Conformational Changes and Oxidation of Membrane Proteins in Senescent Human Erythrocytes

Abstract

Human red cells spend 120 days in the circulation and are then removed in an age-dependent manner (1). Since cell destruction is age-dependent, studies about red cell senescence focused on the mechanisms by which the aging of the cell leads to its destruction. The presence of autoantibodies on the surface of senescent cells produced the development of the autoimmune hypothesis for senescent cell removal from the circulation (2–4), and raised questions about the presence of senescence markers on the cell surface that permit such recognition and the mechanisms of their development during red cell life span. Studies on surface changes taking place during red cell senescence have been carried out mainly on density-separated red cells (5). A reduction in membrane surface area in the dense cell population is evident as a decrease in membrane cholesterol and phospholipid content (6,7) and in acetylcholinesterase activity and sialic acid content (8). Cell deformability decreases (9–12) and at the level of the membrane slight modifications of the covalent structure of some components have been described, produced by processes like oxidation (13–15), proteolysis (16, 17), glycation (18), methylation and transamidation (19), phosphorylation (20), and modifications of phospholipid asimmetry (21) and of topology and topography of proteins have been reported or hypothesized (22–25). Most of these modifications are effective in promoting autoantibody binding and/or phagocytosis in vitro, thus supporting a possible role of these mechanisms in determining recognition and removal of senescent cells. Investigations carried out with in vivo (26,27) and in vitro models (28,29) for red cell senescence and studies with mutant erythrocytes showed that oxidation plays a relevant role in determining surface properties of senescent cells and of many pathological cells with a decreased life span (30–32). Since the oxidative state of membrane proteins in human red cells of different age has not been investigated in detail in the past, we tried to quantitate the oxidative lesion the membrane proteins undergo during red cell life-span, in an attempt to understand what kind of membrane processes expressed in senescent red cells can be related to oxidation.