Lipofuscin is a general term assigned to material that exhibits greenish- yellow fluorescence and accumulates in cells as they age. Because of its age-related increase and seemingly universal occurance, lipofuscin is considered a hallmark of aging. However, relatively little direct chemical evidence as to the identity of lipofuscin exists. The goal of our study was to isolate and identify, from livers of Fisher-344 rats, those fluorescent materials which increase with age. We report that the fluorescent intensity of detergent-solubilized liver from Fisher-344 rats increased as a function of age. The fluorophore responsible for this increase was extracted with methanol and could be resolved from other cellular components when the methanol extracts were chromatographed over a reverse phase column. FAB-MS of this substance showed a quasi-molecular ion of 525, indicating retinyl palmitate as a possible structure. The compound was further identified as retinyl palmitate by coelution with authentic retinyl palmitate and by inspection of its proton NMR spectrum which was in full accord with that reported previously. The level of retinyl palmitate in the liver was found to increase throughout the entire life of the Fisher-344 rat (2 to 24 months) in a nearly linear fashion, from a value of 0.26 to 1.77 mg/g of liver. As has been reported for lipofuscin, retinyl palmitate accumulates in intracellular granules and exhibits greenish-yellow fluorescence. Due to these similarities, the relationship between retinyl palmitate and lipofuscin warrants further investigation. Retinyl palmitate is a lipophilic compound containing five conjugated double bonds. One intriguing possibility is that retinyl palmitate protects the cell from free radical damage, a process which has been implicated in cellular aging. The ability of retinyl palmitate to protect unsaturated fatty acids, and thus cell membranes, from peroxidative damage will be investigated.
