Hcysh

Mixed disulfide

Homocystine

Free reduced

HCY, homocysteine; CYS, cysteine.

Figure 1 The biosynthesis and metabolism of homocysteine. Reactions that are regulated by S-adenosylmethionine (SAM) are indicated by positive and negative signs. Key enzymes: (1) methyltetrahydrofolate-homocysteine methyltransferase or methionine synthase; (2) betaine-homocysteine methyltransfer-ase; (3) cystathionine ^-synthase; (4) methylenetetrahydrofolate reductase. Abbreviations: THF, tetrahydrofolate; PLP, pyridoxal-5'-phosphate (vitamin B6).

Figure 1 The biosynthesis and metabolism of homocysteine. Reactions that are regulated by S-adenosylmethionine (SAM) are indicated by positive and negative signs. Key enzymes: (1) methyltetrahydrofolate-homocysteine methyltransferase or methionine synthase; (2) betaine-homocysteine methyltransfer-ase; (3) cystathionine ^-synthase; (4) methylenetetrahydrofolate reductase. Abbreviations: THF, tetrahydrofolate; PLP, pyridoxal-5'-phosphate (vitamin B6).

neurotransmitters, and the synthesis of creatine. A product of all SAM-dependent methylation reactions is S-adenosylhomocysteine (SAH), which in turn is metabolized to form adenosine and homocysteine. Homocysteine is then at a metabolic crossroad: it can be remethylated to form methionine or cata-bolized through cystathionine synthesis.

In remethylation, homocysteine reacquires a methyl group in a reaction catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyltrans-ferase) (EC 2.1.1.13) with methyltetrahydrofolate serving as the methyl donor and vitamin B12 serving as a cofactor. This reaction occurs in all mammalian cells. Alternatively, homocysteine can be remethylated in a folate- and vitamin B12-independent reaction utilizing betaine as the methyl donor and catalyzed by betaine-homocysteine methyltransferase (EC 2.1.1.5). This reaction occurs primarily in the liver, and to a lesser extent in the kidney and possibly the brain.

Homocysteine catabolism occurs through cystathionine synthesis in a condensation reaction with serine. This reaction is catalyzed by cystathio-nine ^-synthase (EC 4.2.1.22), which requires vitamin B6 in the form of pyridoxal-5'-phosphate (PLP) as a cofactor. Cystathionine is then cleaved to form a-ketobutyrate and cysteine in a second PLP-dependent reaction catalyzed by cystathionase (EC 4.4.1.1). Further metabolism of cysteine leads to the formation of glutathione or inorganic sulfate.

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