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Homocystinuria: Vitamin B6 Dependent or Not?

Ann Intern Med. 1969;71(1):209-211. doi:10.7326/0003-4819-71-1-209
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Homocysteine arises from methionine, after "activation" as S-adenosylmethionine, and loss of the methyl group. Homocysteine normally combines with serine under influence of the enzyme cystathionine synthase, to form cystathionine. The last is cleaved on the other side of the sulfur to yield the shorter-chain sulfur amino acid, cysteine, plus homoserine. (The prefix "homo" means "one more than" or that those compounds have one more methylene, -CH2-, group than the root-name compound. Homocysteine is half of homocystine. The former, HS-CH2CH2CHNH2COOH, is one half of the latter, joined by oxidation to the disulfide to produce a symmetrical diamino, dicarboxylic acid.)

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