Techniques for the Localization of Phospholipids in Biological Membranes

Abstract

It is only about one decade ago when it was for the first time proposed that the phospholipids in a biological membrane might be distributed over both halves of the bilayer in a highly asymmetric fashion. This proposal of Bretscher was based on the observations that treatment of intact erythrocytes with NH2-group specific reagents does not result in the labeling of appreciable quantities of the amino-phospholipids, phosphatidylethanolamine (PE) and phosphatidylserine (PS), whereas these phospholipids could be easily labeled when open ghost membranes were exposed to the reagents (1). It was concluded from these observations that the majority of the aminophospholipids (PE and PS) should be located at the inner aspect of the erythrocyte membrane. A logical consequence was then to suppose that the other two major phospholipids of the human red cell, the choline-containing phospholipids phosphatidylcholine (PC) and sphingomyelin (SM), are predominantly located in the outer half of the bilayer. Soon after Bretscher had launched this ‘revolutionary’ idea, Zwaal and coworkers were able, not only to confirm Bretscher’s observations, but also to complete the picture of the asymmetric distribution of the phospholipids in the human red cell membrane by using various highly purified phospholipases (see Refs. 2, 3 and the subsequent chapter). Since then, such studies, involving a great variety of different probes, have been extended to many other membrane systems but, unfortunately, not always as successful as in case of the erythrocyte membrane. It is the aim of this chapter to review the various types of probes that are used nowadays for phospholipid localization studies and to discuss their specific possibilities and limitations.