Involved in the regulation of homocysteine metabolism. Converts betaine and homocysteine to dimethylglycine and methionine, respectively. This reaction is also required for the irreversible oxidation of choline. glycine betaine + L-homocysteine = L-methionine + N,N-dimethylglycine Binds 1 zinc ion per subunit. Amine and polyamine degradation; betaine degradation; sarcosine from betaine: step 1/2. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-methionine from L-homocysteine (BhmT route): step 1/1. Homotetramer. Predominantly localized in the cytoplasm with a small fraction detected in the nucleus. Translocates into the nucleus upon oxidative stress. Highly expressed in liver and kidney (at protein level). Expressed at lower levels in testis, lung, cerebellum, skeletal muscle and pancreas (at protein level).