SPECTRAL STUDIES OF THE LIPIDS AND PROTEINS OF OCULAR-TISSUES .3. FOURIER-TRANSFORM INFRARED STUDY OF THE ROD OUTER SEGMENT DISK AND PLASMA-MEMBRANES OF VERTEBRATE RETINA

Phospholipid composition and structure of disk and plasma membranes pu rified from bovine rod outer segments (ROS) are examined using Fourier transform infrared spectroscopy. Vibrational data indicate that both disk and plasma membranes lack sphingophospholipids, in contrast to th e lens membranes. The hydrocarbon chains of the disk lipids are unsatu rated by a factor of 5 over the acyl chains of the plasma lipids. The plasma lipids with 3-fold higher cholesterol and 5-fold higher saturat ion melt at a higher temperature (26 degrees C) than the disk lipids w hich melt at 16 degrees C. The transition temperature decreases by mor e than 20 degrees C in going from disk lipids to disk membrane, indica ting a large drop in the enthalpy of the ROS membrane-matrix, presumab ly due to enhanced rhodopsin-lipid interaction. The lipid composition predisposes the disk and plasma membranes to be fluid and structurally disordered (about 84%) around physiological temperature. The fluid ph ospholipid environment of the disk membrane (i.e., just a few degrees above subzero temperatures) is considered to be vital for the ROS phot oreceptor function. The amide I band profile of rhodopsin indicates an extensive alpha-helical (53%) peptide chain, with little beta-sheet ( 21%) and beta-turns (18%) in ROS membranes. This structure and/or conf ormation is conserved between 0-60 degrees C even though disk and plas ma lipids undergo a phase change. The H-D exchange data indicate that as much as 84% of the peptide residues of ROS membranes in partially b leached retinas is accessible to D2O solvent after 1 h. This (3-4)-fol d higher exchange compared to the rate observed in the dark-adapted re tinas is understood primarily from the breakdown of hydrophobic intera ctions and/or tertiary and quaternary structures which stabilize the R OS membranes. It appears that photobleaching not only transduces photo ns but opens up the ''hydrophobic heart'' of the rhodopsin which leads to solvent accessibility. The rhodopsin structure may be stabilized t hrough hydrogen bridges existing between phospholipid carbonyls with r hodopsin and/or bilayer water, but hydrophobic forces appear to play a dominant role in stabilizing the ROS membrane-matrix, Subjection of R OS lipids to oxidative stress results in a substantial increase in con jugated polyenes, characteristic of the LPO-induced free-radical damag e involving pathological process in retinal disorders. The data indica te structural and chemical modifications of the membrane phosphate hea dgroup and degradation of the hydrocarbon region similar to those obse rved in the cataractous lens membranes. A change in lipid order (fluid ity) was observed that may derange ROS membrane function. Addition of low amounts of alpha-tocopherol inhibits lipid oxidation, maintaining the functional integrity of the ROS.