Studies on the unmasking of membrane-bound alkaline phosphatase during the differentiation of Dictyostelium discoideum

Evidence has been presented recently to suggest that the 5′-nucleotidase and alkaline phosphatase activities of culmination phase cells of Dictyostelium discoideum are due to a single protein. However, we find that the membrane bound 5′-nucleotidase activity is only marginally activated by either 50 °C treatment or by dialysis, conditions that markedly activate alkaline phosphatase activity. In contrast, the 5′-nucleotidase activity of Triton X-100 extracts is activated by dialysis to the same extent as alkaline phosphatase activity. The available evidence suggests that, although a single protein is responsible for both alkaline phosphatase and 5′-nucleotidase activities, the characteristics of binding of the two substrates p-nitrophenyl phosphate and AMP to this enzyme are somewhat different. The alkaline phosphatase activity of dialyzed vegetative cell membranes is inhibited by the addition of concentrated dialyzate and this inhibition is reversed by further dialysis. The culmination phase enzyme is also inhibited by concentrated dialyzate from vegetative cells, suggesting that the removal of inhibitor from the enzyme can completely account for the developmental regulation. Dialyzates from culmination phase membranes do not inhibit enzyme activity, indicating the absence of inhibitor in these preparations. Alkaline phosphatase and 5′-nucleotidase activities of a partially purified enzyme preparation are equally inhibited by the addition of the concentrated dialyzate.