Effects of n-3 fatty acid depletion on microsomal PS biosynthetic activity. Microsomes (750 μg protein) obtained from the rats raised with n-3 fatty acid sufficient or deficient diets were incubated with exogenously added deuterium labeled PC substrates (50 μM each) in the presence of 1.5 mM serine for 1 h. Deuterium labeled PS products were measured by liquid chromatography/electrospray ionization-mass spectrometry. Neither PS biosynthetic capacity nor substrate specificity was significantly altered due to the replacement of DHA with DPAn-6. (Z. Wen and H.Y. Kim, unpublished data).