Vitamin D-dependent phosphorylation of an intestinal protein

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Abstract
The complete pattern of response of target tissues to steroid hormones is still unclear; the initial biochemical events may involve the transcriptional control of protein synthesis. A still outstanding problem concerns the role of these hormone-dependent proteins in the physiological response. Synthesis of several intestinal proteins is stimulated by 1,25-dihydroxyvitamin D (1,25-(OH)2D), at least one, Ca-binding protein (CaBP), by a transcriptional event. Steroid hormones stimulate protein phosphorylation 1,25-(OH)2D acting in vivo stimulates the ATP phosphorylation of a chick intestinal brush-border membrane protein of MW 84,000. The phosphate is attached to the protein through a serine or threonine residue. The reaction is stimulated by Ca2+ but does not require Mg2+, and kinetic studies showed that 1,25-(OH)2D acts on this process by increasing the number of sites rather than the affinity of the substrate for the phosphate group. This phosphorylated protein has similar characteristics to those of the intestinal membrane protein whose synthesis is stimulated by 1,25-(OH)2D.

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