Synthesis of methylated ethanolamine moieties: regulation by choline in soybean and carrot

Cultured cell suspensions of both carrot (Daucus carota L.) and soybean (Glycine max) take up exogenous choline efficiently from their respective growth media. During sustained growth at a concentration near 50 micromolar choline, this compound was taken up at rates which exceeded those at which phosphatidylcholine, is synthesized by cells growing in standard (i.e. choline-free) media. In 50 micromolar choline, both types of cells metabolized this compound to phosphocholine and phosphatidylcholine, but not to other detected metabolites, and marked accumulations of phosphocholine and choline occurred relative to phosphatidylcholine. Pregrowth in 50 micromolar choline for several doublings decreased the rate at which carrot cells transferred (3)H from l-[(3)H(3)C] methionine into the network of all methylated derivatives of ethanolamine by some 98%. With soybean cells, a decrease of 77% was observed. In both cell types, transfer of (3)H into S-methylmethionine, pectin methyl esters, methylated nucleic acids, and nonpolar lipid continued unabated. Gel-filtered extracts of carrot cells pregrown in 50 micromolar choline had marked decreases in the specific activities of S-adenosylmethionine-dependent phosphoethanolamine, phosphomethylethanolamine, and phosphodimethylethanolamine N-methyltransferases; extracts of soybean cells had a similar decrease in phosphoethanolamine N-methyltransferase. The significance of these findings for regulation of the rate of synthesis of methylated ethanolamine moieties is discussed.