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 o f 22-oxa-1 alpha,25-dihydroxyvitamin D-3 (22-oxacalcitriol; OCT), a no ncalcemic 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 vit amin D binding protein (DBP) is extremely low and OCT mainly circulate s in the blood as an intact form nonspecifically bound to lipoproteins , especially to chylomicrons and low density lipoprotein (LDL). OCT in travenously injected into normal rats rapidly disappeared from the blo od, and rapidly appeared in the bile as glucuronides of intact OCT and Icr, 3 20(S)-trihydroxy-9,10-secopregna-5,7,10(19)-triene (23,24,25,2 6,27-pentanorOCT; pentanorOCT) as an OCT metabolite. When OCT or 1,25( OH)(2)D-3 was injected into normal rats, significant amounts of OCT an d 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, thym us, adrenals, liver, small intestine, and kidneys were significantly h igher than the respective values for 1,25(OH)(2)D-3 2 and/or 10 min af ter injection, while those of OCT in the plasma and calvaria were sign ificantly lower than those of 1,25(OH)(2)D-3. The in vivo rapid turn-o ver, nonspecific transportation, and incorporation of detectable amoun ts into the tissues are typical characteristics of OCT which may accou nt for its specific activities.