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Cholecalciferol 25-Hydroxylation is Similar in Liver Microsomes from Male and Female Rats when Cholecalciferol Concentration is Low^1,2,3,

Department of Biochemistry, Morehouse School of Medicine, Atlanta, GA 30310-1495

We compared cholecalciferol 25-hydroxylation in liver microsomes of male and female rats. The rate of production of 25-hydroxycholecalciferol was similar in liver microsomes from female rats and those from male rats when cholecalciferol concentration ranged from 50 to 200 nmol/L. The liver cytosolic fraction stimulated the 25-hydroxylase activity of the microsomes up to 100% in both male and female rats at 44 nmol/L cholecalciferol. Cytosol metabolized cholecalciferol to a currently unidentified metabolite. At 300 nmol/L cholecalciferol, synthesis of the cytosolic metabolite was 100% greater than at 100 nmol/L and coincided with 32% lower synthesis of 25-hydroxy-cholecalciferol. These results suggest similar 25-hydroxy-lase activity in liver microsomes from male and female rats and similar ability of liver cytosol from these rats to stimulate 25-hydroxylation at low nanomolar concentrations of cholecalciferol, whereas inhibitory effects of cytosol at higher concentrations of cholecalciferol were shown.

KEY WORDS: • vitamin D • cytosol • rats • cholecalciferol • liver microsomes

¹ Presented in part in abstract form at the September 1993 meeting of the American Society for Bone and Mineral Research, Tampa, FL [Thierry-Palmer, M., Nagappan, P., Kidd, L., Winston, B., & Free, A. L. (1993) Function of a 100–300 kD fraction of cytosol in the metabolism of cholecalciferol by the liver. J. Bone Min. Res. 8 (Suppl. 1): S209 (abs.); Thierry-Palmer, M., Free, A. L., & Nagappan, P. R. (1993) Comparison of cholecalciferol 25-hydroxylase activity in liver microsomes of male and female rats. J. Bone Min. Res. 8 (Suppl. 1): S209 (abs.)]. Presented in part in abstract form at Experimental Biology 94, April 1994, Anaheim, CA [Thierry-Palmer, M., Free, A. L., Nagappan, P. R. (1994) Comparison of cholecalciferol 25-hydroxylase activity in liver microsomes of male and female rats. FASEB J. 8: A190 (abs.)].

² Supported by National Science Foundation Grants DCB-8409838 and IBN-9100393.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed.

⁵ Current address: Department of Medicine, Cleveland Clinic Foundation, Cleveland, OH 44195.

Manuscript received 8 March 1994. Revision accepted 6 July 1994.