The early studies of Huldschinsky (l), Mellanby (2), McCollum et al.(3), Steenbock and Black (4). Askew et al.(5). and others indicated that an antirachitic agent capable of curing rickets was present in cod-liver oil and in irradiated foods. The culmination of these investigations was the isolation and identification of vitamin Ds as the antirachitic factor followed by elucidation of its biosynthesis from cholesterol (6, 7). In the ensuing three decades, relatively little additional work on vitamin D metabolism was done. Nonetheless, during this time there were great advances in methodology applicable to the study of vitamin D metabolism, which set the stage for a new era of vitamin D research. The advances included reverse-phase chromatography for vitamin D, development of a method for the biosynthesis of 14C-labeled vitamin D, and competitive binding assays for quantification of vitamin D (7, 8). In 1968, Blunt, DeLuca, and Schnoes (9) reported the isolation of 25hydroxyvitamin Ds (25-OH-DJ). They also reported that this metabolite ofvitamin Dj had a slightly greater biological activity than its precursor, vitamin DJ. Until this time, all known metabolites of vitamin Ds isolated had been reported to be inactive (10). Lawson, Wilson, and Kodicek (11) studied the metabolism of 25-OH-Ds using a doubly labeled compound with a tritium label at carbon-1 and I4C at carbon4 They observed that the tritium label, but not the carbon label, was lost in one of the metabolites of the compound found in intestinal nuclear fractions of chicks, a subcellular fraction known to retain the highest radioactivity

Abbreviations, trivial names, and systemic names (in brackets): vitamin Ds [calcidol]; 25-OH-DJ, 25hydroxyvitamin DJ [calcidiol]; Ia-OH-DJ. la-hydroxyvitamin DJ; 1, 25 (OHhDj, la. 25dihydroxyvitamin DJ [calcitriol]; 24, 25 (OH) zD~, 24R. 25-dihydroxyvitamin Ds; 1, 24, 25 (OH) sDs, la, 24R. 25-tnhydroxyvitamin Ds [calcitetrol]; 24-oxo-25-OH-Ds. 24-oxo-29hydroxyvitamin Ds; 24-oxo-23, 25 (OHhDs, 23S. 25-dihydroxy-Z4xo-~~ min Ds; 24-oxo-l, 25 (OH) zDs, la. 25-dihydroxy-24-oxc-vitamin Ds; 24-oxo1, 23, 25 (OH) jDs. la, ZSS, 25-trihydroxy-24-oxo-vitamin DJ; HPLC. high performance liquid chromatoyaphy; PCR, polymerase chain reaction. Formerly from the Department of Biochemistry, Hiroshima University School of Dentistry, Name was previously spelled as Kyuichiro Okuda.