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. 2018 May 24;9:1070. doi: 10.3389/fimmu.2018.01070

Figure 1.

Figure 1

(A) Molecular structure of betaine. (B) Metabolism of betaine and related sulfur amino acids (SAAs). Betaine is a substrate of choline and can be converted to DMG via demethylation to ultimately become glycine. Most of these reactions occur in the mitochondria. The demethylation reaction converts homocysteine to methionine and can be replaced by 5-methyl-THF, which can catalyze methylation to form THF. Then, methionine is successively converted to SAM and finally to homocysteine to form the methionine cycle. Homocysteine can also go through the transsulfuration pathway to form cystathionine, cysteine, taurine, or glutathione. The enzymes mentioned in this review are shown and marked in the cycle with individual numbers. 1. Betaine-homocysteine methyltransferase (BHMT); 2. Methionine synthase (MS); 3. Methionine adenosyltransferase (MAT); 4. SAM-dependent methyltransferases; 5. S-adenosylhomocysteine hydrolase; 6. Cystathionine β-synthase (CBS); 7. Cysteine dioxygenase (CDO); 8. γ-glutamylcysteine synthetase (GCS). THF, tetrahydrofolate; SAM, S-adenosyl-L-methionine; SAH S-adenosyl-L-homocysteine; DMG, N,N-dimethylglycine.