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Article

Ineffective Vitamin D Synthesis in Cats Is Reversed by an Inhibitor of 7-Dehydrocholestrol-⁷-Reductase^{1, ,2}

Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616

Changes in plasma 25-hydroxyvitamin D (25-OHD) were used as an index of vitamin D status of cats. Plasma 25-OHD concentration of kittens given a purified vitamin D-free diet and exposed to direct summer sun for 15 h/wk declined at a similar rate as kittens given the same diet kept indoors. Similarly, plasma 25-OHD of kittens exposed to ultraviolet (UV) lamps declined at a similar rate as kittens not exposed, and these kittens developed clinical signs of vitamin D deficiency. Eight weaned kittens were given the vitamin D-free purified diet until their plasma concentrations of 25-OHD were < 5 nmol/L. They then had the hair on their backs clipped at weekly intervals and were paired on the basis of skin color and exposed to UV light for 2 h/d. One member of each pair was given an inhibitor of 7-dehydrocholesterol (5,7-cholestradien-3ß-ol)-⁷-reductase (EC 1.3.1.21) in the diet. Cats receiving the inhibitor had a progressive increase in 25-OHD concentration of plasma with time to 91 ± 22 nmol/L (mean ± SEM), whereas cats not receiving the inhibitor had plasma 25-OHD concentrations that were not detectable (P < 0.001). Biopsy samples of skin from cats receiving the inhibitor had more than five times the concentration of 7-dehydrocholesterol (P < 0.001) than the skin of control cats. Low concentration of 7-dehydrocholesterol (presumably due to high activity of the reductase) in the skin of cats is the major impediment to effective vitamin D synthesis. Analysis of wild caught potential prey of cats indicated that these animals could supply adequate vitamin D to meet the requirement of growing kittens.

KEY WORDS: • cholecalciferol • 7-dehydrocholestrol-⁷-reductase • vitamin D synthesis • Cats

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