Lactose stereoisomers. Two epimers of lactose occur in nature, α-lactose and β-lactose. The epimers differ by the configuration of structural groups around a single stereogenic carbon atom (top right). (a) In KEGG Compound the synonyms lactose and milk sugar are assigned to a generic stereoisomer, where the configuration around this stereogenic carbon is not specified (C00243). Reactions, enzymes and pathways involving lactose are linked to this entry in KEGG. (b) The same synonyms and most lactose-related data are linked to the α-epimer in HMDB (HMDB00186). There is neither an entry for the generic stereoisomer in HMDB, nor an entry for the α-epimer in KEGG Compound.