To ascertain the physiologic function of vitamin D2 conjugates in calcium homeostasis, we synthesized vitamin D2 3β-glucosiduronate and vitamin D2 3β-sulfate in pure form and tested their biological activity in vitamin D deficient rats fed a low calcium diet. Vitamin D23β-glucosiduronate was active in promoting calcium transport in the intestine at a dose of 100 pmol per rat. It increased calcium mobilization from bone and soft tissue at a dose of 1000 pmol per rat. This conjugate was less active than equimolar doses of vitamin D2. These results demonstrate that vitamin D2 3β-glucosiduronate can be utilized by the rat as a source of vitamin D. In contrast, vitamin D23β-sulfate was biologically inert. It failed to increase calcium transport in the duodenum of vitamin D deficient rats except at the highest doses tested (≫100,000 pmol/rat). It was similarly ineffective in increasing calcium mobilization from bone and soft tissue. Our results lead us to conclude that vitamin D23β-glucosiduronate is probably utilized by the rat after hydrolysis to the free sterol; on the contrary, the sulfate is not biologically active except at the highest doses tested.
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