Protein binding properties of 22-oxa-lα, 25-dihydroxyvitamin D
3(22-oxa-1, 25-D
3), a synthetic analogue of 1α, 25-dihydroxyvitamin D
3(1, 25-D
3), were compared with those of vitamin D
3derivatives. The order of binding affinity to the chick embryonic intestinal receptor was 1, 25-D
3>22-oxa-1, 25-D
3>25-hydroxyvitamin D
3(25-D
3) >24R, 25-dihydroxyvitamin D
3(24, 25-D
3) >vitamin D
3(D
3), while that to the rat plasma vitamin D-binding protein (DBP) was 25-D
3>24, 25-D
3>D
3>1, 25-D
3>22-oxa-1, 25-D
3. The binding potencies of 22-oxa-1, 25-D
3to the receptor and DBP were about 1/8 and 1/600 of the respective values of 1, 25-D
3. When the distribution of the tritiated compounds in human plasma components was examined by an
in vitro polyacrylamide gel electrophoretic method, [
3H]-22-oxa-1, 25-D
3was found to bind only to the lipoproteins including chyromicron. These results suggest that the replacement of a carbon atom into an oxygen atom in the side chain structure of 1, 25-D
3results significant decrease in the binding affinity to DBP and that 22-oxa-1, 25-D
3is transported as a complex-form not with DBP but with lipoprotein to the target tissues.
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