Phospholipid transmethylation in the membrane of human neutrophils and lymphocytes

Phospholipid transmethylation in the microsomal fraction of stimulated and unstimulated human leukocytes was measured in a recently developed assay system. Microsomal fraction was prepared from neutrophils, unseparated lymphocytes, T lymphocytes, and non-T lymphocytes by sonication and subsequent ultracentrifugation. Two hundred micrograms of microsomal protein was reacted with S-adenosyl-L-[methyl-3H]methionine. In unstimulated cells, incorporation of methyl-3H into phospholipid was 0.60 +/- 0.06 pmol min-1 mg protein in neutrophil membrane, 0.84 +/- 0.075 in unseparated lymphocytes, 1.23 +/- 0.17 in T lymphocytes, and 0.71 +/- 0.085 in non-T lymphocytes (mean +/- SE). Stimulation of neutrophils with opsonized zymosan or concanavalin A (Con A), and of lymphocytes with Con A, phytohemagglutinin, or pokeweed mitogen increased 15 to 30%. The resulting methylated phospholipids were identified and quantitated by two-dimensional thin-layer chromatography. The inhibitor 5'-S-isobutyl-5'-deoxyadenosine (SIBA) inhibited transmethylation 47-55%. This assay system appears to measure specifically the activity of methyltransferases which mediate the transmethylation of membrane phospholipid; the assay should find important applications in the study of membrane lipid metabolism in human health and disease.