THE BINDING-PROPERTIES, WITH BLOOD PROTEINS, AND TISSUE DISTRIBUTION OF 22-OXA-1-ALPHA,25-DIHYDROXYVITAMIN D-3, A NONCALCEMIC ANALOG OF 1-ALPHA,25-DIHYDROXYVITAMIN D-3, IN RATS

The binding properties, with blood proteins, and tissue distribution of 22-oxa-1 alpha,25-dihydroxyvitamin D-3 (22-oxacalcitriol; OCT), a noncalcemic analogue of 1 alpha,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3], in rats were investigated. The binding affinity of OCT to plasma vitamin D binding protein (DBP) is extremely low and OCT mainly circulates in the blood as an intact form nonspecifically bound to lipoproteins, especially to chylomicrons and low density lipoprotein (LDL). OCT intravenously injected into normal rats rapidly disappeared from the blood, and rapidly appeared in the bile as glucuronides of intact OCT and Icr, 3 beta,20(S)-trihydroxy-9,10-secopregna-5,7,10(19)-triene (23,24,25,26,27-pentanorOCT; pentanorOCT) as an OCT metabolite. When OCT or 1,25(OH)(2)D-3 was injected into normal rats, significant amounts of OCT and 1,25(OH)(2)D-3 were quickly detected in the thyroid and parathyroid glands, thymus, adrenals, liver, plasma, small intestine, kidneys, and calvaria. The detected amounts of OCT in the parathyroid glands, thymus, adrenals, liver, small intestine, and kidneys were significantly higher than the respective values for 1,25(OH)(2)D-3 2 and/or 10 min after injection, while those of OCT in the plasma and calvaria were significantly lower than those of 1,25(OH)(2)D-3. The in vivo rapid turn-over, nonspecific transportation, and incorporation of detectable amounts into the tissues are typical characteristics of OCT which may account for its specific activities.