The normal mammal requires large amounts of choline for maintenance and growth of tissue mass. Since milk, the only food for neonates, has many-fold higher free choline concentration than does maternal plasma, it is possible that mammary gland can synthesize choline molecules. The only known mammalian pathway for the synthesis de novo of choline molecules is catalysed by phosphatidylethanolamine N-methyltransferase (PeMT), which synthesizes phosphatidylcholine (PtdCho) via sequential methylation of phosphatidylethanolamine (PtdEtn) using S-adenosylmethionine (AdoMet) as a methyl donor. We identified PeMT activity in rat mammary tissue, and differences in affinities for substrate, as well as in activities as a function of pH, suggest that at least two distinct enzyme activities are involved [i.e. one catalysing the methylation of PtdEtn to form phosphatidyl-N-methylethanolamine (PtdMeEtn) and the other catalysing the methylation of PtdMeEtn and phosphatidyl-NN-dimethylethanolamine (PtdMe2Etn) to form PtdMe2Etn and PtdCho, respectively]. The relationships between AdoMet concentrations and PtdCho formation from endogenous PtdEtn in rat mammary homogenate were complex: a sigmoidal component (with a Hill coefficient of 2.2), requiring 55 microM-AdoMet for half saturation (Vmax. = 9 pmol/h per mg of protein), and a high affinity component (Kapparent = 8.7 microM and Vmax. = 3.8 pmol/h per mg of protein) were identified. When exogenous PtdMe2Etn was added as substrate, PtdCho formation exhibited Michaelis-Menten kinetics for AdoMet, and its affinity for AdoMet was high (Kapparent = 9 microM, Vmax. = 85 pmol/h per mg of protein). In the presence of endogenous substrates, the rates of PeMT-catalysed PtdCho formation within homogenates of rat mammary tissue were similar in tissue from lactating and non-lactating animals. When exogenous PtdMe2Etn was added to homogenates of rat mammary tissue, tissue from lactating rats made twice as much PtdCho as did tissue from non-lactating rats. Isolated mammary epithelial cells also exhibited PeMT activity; the rate of formation of PtdCho was much greater in intact versus broken cells. We also identified PeMT activity in homogenates of mammary tissue from non-lactating humans. The rate of PtdCho formation was of similar magnitude to that seen in rat tissue. This evidence supports the hypothesis that some of the choline found in milk could have been synthesized de novo in the mammary gland.

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