Differentiation of the Changes in Alkaine Phosphatase from Calcium Ion - Activated Adenosine Triphosphatase Activities Associated

The importance of cholecalciferol in calcium absorption is well established, but the manner in which the vitamin exerts its effect at a molecular level is still unclear. The active form of cholecalciferol in the intestine is a steroid hormone 1,25-dihydroxycholecalciferol, which is known to be necessary for calcium absorption and the synthesis of the cytoplasmic calcium-binding protein (Lawson & Emtage, 1974; Wasserman etal., 1974). However, comparative studies of the sequence and time scale of changes in calcium absorption and calcium-binding-protein concentrations observed in response to 1,25dihydroxycholecalciferol have shown that calciumbinding-protein production is not a primary event in the absorption of calcium. Thus other factors in addition to this protein must be involved in this process and these factors must in some way be affected by the hormone (Spencer et al., 1976). Such factors are at present unidentified, but a number of suggestions have been made. It has recently been shown that the synthesis of two membrane-bound proteins stimulated by 1,25-dihydroxycholecalciferol occurs before the onset of calcium-binding-protein production (Wilson & Lawson, 1977). One of these proteins has been identified as an actin-like protein (Wilson & Lawson, 1978) and the other, at present unidentified, has an apparent mol.wt. of 84000. In addition the activities of the membrane-bound enzymes alkaline phosphatase and Ca-ATPase* are increased by cholecalciferol and the possibility has often been considered that they are involved in the calcium absorption process (Haussler et al., 1970; Holdsworth, 1970; Melancon & DeLuca, 1970; Norman et al., 1970; Kowarski & Schacter, 1972). Some confusion has arisen as to whether these two