Clarification and Quantitation of Primary (Tissue) and Secondary (Microbial) Catabolites of Riboflavin That Are Excreted in Mammalian (Rat) Urine

Abstract

Riboflavin derivatives were quantitated and identified in urine of rats fed 0, 2 and 6 µg riboflavin/g diet per day both with and without added succinyl sulfathiazole for 6 wk. Two rats from each dietary group were placed in metabolic cages and urine was collected in the dark for 24 h. On the fourth week, a third animal from each group received an i.p. injection of [2-14C]riboflavin before being placed in a metabolic cage and urine collected in the dark for 48 h. Urine samples were extracted with phenol for flavin components and with chloroform for lumichrome and derivatives. Riboflavin was the predominant flavin excreted by rats in all dietary groups, followed by hydroxymethylriboflavins and smaller amounts of flavin mononucleotide (FMN), lumiflavin and 10-hydroxyethylflavin. Carboxylumichromes accounted for 5–10% of the total flavin-derived fluorescence in urine of rats fed 2 and 6 µg riboflavin/g diet and were reduced to ∼3% when sulfathiazole was added to the base diets. Carboxylumichromes were absent from urine of riboflavin-deficient rats. Riboflavin accounted for 85–90% of the recovered radioactivity of all radioactive urine extracts; no radioactively labeled carboxylumichromes were detected. These results indicate that hydroxymethylriboflavins are primary catabolites of riboflavin derived from tissue microsomal oxidations, whereas carboxylumichromes reflect the continued oxidation of ring hydroxymethyl functions plus gut microbial cleavage of the side chain of flavin.