homocysteinePronunciation: /ˌhɒmə(ʊ)ˈsɪstɪiːn, ˌhəʊm-, -tɪɪn, -teɪn, -tiːn/Definition of homocysteine noun [mass noun] Biochemistry an amino acid which occurs in the body as an intermediate in the metabolism of methionine and cysteine. Chemical formula: HSCH2CH2CH(NH2)COOH homocysteine in other Oxford dictionaries Definition of homocysteine in the US English dictionary