Utilization, metabolism, significance and application of D-vitamins in thedog and cat

Because there is a lack of free 7-dehydrocholesterol in the epidermis dogs and cats can not form Vitamin D-3 on exposure to W-light. For the activatio n of the D-vitamins a hydroxylation to 25-hydroxyvitamin D-2 and D-3 in the liver as well as to the biologically most effective 1,25- and 24.25-dihydr oxyvitamin D-2 and D-3 in the kidneys is necessary. The dihydroxyvitamin-D- molecules are bound to receptors and in complex with other proteins active in the regulation of the transcription of more than 50 genes. A survey on t he effects of the activated D-vitamins in the various cell types as well as on the consequences of a lack or surplus is given. The problem of the appl ication of vitamin D-3 is discussed.