Department of Biochemistry, Memorial University of Newfoundland, St John's, Newfoundland and Labrador, Canada.
Curr Opin Clin Nutr Metab Care. 2013 Jan;16(1):102-8. doi: 10.1097/MCO.0b013e32835ad2ee.
Methyl group metabolism is a metabolically demanding process that has significant nutritional implications. Methionine is required not only for protein synthesis but also as the primary source of methyl groups. However, demethylated methionine can be remethylated by methyl groups from methylneogenesis (via folate) and betaine (synthesized from choline). This review discusses the impact of methylation precursors and products on the methionine requirement.
Recent evidence has clearly demonstrated that transmethylation reactions can consume a significant proportion of the flux of methionine. In particular, synthesis of creatine and phosphatidylcholine consume most methyl groups and their dietary provision could spare methionine. Importantly, methionine can become limiting for protein and phosphatidylcholine synthesis when creatine synthesis is upregulated. Other research has shown that betaine and choline seem to be more effective than folate at reducing hyperhomocysteinemia and impacting cardiovascular outcomes suggesting they may be limiting.
It appears that methyl groups can become limiting when dietary supply is inadequate or if transmethylation reactions are upregulated. These situations can impact methionine availability for protein synthesis, which can reduce growth. The methionine requirement can likely be spared by methyl donor and methylated product supplementation.
甲基代谢是一个代谢需求很高的过程，对营养有重要影响。蛋氨酸不仅是蛋白质合成所必需的，也是甲基的主要来源。然而，去甲基蛋氨酸可以被甲基供体重新甲基化，来自于甲基供体的合成物（通过叶酸）和甜菜碱（由胆碱合成）。这篇综述讨论了甲基化前体和产物对蛋氨酸需求的影响。
最近的证据清楚地表明，转甲基反应可以消耗大量的蛋氨酸通量。特别是肌酸和磷脂酰胆碱的合成消耗了大部分甲基，它们的饮食供给可以节省蛋氨酸。重要的是，当肌酸合成上调时，蛋氨酸可能会限制蛋白质和磷脂酰胆碱的合成。其他研究表明，与叶酸相比，甜菜碱和胆碱似乎更能有效降低高同型半胱氨酸血症和影响心血管结局，这表明它们可能是有限的。
当饮食供应不足或转甲基反应上调时，甲基基团可能会变得有限。这些情况会影响蛋氨酸用于蛋白质合成的可用性，从而降低生长速度。通过补充甲基供体和甲基化产物，蛋氨酸的需求可能会得到节省。
