Acid phosphatase and adenosine triphosphatase activities in the cell wall of baker's yeast

In order to establish whether a specific adenosine triphosphatase is present in yeast cell wall, hydrolysis rates for p-nitrophenylphosphate (acid phosphatase activity) and for ATP (ATPase activity) were compared under various conditions. Rate determinations were made with both, intact cells and with preparations containing secreted enzymes from protoplasts. Acid phosphatase and ATPase activities had the same pH profile and were susceptible in the same way to the repression by orthophosphate and to the inhibition by 2-deoxyglucose. The Lineweaver-Burk plot shows biphasic kinetic behaviour for the hydrolysis of either p-nitrophenylphosphate or ATP. This suggests the existence of two enzymes with different affinities for the substrates, or one enzyme with at least two active sites. The two activities differ in thermostability and only one activity could be completely abolished by heat treatment. The thermostable enzyme activity had K-m values of 0.475 mM for p-nitrophenylphosphate, and 0.040 mM for ATP. ATP behaved as a partially competitive inhibitor of p-nitrophenylphosphate hydrolysis. Substrate competition studies showed that only a non-specific acid phosphatase is responsible for the hydrolysis of ATP.