Eliciting possible reaction equations and metabolic pathways involving orphan metabolites.

The development of metabolomics has resulted in the discovery of an increasing number of orphan metabolites, which are defined as compounds that are known to be present in living organisms but whose synthetic/degradation pathways are unknown. In this paper, we describe a procedure for identifying possible products and/or precursors of such orphan metabolites and for suggesting complete reaction equations and the corresponding EC (Enzyme Commission) number simultaneously. Chemical structure comparison is performed for a pair of compounds consisting of a reported substrate and its corresponding product and also for pairs of randomly selected compounds. Possible combinations of compounds registered in the KEGG database were used for generating putative enzyme reaction equations, which resulted in 77% of the reported equations being generated, as most of the remainder represent classes of compounds, rather than specific compounds, or contain Markush structures. The quality was checked using chemical structure...