Urinary lumichrome-level catabolites of riboflavin are due to microbial and photochemical events and not rat tissue enzymatic cleavage of the ribityl chain.

The occurrence of 7- and 8-carboxylumichromes as a significant fraction of riboflavin-derived material in rat urine has led to a reexamination of tissues for possible enzymatic activity that could contribute to cleavage at position 10 of the D-ribityl chain of the vitamin. For this, homogenates were incubated in the dark under various conditions with pure [2-14C]riboflavin with and without cofactors prior to extractions with phenol for flavins or chloroform for lumichromes, and high pressure liquid chromatography was used to separate and to quantitate all radioactive compounds. Only unmodified riboflavin and small amounts of flavin mononucleotide (FMN) were found even though the level of detection of known derivatives including 10-(2'-hydroxyethyl)flavin and lumichrome was shown to be extremely sensitive. Hence, rat tissue per se does not appear responsible for significant cleavage of the D-ribityl chain of riboflavin. Rather such cleavage to the lumichrome level must result from the known actions of intestinal microflora and potentially from photolysis.