Synthesis and biological activity of 1 alpha,23,25,26-tetrahydroxyvitamin D3

The metabolic pathway from 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3] to 1 alpha,25-dihydroxyvitamin D3-26,23-lactone includes the formation of 1 alpha,23,25-26-tetrahydroxyvitamin D3 [1 alpha,23,25,26-(OH)4D3]. The aim of the current study was to explore the as yet unknown biological properties of this vitamin D3 sterol. The four diastereoisomers of 1 alpha,23,25,26-(OH)4D3 were chemically synthesized. They were compared to 1 alpha,25-(OH)2D3 in terms of their affinity for the chick intestinal 1 alpha,25-(OH)2D3 receptor and their biologic activity in vivo (stimulation of intestinal calcium absorption and mobilization of calcium from bone in vitamin D-deficient rats). The 1,25-(OH)2D3 receptor binding affinities of 1 alpha,23(R)25(R)26-(OH)4D3, 1 alpha,23(S)25(S)26-(OH)4 D3, 1 alpha,23(S)25(R)26-(OH)4D3, and 1 alpha,23(R)25(S)26-(OH)4D3 were 11, 100, 216, and 443 times weaker than the binding affinity of 1 alpha,25-(OH)2D3, respectively. Compared to 1 alpha,25-(OH)2D3, the relative capacities of the 1 alpha,23,25,26-(OH)4D3 compounds to stimulate intestinal calcium absorption were 1/4 for 1 alpha,23(R)25(R)26-(OH)4D3; 1/19 for 1 alpha,23(S)25(S)26-(OH)4D3; 1/90 for 1 alpha,23(S)25(R)26-(OH)4D3; and 1/136 for 1 alpha,23(R)25(S)26-(OH)4D3. Maximal stimulation of intestinal calcium transport occurred 8 h after administration of vitamin D3 metabolites. Mobilization of calcium from bone was quantitated by serum calcium concentration measurements. The activities of 1 alpha,23(R)25(R)26-(OH)4D3, 1 alpha,23(S)25(S)26-(OH)4D3, 1 alpha,23(S)25(R)26-(OH)4D3, and 1 alpha,23(R)25(S)26-(OH)4D3 to increase serum calcium were estimated to be 4, 13, 43, and 69 times weaker than that of 1 alpha,25-(OH)2D3, respectively. These results illustrate the stereospecificity of the chicken intestine 1 alpha,25-(OH)2D3 receptor for binding of 1 alpha,23,25,26-(OH)4D3 and suggest that the 1 alpha,23,25,26-(OH)4D3 exerts its biological activity in the rat through an interaction with 1,25-(OH)2D3 receptors. In summary, the 1 alpha,23,25,26-(OH)4D3 had a markedly lower biological activity than 1 alpha,25-(OH)2D3.