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

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 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 calcium absorption. After this initial stimulation, there was a rapid decline in sialic acid incorporation into microsomes decreasing below control levels at 24 hr. Calcium 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 calcium in vitro. These results suggest that the initial action of 1,25-(OH)2D3 is to change the membrane permeability to calcium and to change the subcellular distribution of calcium in the small intestine. The accumulated calcium in the microsomes then stimulates the sialic acid incorporation into desialoenzyme. This results in the changes of isozyme pattern of alkaline phosphatase, viz, the transformation of desialoenzyme to sialoenzyme. The transformed alkaline phosphatase might be one of the factors involved more directly in the regulation of calcium transport in intestine. (auth.)