The methyl balance of growing Lemna paucicostata has been studied by experiments with radioactive methionine. Quantitatively the most important methylated end-products (and the amounts accumulated per colony, in parentheses) are: phosphatidylcholine, choline, and phosphocholine (together, 10 nmole); pectin methyl ester (3.7 nmole); chlorophyll methyl ester (1.7 nmole); neutral lipid, as yet uncharacterized, but possibly cyclopropane fatty acids (1.5 nmole). Other possible methylated end-products are quantitatively small in comparison, and, together, the above plus the 4.4 nmole/colony accumulated as protein methionine account for the total methylneogenesis of the plant. About 10% of the total S-adenosylmethionine consumption of the plant is utilized in forming S-methylmethionine. Since the latter compound is formed about 30 times faster than it accumulates, it appears to be turning over rapidly metabolically. Tracer experiments show the methyl groups of S-methylmethionine are rapidly transferred back go methionine. Phosphatidylcholine is formed by sucessive methylations of phosphoethanolamine, rather than by methylation of phosphatidylethanolamine, as is commonly held to be the case for mammals and microorganisms. If exogenous choline is available in amounts sufficient to provide for the total choline needs of the plant, transfer of methyl groups to phosphoethanolamine and is products is decreased about 90%. Overall transmethylation decreases about 50%. A series of major metabolic rearrangements is set in motion. These regulatory phenomena are now being studied.
