“…This variation might have evolved fortuitously, but there could be a functional significance if, as seems possible, alkaline phosphatase is in general vitamin A-dependent and if these isoenzymes have different threshold requirements for synthesis. This would explain why, for example, neurones and the blood vessel endothelium are rich in alkaline phosphatase in the presence of relatively low vitamin A concentrations (-100 ug/Il00 ml) (18), whereas the epidermal prickle cells require , 50,000 ug/ 100 ml for comparable synthesis to occur. In view of this, it is interesting to note that Fell and Mellanby (19) Abstract.…”