The phospholipid composition of human lens membranes is not only unusually rich in sphingolipids but also very peculiar in the nature of its major (50%) sphingolipid component, 4, 5 dihydrosphingomyelin (DHSM). Most plasma membranes in animal cells contain sphingolipids. However, the main sphingolipid is sphingomyelin (SM), with a trans double bond between carbons 4 and 5 of the sphingosine moiety. The first goal of this investigation is to determine the effects of the presence (in SM) and the absence (in DHSM) of this double bond on the structural arrangement of the sphingolipids in the membrane. The formation of hydrogen bonds and their relevance in lipid-lipid and lipid-water interactions will be studied by infrared spectroscopy. Confirmatory studies will be performed using one- and two-dimensional nuclear magnetic resonance spectroscopy. The second objective is to study the biological role of DHSM, considered until about a year go as only a very minor component of plasma membranes. As the amount of DHSM in human lens membrane is about five and ten times greater than in lenses of cows and rabbits, respectively, which have a much shorter life span, it is hypothesized that DHSM adds stability to the lens membranes by increasing their resistance to oxidative and metabolic processes. This hypothesis will be tested on prepared unilamellar vesicles as well as whole lenses using UV radiation and hydrogen peroxide as oxidants. Furthermore, the composition of lens membranes from species with different life spans, from rabbits (~3 years) to elephants (~60 years) will be analyzed by 31P NMR to test for an expected increase in %DHSM with life expectancy. Along with their unusual phospholipid composition, human lens membranes exhibit molar ratios of cholesterol (CHol) to phospholipid higher than in any other tissue. The nature of the molecular interactions between CHol and both sphingolipids will be studied. In particular, the role of the OH group of CHol in the formation of H-bonds will be tested by carrying out experiments with CHol and with cholestene, which lacks the OH group. The impact of CHo1 on the function of membranes containing SM and DHSM will be studied. The activity of Ca-ATPase as well as permeability to Ca will be measured. With their unusual composition and unique functional demands, human lens membranes offer an ideal model system to establish the structural and functional roles of sphingolipids in biomembranes.
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