Interactions in glycine and methionine uptake, conversion and incorporation into proteins in the preimplantation mouse embryo

Abstract

SummaryGlycine is the most concentrated amino acid in the female genital tract. In this study, we report its conversion and incorporation into proteins in the presence or absence of methionine, in borth 1-cell and blastocyst mouse embryos. The uptake, incorporation and conversion of radiolabelled glycine were studied in the presence or absence of unlabelled methionine. For control purposes, the reciprocal experiment was performed with labelled methionine in the presence or absence of unlabelled glycine. At the 1-cell stage neither glycine uptake nor its incorporation into proteins is inhibited by methionine. Glycine is, however, highly used as an oxidisable energy substrate, via glycolate. At the blastocyst stage, glycine conversion into other amino acids is high and mainly utilised in the formation of glutamic acid. Glycine is highly incorporated into proteins, resulting in a poor exchange of glycine from the preloaded embryos. Methionine competes for glycine uptake and consequently reduces its overall incorporation into proteins. For methionine, neither its uptake nor its incorporation into proteins is reduced in the presence of glycine for the two embryonic stages tested here. The embryo has different mechanisms for incorporation and utilisation of methionine and glycine. Glycine, which has an important function in the embryo, has an inefficient transport system compared with methionine. We were unable to demonstrate the presence of methyiglycine since SAM-glycine-methyltransferase (EC 2.1.1.20) was not detected. The same results were obtained when exogenous methionine was added. We therefore concluded that glycine does not compete in transmethylation within the embryo.