Modification by liposomes of the adenosine triphosphate-activating effect on adenylate deaminase from pig heart

Adenylate deaminase (AMP deaminase, EC 3.5.4.6) of a high substrate specificity was purified from pig heart by chromatography on cellulose phosphate. The enzyme shows a co-operative binding of AMP [h (Hill coefficient) 2.35, with SO.5 (half-saturating substrate concentration) 5mM]. ATP and ADP act as positive effectors, lowering h to 1.55 and SO.5 to 1 mM. The addition of liposomes (phospholipid bilayers) to ATP-activated or ADP-activated enzyme causes a further shift of the h value to 1.04 and SO.5 to 0.5 mM. For ATP-activated enzyme the addition of liposomes increases Vmax. by about 100%, and for ADP-activated enzyme by 50%. Liposomes have no effect on the kinetics of AMP deaminase in the absence of ATP and ADP, and neither do they influence the inhibitory effect of orthophosphate on heart muscle AMP deaminase. Metabolic implications of these findings are discussed.