No abstract

Membrane phospholipid asymmetry is considered to be a general property of biological membranes. Detailed information is presently available on the non-random orientation of phospholipids in red cell- and platelet membranes. The outer leaflet of the lipid bilayer membrane is rich in choline-phospholipids, whereas amino-phospholipids are abundant in the inner leaflet. Studies with blood platelets have shown that these asymmetries are not maintained when the cells are activated in various ways. Undoing the normal asymmetry of membrane phospholipids in activated blood cells is presumably mediated by increased transbilayer movement of phospholipids. This process, which leads to increased exposure of negatively charged phosphatidylserine at the outer surface, plays an important physiological role in local blood clotting reactions. A similar phenomenon occurs in sickled red cells. Phospholipid vesicles breaking off from reversibly sickled cells contribute similarly to intravascular clotting in the crisis phase of sickle cell disease. The loss of membrane phospholipid asymmetry in activated platelets seem to be strictly correlated with degradation of cytoskeletal proteins by endogenous calpain. It is remarkable that membrane phospholipid asymmetry can be (partly) restored when activated platelets are treated with reducing agents. This leads to disappearance of phosphatidylserine from the outer leaflet where it was previously exposed during cell activation. These observations will be discussed in relation to two mechanisms which have been recognized to play a role in the regulation of membrane phospholipid asymmetry; i.e. the interaction of amino-phospholipids to cytoskeletal proteins, and the involvement of a phospholpid-translocase catalyzing outward-inward transbilayer movement of amino-phospholipids.

Phosphatidylserine (PS) in the plasma membrane of nonactivated human platelets is almost entirely located on the cytoplasmic side. Stimulation of platelets with the Ca2+ ionophore A23187 or combined action of collagen plus thrombin results in a rapid loss of the asymmetric distribution of PS. Also, treatment with the sulfhydryl-reactive compounds diamide and pyridyldithioethylamine (PDA) causes exposure of PS at the platelet outer surface. PS exposure is sensitively measured as the catalytic potential of platelets to enhance the rate of thrombin formation by the enzyme complex factor Xa-factor Va, since this reaction is essentially dependent on the presence of a PS-containing lipid surface. In this paper we demonstrate that endogenous PS, previously exposed at the outer surface during cell activation or sulfhydryl oxidation, can be translocated back to the cytoplasmic leaflet of the membrane by addition of dithiothreitol (DTT) but not by nonpermeable reducing agents like reduced glutathione. Treatment of platelets with trypsin or chymotrypsin, prior to addition of DTT, inhibits the inward transport of exposed PS. Moreover, severe depletion of metabolic ATP, as obtained by platelet stimulation with A23187 in the presence of metabolic inhibitors, though not inhibiting PS exposure at the outer surface, blocks the translocation of endogenous PS to the internal leaflet of the plasma membrane. These results strongly indicate the involvement of a membrane protein in the inward transport of endogenous PS. Recently, an aminophospholipid-specific translocase in the platelet membrane was postulated on the basis of the inward transport of exogenously added PS (analogues) [Sune, A., Bette-Bobillo, P., Bienvenue, A., Fellmann, P., & Devaux, P.F. (1987) Biochemistry 26, 2972-2978].(ABSTRACT TRUNCATED AT 250 WORDS)

No abstract