Calcidiol synthesis

Sunlight is not strictly essential for cutaneous synthesis of cholecalciferol, because UV-B penetrates clouds reasonably well complete cloud cover reduces the available intensity by about 50%. It also penetrates light clothing. However, low-intensity irradiation (below 20 ml per cm in vitro) does not result In significant photolysis of 7-dehydrocholesterol to previtamin D. Acute whole-body exposure to UV-B irradiation below 18 ml per cm does not result in any detectable increase in plasma cholecalciferol or calcidiol. In temperate regions (beyond about 40°N or S), the intensity of UV-B is below this threshold in winter, so there is unlikely to be any significant cutaneous synthesis of the vitamin in winter, and plasma concentrations of calcidiol show a marked seasonal variation in temperate regions (Holick, 1995 see Table 3.2). 

The active metabolite of vitamin D, calcitriol, is formed in the proximal tubules of the kidneys from calcidiol. There are three cytochrome P450-dependent enzymes in kidneys that catalyze 1-hydroxylation of calcidiol CYP27A and CYP27 in mitochondria and a microsomal la-hydroxylase, which is ferredoxin-dependent. It is likely that the microsomal enzyme is the most important its synthesis is induced by cAMP in response to parathyroid hormone (Section 3.2.8.2) and repressed by calcitriol (Omdahl et al., 2001 Wikvall, 2001). 

Calcidiol la-hydroxylase is not restricted to the kidney, but is also found in placenta, bone cells (in culture), mammary glands, and keratinocytes. The placental enzyme makes a significant contribution to fetal calcitriol, but it is not clear whether the calcidiol 1-hydroxylase activity of other tissues is physiologically significant or not. Acutely nephrectomized animals given a single dose of calcidiol do not form any detectable calcitriol, but there is some formation of calcitriol in anephric patients, which increases on the administration of cholecalciferol or calcidiol. However, thus extrarenal synthesis is not adequate to meet requirements, so that osteomalacia develops in renal failure (Section 3.4.1). The enzyme is inhibited, or possibly repressed, by strontium ions this is the basis of strontium-induced vitamin D-resistant rickets, which responds to the administration of calcitriol or la-hydroxycalciol, but not calciferol or calcidiol (Omdahl and DeLuca, 1971). 

Calcidiol la-hydroxylase also acts on 24-hydroxycalcidiol, yielding cal-citetrol indeed, it has a relatively low specificity and will act on any secosteroid with hydroxyl groups at C-3 and C-25. Calcitriol has a short metabolic half-life after injection of the order of 4 to 6 hours (Holick, 1990). But, under normal conditions, the regulation of its synthesis means that the plasma concentration remains fairly constant, depending on the state of calcium balance (Hewison et al., 2000). 

Both calcidiol and calcitriol are substrates for24-hydroxylation, catalyzed by a cytochrome P4so-dependent enzyme in kidneys, intestinal mucosa, cartilage, and other tissues that contain calcitriol receptors. This enzyme is induced by calcitriol the activities of calcidiol 1-hydroxylase and 24-hydroxylase in the kidney are subject to regulation in opposite directions, so that decreased requirement for, and synthesis of, calcitriol results in increased formation of 24-hydroxycalcidiol. Kidney epithelial cells in culture show increased formation of 24-hydroxycalcidiol, and decreased formation of calcitriol, after the addition of calcitriol or high concentrations of calcium to the culture medium. 

There is evidence that 24-hydroxycalcidiol has physiological functions distinct from those of calcitriol, and the regulation of the 24-hydroxylase suggests that it functions to provide a metabolically active product, as well as diverting calcidiol away from calcitriol synthesis (Henry, 2001). Studies of knockout mice lacking the 24-hydroxylase show that 24-hydroxycalcidiol has a role in both in-tramembranous bone formation during development and the suppression of parathyroid hormone secretion (St-Arnaud, 1999 van Leeuwen et al., 2001). 

In the kidneys, parathyroid hormone increases 1 -hydroxylation of calcidiol and reduces 24-hydroxylation. This is not the result of de novo enzyme synthesis, but an effect on the activity of the preformed enzymes, mediated by cAMP-dependent protein kinases. In turn, calcitriol has a direct role in the control of parathyroid hormone, acting to repress expression of the gene. In chronic renal failure, there is reduced synthesis of calcitriol, leading to the development of secondary hyperparathyroidism that results in excess mobilization of bone mineral, hypercalcemia, hypercalciuria, hyperphosphaturia, and the development of calcium phosphate renal stones. 

Elderly people are at risk of osteomalacia, because of both decreased synthesis of 7-dehydrocholesterol in the skin with increasing age and low exposure to sunlight. Plasma concentrations of calcidiol below 10 nmol per L are commonly seen in people over 75 years of age, not rising above 20 nmol per L at any time of the year. Histologically proven osteomalacia is observed in 2% to 5% of elderly people presenting to the hospital in Britain. 

Parathyroid hormone is secreted in response to a fall in plasma calcium. In the kidney it acts to increase the activity of calcidiol 1-hydroxylase and decrease that of the 24-hydroxylase. This is not an effect on protein synthesis, but the result of changes in the activity of existing enzyme protein, mediated by cAMP (section 10.3.2). In turn, both calcitriol and high concentrations of calcium repress the synthesis of parathyroid hormone. 

Calcium exerts its main effect on the synthesis and secretion of parathyroid hormone. However, calcium ions also have a direct effect on the kidney, reducing the activity of calcidiol 1-hydroxylase (but with no effect on the activity of 24-hydroxylase). 

It is difficult to determine requirements for dietary vitamin D, as the major source is synthesis in the skin. The main criterion of adequacy is the plasma concentration of calcidiol. In elderly subjects with little sunlight exposure, a dietary intake of 10 fg of vitamin D per day results in a plasma calcidiol concentration of 20 nmol/L, the lower end of the reference range for younger adults at the end of winter. Therefore, the reference intake for the elderly is 10 Jg/day. Average intakes of vitamin D are less than 4 Jg/day, so to achieve an intake of 10 Xg/day will almost certainly require either fortification of foods or the use of vitamin D supplements. 

---