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25-Hydroxyvitamin D-24-Hydroxylase in Phytohemagglutinin-Stimulated Lymphocytes: Intermediate Bioresponse to 1,25-Dihydroxyvitamin D₃ of Cells from Parents of Patients with Vitamin DDependent Rickets type II^*

Department of Pediatrics, University of Tokushima School of Medicine Tokushima, Japan

Address all correspondence and requests for reprints to: Eiji Takeda, M.D., Department of Pediatrics, University of Tokushima School of Medicine, Kuramoto-Cho 2, Tokushima City, Tokushima 770, Japan.

A method for assay of 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) activity in phytohemagglutinin (PHA)-stimulated lymphocytes was applied to determine whether vitamin D-dependent rickets type II (VDDR II) is hereditary. In normal lymphocytes incubated with PHA for 3 days, maximal and half-maximal responses of 24-hydroxylase were observed after exposure to 10^–8 mol/L and (1.3 ± 0.4) x 10^–9 mol/L 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂D₃], respectively. These responses were similar to those of cultured skin fibroblasts. In contrast, after exposure to 10^–8, 10^–7, and 10^–6 mol/L 1,25-(OH)₂D₃, no 24-hydroxylase activity was detected in cells from patients with VDDR II, and intermediate activity was observed in cells from their parents. These findings indicated the presence of an intracellular receptor-effector system for 1,25-(OH)₂D₃ in peripheral lymphocytes. Heterozygotes of VDDR II could be identified, and autosomal recessive inheritance of the disease was demonstrated. Detection of heterozygotes of this disease was not possible by assay of inhibition of thymidine incorporation, another marker of the function of 1,25-(OH)₂D₃ in PHA-stimulated lymphocytes. Therefore, assay of 24-hydroxylase induction reflected the receptor status more closely than assay of inhibition of DNA biosynthesis. The assay of 24-hydroxylase activity in PHA-stimulated lymphocytes described here will be useful for diagnosis of VDDR II and study of families of patients with this disease.

^* This work was supported by Grant 62570428 from the Ministry of Education, Science, and Culture of Japan and a grant from the Mother and Child Health Foundation.

Received February 13, 1989.

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