There is no report on detailed fatty acid composition of retinal NSL thus far. After categorizing the retinal NSLs into seven CER classes, we determined the fatty acid composition of total NSL and each of the NSL fractions. At the same time, for a comparative study, we determined fatty acid compositions of NSLs in brain, liver, skin, and testis. What appears to be unique in retinal NSL fatty acid composition is the level of saturated fatty acids. Retinal NSLs contain the highest levels of saturated fatty acids (90%) among all the tissues tested, with 18:0 and 16:0 being the two major species (63% and 11%, respectfully;
Table 1). The composition of free retinal CERs (S1, S2, and S3) was also very similar, of which 18:0+16:0 accounts for 64% to 76% (
Table 3). This estimation matches closely the estimation by Fox et al.,
16 who reported that rat retinal CER contains 79% 16:0+18:0. Also, our measurement of the 16:0+18:0 composition in glucosyl CER species (62%–69%;
Table 3, S4, S5, and S6) matches very closely the reported values of 63% to 70%.
16 The SM fraction, on the other hand, contributed >70% of total retinal NSL and contained >96% saturated FA (
Table 3). SM in other tissues is known to contain higher quantities of unsaturated FAs, especially in testis,
44 but a composition of exclusive saturates in the retina may have some physiological importance. On the other hand, retinal levels of unsaturated amide-linked fatty acids are very low. The retinal value is the lowest of all tissues for n6 fatty acids, which is >10 times lower than skin. Among the polyunsaturated fatty acids, DHA contributes half of the total of only 4% in the retinal NSL. This finding is not surprising, since retinal ROS membranes possess some of the highest levels of DHA of any mammalian cells.
40 Low levels of amide-linked unsaturated fatty acids in the retina may indicate that NSL metabolic pools are different from phospholipid metabolic pools.