Abstract

Human erythrocyte contains an AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) which requires the presence of certain alkali cations. ATP modifies the selective response of this enzyme to various alkali cations. The object of this work was to determine whether anions other than ATP could have similar effects.

AMP deaminase activities in the presence of various anions (SO₄^2-, Cl^-, Br^-, I^-, NO₃^-, and PO₄^- — PO₄^2-) were measured. The results showed that a change in the major anion of the system modifies the enzyme activity, and that the relative effects of anions are influenced by the nature of the activating cation. The effects of the substrate concentration and the activator-cation concentration on the enzyme activity were studied under conditions of constant and maximal anion effects. The data did not conform to simple Michaelis-Menten kinetics. At lower ranges of substrate and cation concentrations the results could be described by the empirical Hill equation. Although some reaction schemes consistent with the kinetic data were suggested, no definite conclusions about the mechanisms of the cation and anion effects could be reached. The studies did prove, however, that a clear distinction between the selective response of the enzyme to various alkali cations and the selective binding of these ions to the enzyme should be made.

Interactions of ATP and orthophosphate with the enzyme were investigated. It was shown that, in the presence of the two anions, decrease in ATP concentration and increase in orthophosphate concentration could have synergistic inhibitory effects on the enzyme activity. Possible significance of such effects in relation to the mechanisms for the control of adenine nucleotide content of erythrocyte was pointed out.