| Spectral overlap and chemical-shift (delta) irreproducibility in 31P-NMR resonances for phospholipids in organic solvents challenge accurate band assignment and quantification. Changes in temperature enable resolution of overlaps and evaluation of temperature coefficients. These coefficients can then be used as another parameter in band assignments. Differences in coefficients suggest that H-bonds between the phosphate and headgroups are responsible for the changes of delta with temperature. To reassess previous assignments of phospholipid resonances in adult human lenses, temperature coefficients were evaluated. The corrected assignments revealed the presence of ceramide-1-phosphate and dihydroceramide-1-phosphate, metabolites of dihydrosphingomyelin and sphingomyelin. Among glycerophospholipids, mainly alkyl-acyl analogs and their lyso-forms were present.; The lipid analysis of a lens from a 33-year-old chimpanzee, the primate most closely related to humans, was pursued. 31P-NMR spectroscopy and matrix-assisted laser desorption-ionization (MALDI) mass spectrometry (MS) revealed the preponderance (50% to 70%) of dihydrosphingomyelin in cortical and nuclear membranes. These levels exceed those present in human lenses of comparable ages. Unlike human lenses, however, the content of phosphatidylcholines in the cortical region was 9%, nearly four times higher than in human cortical membranes of lenses of similar ages. These trends are consistent with previously observed correlations between phospholipid composition and lens growth. Compared to humans, the higher contents of phosphatidylcholines in the chimpanzee lens may contribute to faster postnatal growth whereas the higher levels of dihydrosphingomyelin may limit the lens weight (0.13 g) to values lower than those in humans (∼0.2 g) of comparable age.; To demonstrate the ability of MALDI-MS to track lipid metabolism, the synthesis of choline-phospholipids was monitored in human-lens epithelial cells in situ and in vitro. As the cells were cultured with deuterated choline, new peaks appeared in the spectra. Initial rates of appearance of deuterated phosphatidylcholines were comparable and four times higher than those for sphingomyelins. These differences are consistent with the initial generation of PCs, their subsequent degradation, and transfer of phosphocholine to sphingomyelins. The addition of myriocin, an inhibitor of sphingomyelin synthesis, caused a two-fold decrease in the contents of new sphingomyelins and the unexpected increase in some phosphatidylcholines, but the mitotic rate was reduced. |
