CONFORMATIONAL ORDER OF PHOSPHOLIPIDS INCORPORATED INTO HUMAN ERYTHROCYTES - AN FTIR SPECTROSCOPY STUDY

Acyl chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d(54)) a nd dimyristoylphosphatidylserine (DMPS-d(54)) were incorporated by inc ubation into human erythrocytes. Light microscopic analysis demonstrat ed that erythrocytes incubated with DMPC-d(54) became echinocytic whil e those incubated with DMPS-d(54) became stomatocytic. This indicates that DMPC-d(54) was incorporated preferentially into the outer monolay er whereas DMPS-d(54) was selectively incorporated into the inner mono layer. Fourier transform infrared (FTIR) spectroscopy was used to moni tor the conformational order of the incorporated phospholipids. The as ymmetric CD2 stretching frequency of the inserted perdeuterated acyl c hains was measured in both isolated membranes and intact erythrocytes as a function of temperature. DMPC-d(54) incorporated into erythrocyte s exhibited a cooperative phase transition at similar to 19 degrees C, i.e., at the same temperature as pure vesicles. In contrast, DMPS-d(5 4) incorporated into red cells exhibited no phase transition, but poss essed conformational order similar to that of the liquid-crystalline s tate. These results suggest that DMPC-d(54) persists in domains in the outer monolayer while DMPS-d(54) is dispersed in the inner monolayer. These experiments are the first to demonstrate that FTIR spectroscopy can be utilized to monitor directly a specific species of lipid molec ule from the entire phospholipid population.