Acylphosphatase induced modifications in the functional properties of erythrocyte membrane sodium pump.
Human red cell acylphosphatase actively hydrolyzes the Na+/K(+)-ATPase phosphoenzyme from erythrocyte membrane. This effect occurred with amounts of acylphosphatase (up to 10 units/mg membrane protein) within the physiological range, and the low value of the apparent Km (0.147 +/- 0.050 microM) indicates that the enzyme has a high affinity for this substrate. When added at the above concentration to inside out vesicles from human erythrocytes, acylphosphatase significantly enhanced the rate of strophantidine-sensitive ATP hydrolysis. The same amounts of acylphosphatase stimulated, although to a lower extent, the rate of ATP-dependent 22Na+ influx (normal efflux). Thus, the calculated stoichiometry for Na+/ATP was 2.68 in the absence of acylphosphatase and 1.06 in the presence of 10 units/mg vesicle protein of the enzyme. Conversely, acylphosphatase addition strongly decreased the rate of ATP-dependent 86Rb+(K+) efflux (normal influx) which, with 10 units/mg vesicle protein, was almost suppressed. As a consequence, the Na+/Rb+ ratio, calculated as 1.52 in the absence of acylphosphatase rose to 72.5 in the presence of 10 units/mg vesicle protein of this enzyme. These results suggest that, because of its hydrolytic activity on the phosphoenzyme intermediate, acylphosphatase 'uncouples' erythrocyte membrane Na+,K+ pump. Possible mechanisms for this effect are discussed.
Dipartimento di Scienze Biochimiche, Università di Firenze, Italy.
P
P
Nassi
E
E
Marchetti
C
C
Nediani
G
G
Liguri
G
G
Ramponi
D017766:Acid Anhydride Hydrolases; D000255:Adenosine Triphosphate / Q000378:metabolism; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D006868:Hydrolysis; D017136:Ion Transport / Q000187:drug effects; D010744:Phosphoric Monoester Hydrolases / Q000494:pharmacology*; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects* / Q000378:metabolism; D000097024:Acylphosphatase
D017766:Acid Anhydride Hydrolases; D000255:Adenosine Triphosphate; D010744:Phosphoric Monoester Hydrolases; D012413:Rubidium; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D000097024:Acylphosphatase