Add time:07/15/2019         Source:sciencedirect.com

The kidney is the major source of the circulating dihydroxylated metabolites of vitamin D, 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] and 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3]. The enzymes which catalyze the production of these two dihydroxylated vitamin D metabolites are the 25(OH)D3-1α-hydroxylase (1α-hydroxylase) and –24R-hydroxylase (24R-hydroxylase), respectively. While there is no controversy regarding the fundamental importance of the 1α-hydroxylase in the production of the steroid hormone 1α,25(OH)2D3, the biologic significance of the 24R-hydroxylase has been the subject of ongoing discussion. Some hold that it is strictly catabolic, leading to side chain oxidation and cleavage of 25-hydroxylated vitamin D sterols, and others hold that it plays a biosynthetic role in the production of 24R,25(OH)2D3 which has biologic activities distinct from those of 1α,25(OH)2D3. The 24R-hydroxylase has properties in common with other multicatalytic steroidogenic enzymes: (1) the enzyme carries out multiple oxidative and carbon-carbon bond cleavages; (2) it utilizes two natural substrates; (3) its regulation varies depending on the cell or tissue in which it occurs. The purpose of this paper is to review the current literature relevant to the characteristics of the 24R-hydroxylase and its regulation in the context of other multicatalytic steroid hydroxylases in order to provide a perspective regarding its possible function as both a catabolic and activating enzyme in the vitamin D endocrine system.

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