Revised Manuscript Received October 9, 1992 abstract: The structural gene encoding 25-hydroxyvitamin D3 24-hydroxylase (P-450cc24) was isolated from the rat genomic DNA. It spans «15 kb, is composed of 12 exons, and was demonstrated by Southern blot analysis to be present as a single copy. One major T residue was identified at the cap site, a putative TATA (ATAAATA) box was located at position-30, and a putative CCAAT box was at-58. Four possible vitamin D responsive elements that may be involved in regulation of 24-hydroxylase expression were found in the S'-flanking region. Alignment with mitochondrial P-450 proteinsshowed that 7 out of 11 intron insertion sites of P-450cc24 gene occupied positions identical with those in the CYP11 family (P-450SCc, P-45OH0). The structure of the gene is discussed in relation to present knowledge about the mechanism of regulation of the 25-hydroxyvitamin D3 24-hydroxylase and calcium homeostasis.

Vitamin D3 is metabolized by a network of P-450-containing hydroxylases. In rat liver, both microsomal and mitochondrial 25-hydroxylases are present: P-450cc25 (Bjorkhem & Holm-berg, 1978; Bjfirkhem et al., 1979; Hayashi et al., 1986; Andersson & Jornvall, 1986) and P-450LMT25 (Bjorkhem et al., 1980; Masumoto et al., 1988; Usui et al., 1990), respectively. In kidney mitochondria, there are both a la-hydroxylase (Moorthy et al., 1991) and a 24-hydroxylase (Pedersen et al., 1983; Ohyama et al., 1989, 1991). Their activities are regulated by the level of serum Ca, phosphate, la, 25-dihydroxyvitamin D3 (l, 25 (OH) 2D3), 1 and/or other physiological factors such as parathyroid hormone (PTH)(Brommage & DeLuca, 1985; DeLuca, 1988; Minghetti & Norman, 1988).