CHARACTERIZATION OF THE CORRELATION BETWEEN ATP-DEPENDENT AMINOPHOSPHOLIPID TRANSLOCATION AND MG2-ATPASE ACTIVITY IN RED-BLOOD-CELL MEMBRANES()

Pseudosubstrates and inhibitors of ATPases were studied with respect t o their capability to modulate the kinetic behavior of Mg2+-ATPase and aminophospholipid translocation in red blood cell ghosts. ATP was sub stituted by the pseudosubstrates of P-type ATPases acetyl phosphate an d p-nitrophenyl phosphate. With both pseudosubstrates, aminophospholip id translocation from the outer to the inner leaflets of resealed eryt hrocyte ghosts could be observed, although with a significantly decrea sed velocity compared to that in presence of ATP, both with respect to phosphate hydrolysis and translocation. Similarly, the apparent affin ities for the pseudosubstrates were much lower than for ATP. Among the inhibitors studied, suramin acted as a competitive inhibitor of ATP t owards both Mg2+-ATPase activity and aminophospholipid translocation. However, the inhibition of translocation occurred at a higher inhibito r concentration than the inhibition of Mg2+-ATPase activity. With elai ophylin, only a partial inhibition of Mg2+-ATPase activity could be de tected, but translocation of labeled phosphatidylserine was almost com pletely abolished. With eosin Y, an almost complete inhibition of both Mg2+-ATPase activity and translocation could be achieved. The observe d responses of aminophospholipid translocation to ATPase inhibitors st rongly suggest that a P-type ATPase, part of which displays a Mg2+-ATP ase activity, is involved in aminophospholipid translocation.