homocysteine Syllabification: (ho·mo·cys·teine)Pronunciation: /ˌhōməˈsistēn/Definition of homocysteine noun Biochemistry an amino acid that 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 British & World English dictionary