kidney, and muscle. J Agric Food Chem 25 1246-1249, 1977. 

P Mattila, V Piironen, E Uusi-Rauva, P Koivistoinen. Determination of 25-hydroxycholecalciferol in egg yolk by HPLC. J Food Comp Anal 6 250-255, 1993. 

KT Koshy, AL VanDerSlik. 25-Hydroxycholecalciferol in cow milk as determined by high-performance liquid chromatography. J Agric Food Chem 27 650-652, 1979. 

Schreurs WH, van Rijn HJ, van den Berg H. Serum 25-hydroxycholecalciferol levels in women using oral contraceptives. Contraception 1981 23(4) 399 t06. 

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... 

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. 

The fractions which crystallize on trituration with aqueous methanol are combined and recrystallized twice from aqueous methanol to give 25-hydroxycholecalciferol hydrate yield 120 mg, MP 81°-83°C (sinters 75°C). 

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. 

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