Receptor-Mediated Action of the Vitamin D Hormone
1,25-Dihydroxyvitamin D3(1,25(OH)2D3) is now considered to be the active hormonal sterol derived from the sunlight vitamin, vitamin D3. As depicted in Fig. 1, 1,25(OH)2D3 is formed in the kidney according to the calcium and phosphorus needs of the organism (Haussler and McCain, 1977). Its main functions are the stimulation of intestinal calcium and phosphate absorption as well as bone remodeling. In addition to its mineral conservation effects in the kidney, 1,25(OH)2D3 induces a 24-OHase enzyme that appears to initiate a catabolic cascade for side chain cleavage and metabolic elimination of both the hormone (Fig. 1) and its 25(OH)D3 precursor (Chandler et al., 1984). Thus the 1,25(OH)2D3 hormone is dynamic in the sense that its production is controlled by calcium/parathyroid hormone (PTH) and phosphate status, and its biodegradation is self-initiated.
KeywordsSide Chain Cleavage Upstream Regulatory Region Unoccupied Receptor Catabolic Cascade Nice Talk
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