The effect of 1,25-dihydroxycholecalciferol on the multiple forms of alkaline phosphatase and the sialic acid incorporation into microsomes of chick duodenum.

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Abstract
Polyacrylamide disc gel electrophoresis of n-butanol solubilized alkaline phosphatase from chick duodenum revealed that the change of alkaline phosphatase induced by 1,25-(OH)2D3 [1,25-dihydroxycholecalciferol] involved the transformation of desialoenzyme to sialoenzyme. The initial stimulation by 1,25-(OH)2D3 of the incorporation of sialic acid into duodenal microsomes corresponded with the initial increase in Ca2+ absorption. After this initial stimulation there was a rapid decline in sialic acid incorporation into microsomes, decreasing below control levels at 24 h. Ca2+ concentration in the microsomes followed a pattern similar to the incorporation of sialic acid into microsomes. The depressed sialic acid incorporation was reversed by the addition of Ca2+ in vitro. Apparently the initial action of 1,25-(OH)2D3 is to change the membrane permeability to Ca2+ and to change the subcellular distribution of Ca2+ in the small intestine. The accumulated Ca2+ in the microsomes then stimulates the sialic acid incorporation into desialoenzyme. This results in the changes of isozyme pattern of alkaline phosphatase, i.e., the transformation of desialoenzyme to sialoenzyme. The transformed alkaline phosphatase might be one of the factors involved more directly in the regulation of Ca2+ transport in intestine.