Vitamin physiological actions

Vitamins are small organic molecules which in small amounts are obligatory nutrients and used by the body as co-factors in a multitude of metabolic processes. They play a role in hormone production, are necessary for blood cell formation and for producing nervous-system constituents, and they are ingredients for the formation of genetic material. There are no chemical relationships between the various vitamins and mostly also their most physiological actions are not related. 

Understand the physiologic actions of vitamin D and its role in calcium and phosphate metabolism. 

Know the physiologic action of vitamin E and its connection to selenium. 

Henera E and Baibas C (2001) Vitamin E action, metabolism and perspectives. Journal of Physiology and Biochemistry 57,43-56. 

It is the most abundant of the noncollagen proteins of bone matrix, accounting for 1% to 2% of total bone protein, or 15% of noncollagen bone protein. Osteocalcin synthesis is induced by physiological concenUations of calcitriol, and the release of osteocalcin into the circulation provides a sensitive marker of vitamin D action and metabolic bone disease (Section 3.5). 

Cobalt is present in human organisms in the form of vitamin Bi2 derivatives, and the physiological actions of NO on vitamin Bi2 derivatives were intensively studied (54). It was reported that the reduced form of vitamin B12, [Cbl(II)], reacts with gaseous NO to form a stable nitrosyl complex. 

Nutritional and health-related applications. Lecithin has long been known in the worldwide nutritional community. Dietary supplement lecithin is generally derived from soybean lecithin. Because of its composition of various phospholipids, vitamins, and fatty acids, lecithin is involved in numerous physiological actions that... 

The normal physiological action of PTH on bone is blunted in patients with renal failure, because their renal cells are not synthesizing the 1,25 dfliydroxy vitamin D required for normal PTH action. It is typical for patients in renal failure to have high serum PTH values this represents secondary hyperparathyroidism, the normal physiological response to vitamin D deficit. Deposition of vM at the bone mineralization front and binding to parathyroid calcium receptors interferes with this physiologic process. The usual parathyroid response to these conditions decreases secretion of PTH. The result is lower-than-expected serum PTH concentration for the degree of renal disease present. 

The unique interactions of vitamin D with the VDRs have been a focus of research and have led to the development of vitamin D analogs, which vary in their affinity for this receptor and thus may result in less hypercalcemia, while retaining the positive physiologic actions on bone and parathyroid tissue. Paricalcitol and doxercal-ciferol are D2 compounds which effectively lower PTH in dialysis patients. Paricalcitol differs from calcitriol by the absence of the exocyclic carbon 19 and the fact that it is a vitamin D2 derivative. Currently this analog is available only for IV administration however, an oral formulation is in development. Doxercalciferol, in contrast to calcitriol and paricalcitol, is a prohormone that needs to be hydrox-ylated in the liver to its active 1,25-dihydroxyvitamin D2 product. Doxercalciferol is available for both IV and oral administration. 

Physiological action— The fat-soluble vitamins are required for the regulation of the metabolism of structural units, and each vitamin appears to have one or more specific roles. Collectively, the water-soluble B vitamins are primarily concerned with the transfer of energy. 

The letter D was used to designate the antirachitic vitamin of liver oil (for its physiological action see Chapt. XXII-4). Windaus first prepared vitamin D by irradiating ergosterol and determined its constitution. 

FIGURE 18.30 The physiological effects of ascorbic acid (vitamin C) are the result of its action as a reducing agent. A two-electron oxidation of ascorbic acid yields dehy-droascorbic acid. 

Structure and physiology of the kidney glomerular filtration tubular activity selective reabsorption and secretion, often using specific carrier mechanisms carbonic anhydrase and acid-base balance. The kidney also produces, and is sensitive to, hormones actions of the hormones ADH, aldosterone and PTH the kidney as a secretory organ erythropoietin, the renin-angiotensin system vitamin D3. 

Retinoids are a family of naturally occurring and synthetic analogues of vitamin A. The skin of subjects deficient in vitamin A becomes hyperplastic and keratotic (phrynoderma, or toad skin). While natural vitamin A is occasionally employed therapeutically, synthetic retinoids are more effective and represent a major advance in dermatological pharmacotherapy. Retinoids have myriad effects on cellular differentiation and proliferation it is likely that nuclear retinoic acid receptors mediate these effects by activating gene expression in a manner analogous to receptors for steroid hormones and thyroid hormones. Despite a common mechanism of action, however, retinoids vary widely in their physiological effects. 

A number of hormones modulate the actions of PTH, FGF23, and vitamin D in regulating bone mineral homeostasis. Compared with that of PTH, FGF23, and vitamin D, the physiologic impact of such secondary regulation on bone mineral homeostasis is minor. However, in pharmacologic amounts, a number of these hormones have actions on the bone mineral homeostatic mechanisms that can be exploited therapeutically. 

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