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Ascorbic Acid Effects on Vitamin D Hormone Metabolism and Binding in Guinea Pigs¹

Institute of Nutrition, Academy of Medical Sciences of the USSR, Moscow 109240, USSR

Ascorbic acid deficiency in guinea pigs fed a vitamin D-replete diet caused a moderate reduction of Ca level in serum and bone; 25-hydroxycholecalciferol or 25-hydroxyergocalciferol (25-OHD) serum concentration tended to decline; renal 25-hydroxycholecalciferol-1-hydroxylase (1-OHase) activity decreased 50%; and 25-hydroxycholecalciferol-24-hydroxylase activity increased 1.6-fold. Chromatin 1,25-dihydroxycholecalciferol [1,25-(OH)₂D₃] receptor concentration in the intestinal mucosa decreased 20–30%, and the percentage of occupied receptors decreased from 12–15% to 6–8%. Receptor affinity for 1,25-(OH)₂D₃ did not change (K_d = 0.24–0.26 nmol/L, K_d2 = 0.06–0.10 nmol/L), but the cooperativity coefficient decreased from 1.7 to 1.4. Vitamin C deficiency potentiated effects of vitamin D deprivation and impaired a restorative action of vitamin D. It was accompanied by a marked delay in the elevation of 25-OHD concentration in serum as well as decreased 1-OHase activity in kidneys and a lower concentration of occupied 1,25-(OH)₂D₃ receptors in the intestinal mucosa. The data demonstrate a critical role for ascorbic acid in vitamin D metabolism and binding.

KEY WORDS: • ascorbic acid • vitamin D metabolism • vitamin D receptor • guinea pigs

¹ Preliminary results from this study were presented at the 7th Workshop on Vitamin D, Rancho Mirage, CA, 24–29 April 1988.

Manuscript received 16 May 1989. Revision accepted 6 June 1990.

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