Revised Manuscript Received October 3, 1988 abstract: About a decade ago calcitroic acid was isolated as a major side chain cleaved water-soluble metabolite of 1, 25-dihydroxyvitamin D3 [Esvelt, R. P., Schnoes,. K., & DeLuca, H. F.(1979) Biochemistry 18, 3977], Presently, calcitroic acid is being considered as the major excretory form of 1, 25-dihydroxyvitamin D3. However, the exact site or sites of calcitroic acid production and the possible side chain modified intermediary metabolites that may be formed during the conversion of 1, 25-dihydroxyvitamin D3 into calcitroic acid are not fully understood. In the mean time there have been many advances in our understanding of the side-chain metabolism of 1, 25-dihydroxyvitamin D3. It is now well established that both the kidney and the intestine metabolize 1, 25-dihydroxyvitamin D3 through the C-24 oxidation pathway accordingto the following steps: 1, 25-dihydroxyvitamin D3—1, 24, 25-trihydroxyvitamin D3—* 1, 25-dihydroxy-24-oxovitamin D3—*· l, 23, 25-trihydroxy-24-oxovitamin D3. Recently, we identified 1, 23-dihydroxy-24, 25, 26, 27-tetranorvitamin D3 (C-23 alcohol) as a major side chain cleaved lipid-soluble metabolite of 1, 25-dihydroxyvitamin D3 and further extended the aforementioned C-24 oxidation pathway in the kidney by demonstrating 1, 23, 25-trihydroxy-24-oxovitamin D3 as the precursor of C-23 alcohol [Reddy, GS, Tserng, K.-Y., Thomas, B. R., Dayal, R., & Norman, AW (1987) Biochemistry 26, 324], In this present study, we investigated the metabolic fate of 1, 25-dihydroxyvitamin D3 (3 X™ 10 M) in the perfused rat kidney and identifiedcalcitroic acid as the major water-soluble metabolite of 1, 25-dihydroxyvitamin D3. Furthermore, we also established C-23alcohol as the immediate precursor of calcitroic acid (C-23 acid) and finally concluded that 1, 25-dihydroxyvitamin D3 is metabolized in the kidney through the C-24 oxidation pathway according to the following steps: 1, 25-dihydroxyvitamin D3-*·

1, 24, 25-trihydroxyvitamin D3 1, 25-di-hydroxy-24-oxovitamin D3-*· l, 23, 25-trihydroxy-24-oxovitamin D3—C-23 alcohol—calcitroic acid. It now appears that the C-24 oxidation pathway for 1, 25-dihydroxyvitamin D3 probably exists in all the target tissues that respond to 1, 25-dihydroxyvitamin D3 and plays an important role in inactivating 1, 25-di-hydroxyvitamin D3 and thereby regulates the tissue concentration of the active hormone, 1, 25-dihydroxy-vitamin D3. e conversion of vitamin D3 into l, 25 (OH) 2D3, 1 the hormonal form of vitamin D3, is now universally accepted. The various factors responsible for the stringent physiological regulation of the production of l, 25 (OH) 2D3 in the kidney are well studied (DeLuca, 1981, 1984; Norman et al., 1982; Ku-mar, 1984; Haussler, 1986). However, as the circulating levels of l, 25 (OH) 2D3 depend not only on its rate of production but also on its rate of breakdown, it is also important to understand the target tissue metabolism and inactivation of l, 25 (OH) 2D3. Frolik and DeLuca (1971, 1972) investigated the further metabolism of l, 25 (OH) 2D3 in vivoand demonstrated for the first time that l, 25 (OH) 2D3 is further metabolized into polar metabolites. Later studies from the same laboratory measured 14C02 in expired air following a dose of l, 25 (OH) 2 [26, 27-14C] D3 in chicks (Kumar & DeLuca, 1976) and rats (Hamden tThis work was presented at the 7thInternational Workshop for