Dietary methyl donors affect in vivo methionine partitioning between transmethylation and protein synthesis in the neonatal piglet

Methionine metabolism is critical during development with significant requirements for protein synthesis and transmethylation reactions. However, separate requirements of methionine for protein synthesis and transmethylation are difficult to define because after transmethylation, demethylated methionine is either irreversibly oxidized to cysteine during transsulfuration, or methionine is regenerated by the dietary methyl donors, choline (via betaine) or folate during remethylation. We hypothesized that remethylation contributes significantly to methionine availability and affects partitioning between protein and transmethylation. 4-8-day-old neonatal piglets were fed a diet devoid (MD-) (n = 8) or replete (MS+) (n = 8) of folate, choline and betaine to limit remethylation. After 5 days, dietary methionine was reduced to 80 % of requirement in both groups of piglets to ensure methionine availability was limited. On day 7, an intragastric infusion of [13C1]methionine and [2H3-methyl]methionine was administered to measure methionine cycle flux. In MD- piglets, in vivo remethylation was 60 % lower despite 23-fold greater conversion of choline to betaine (P