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aden·o·syl·co·ba·la·min (AdoCbl) (ə-den″o-səl-ko-balґə-min) a cobalamin derivative in which the substituent is deoxyadenosyl. It is one of two metabolically active forms synthesized upon ingestion of vitamin B12 and is the predominant form in the liver; it acts as a coenzyme in the reaction catalyzed by methylmalonyl-CoA mutase.Medical dictionary. 2011.