The generation and maintenance of transmembrane phospholipid asymmetry are essential for the function of biological membranes, yet the mechanism underlying these fundamental processes remains unclear. Our research effort is designed to understand how phospholipids are assembled in biological membranes and how the resulting phospholipid asymmetry is maintained. Phospholipid transporters, or "flippases," such as the recently discovered amino phospholipid translocator, represent an interesting new class of proteins that may play a key role in the assembly and organization of phospholipids in biological membranes. These enzymes require energy in the form of ATP, have strict phospholipid structural requirements, and are unique in their ability to transport lipids across membranes. Our goal is to elucidate the structure, function, and biological significance of these proteins.
Our studies employ a variety of biochemical, biophysical, and spectroscopic methods, including protein chemistry, radiolabel, and fluorescent techniques. Part of our work is directed at purifying the amino phospholipid flippase from human erythrocyte membranes. We have purified an ATPase that bears physical characteristics consistent with its involvement in amino phospholipid transport. Our enzymological studies have shown that this enzyme is specifically simulated by phosphatidylserine, the primary substrate of the amino phospholipid flippase. Once this transporter is reconstituted into model membranes, further biophysical studies of lipid-protein interactions and molecular mechanisms of phospholipid transport will be performed.
Concurrently, we are investigating the role of blood cell membrane structure in cardiovascular disease. Specifically, we are studying the loss of transmembrane phospholipid asymmetry observed in diabetic red blood cells to determine the relationship between the vascular complications associated with diabetes and membrane structural perturbations. Our studies indicate that hyperglycemic treatment of non-diabetic cells duplicates this loss of asymmetry by increasing passive lipid flip-flop, without affecting amino phospholipid flippase activity. Antioxidants suppress this loss of asymmetry, implicating a role for glucose-mediated lipid oxidation. Ongoing studies are designed to determine the mechanism by which lipid oxidation induces membrane lipid scrambling, including studies with animal models of diabetes and human diabetics. In related work, we are studying phospholipid transport in normal and diabetic blood platelets to understand the role of oxidative inhibition of the flippase in amino phospholipid externalization, a process required for normal blood clotting. These studies may lead to the development of new strategies for the treatment and prevention of heart disease.