Effects of a methyl-deficient diet on rat liver phosphatidylcholine biosynthesis

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Abstract
To produce a severe choline-methionine deficiency, a synthetic L-amino acid diet, free of choline, methionine, vitamin B12, and folic acid and supplemented with guanidoacetate acid, a methyl group acceptor, was fed to female rats for 2 wk. The in vitro activity of liver microsomal phosphatidylethanolamine methyltransferase was stimulated 2-fold when compared with basal diet controls. The activity of choline phosphotransferase was depressed by 86%; thus, the contribution of the methyltransferase in the overall synthesis of phosphatidylcholine apparently increased. Measurement of the in vivo methylation of phosphatidylethanolamine by incorporation of [1,2-14C]ethanolamine into phosphatidylcholine indicates that the methylation pathway is markedly depressed in methyl deficiency. Hepatic concentrations of the methyltransferase substrate, S-adenosylmethionine, and the inhibitory metabolite, S-adenosylhomocysteine, were significantly altered such that an unfavorable environment for methylation was present in the deficient animal. The ratio of substrate to inhibitor was depressed from 5.2:1 in the controls to 1.7:1 in the livers of methyl-depleted rats. Control of transmethylation in accordance with the availability of substrates, phosphatidylethanolamine, or S-adenosylmethionine, and the level of S-adenosylhomocysteine is discussed.