Laboratory of Nutrition, Faculty of Pharmacy, University Louis Pasteur, 74 Route du Rhin, 67401 Illkirch, France.
Nutr Res Rev. 2004 Dec;17(2):135-51. doi: 10.1079/NRR200489.
N and S metabolisms are closely interwoven throughout both the plant and animal kingdoms. The essentiality of S relates to its participation in the structure of S-containing amino acids (SAA), to its inclusion in many sulfonated molecules, and to a myriad of metabolic and catalytic reactions of vital importance. Methionine (Met) is the indispensable SAA supplied by food proteins and its plasma homeostasis is achieved via a number of highly efficient regulatory mechanisms. In all conditions characterised by a negative body protein balance such as in dietary restriction or cytokine-induced hypercatabolic losses, N and S endogenous pools manifest parallel tissue depletion rates. Adaptive conservation of N and S body stores is reached by a functional restraint of the trans-sulfuration cascade, through the depression of cystathionine beta-synthase activity. As a result, upstream accumulation of homocysteine favours its re-methylation conversion to Met which helps maintain metabolic pathways of survival value. In addition to the measurement of vitamin indices, that of plasma transthyretin, a sensitive marker of protein nutritional status, is proposed to identify the fluctuations of the total body N component accountable for the alterations of homocysteine concentrations in body fluids.
N 和 S 代谢在植物界和动物界中紧密交织。S 的必需性与其参与含 S 氨基酸（SAA）的结构、包含在许多磺化分子中以及参与许多至关重要的代谢和催化反应有关。蛋氨酸（Met）是食物蛋白质提供的不可或缺的 SAA，其血浆稳态通过许多高效的调节机制来实现。在所有以负体蛋白平衡为特征的情况下，如饮食限制或细胞因子诱导的高分解代谢损失，N 和 S 内源性池表现出平行的组织耗竭率。通过功能性抑制转硫途径，降低半胱氨酸β-合酶活性，实现了对 N 和 S 体存储的适应性保护。结果，同型半胱氨酸上游的积累有利于其重新甲基化转化为 Met，有助于维持具有生存价值的代谢途径。除了维生素指数的测量外，还建议测量血浆转甲状腺素，这是一种蛋白质营养状况的敏感标志物，以识别总体 N 成分的波动，这是造成体液中同型半胱氨酸浓度变化的原因。
