Acylphosphatase: a potential modulator of heart sarcolemma Na+,K+ pump.
Acylphosphatase, purified from cardiac muscle, catalyzes the hydrolysis of the phosphorylated intermediate of heart sarcolemmal Na+,K(+)-ATPase. This effect was remarkable even using acylphosphatase amounts (100-300 units/mg of membrane protein) near the lower limit of the physiological range; besides the low value of the apparent Km, on the order of 10(-7) M, indicates that the enzyme has a high affinity for this special substrate. The results of a dot-immunobinding assay suggest the possibility of an interaction between acylphosphatase and native, undenaturated Na+,K(+)-ATPase. Moreover, when added to sarcolemmal vesicles, acylphosphatase was found to affect the functional properties of the Na+,K+ pump with regard to the rate of both ATP hydrolysis and cation transport. However, while ATPase activity and Na+ uptake were stimulated, the last at a greater extent, the active K+ transport was inhibited, so that the Na+/K+ ratio, which was calculated as 1.50 without acylphosphatase, rose to 6.68 in the presence of 300 units/mg of vesicle protein of this enzyme. Taken together, the reported results indicate that acylphosphatase, because of its hydrolytic activity on the phosphoenzyme intermediate, induces a sort of "uncoupling" effect on the heart sarcolemmal membrane Na+,K+ pump. Possible mechanisms for such an effect, which suggests a potential role of acylphosphatase in the control of this active transport system, are discussed.
Nediani
C
C
Dipartimento di Scienze Biochimiche, Università di Firenze, Italy.
Fiorillo
C
C
Marchetti
E
E
Bandinelli
R
R
Degl'Innocenti
D
D
Nassi
P
P
D017766:Acid Anhydride Hydrolases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active; D002417:Cattle; D006868:Hydrolysis; D007118:Immunoassay; D007700:Kinetics; D009206:Myocardium / Q000201:enzymology*; D010710:Phosphates / Q000378:metabolism; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D012508:Sarcolemma / Q000201:enzymology*; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000097024:Acylphosphatase
D017766:Acid Anhydride Hydrolases; D000255:Adenosine Triphosphate; D010710:Phosphates; D011188:Potassium; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D000097024:Acylphosphatase