Urinary Riboflavin Metabolites in the Rat

: Male rats were fed a riboflavin-deficient diet for 25 days and then fed [2-14C]riboflavin to replete their riboflavin pool(s). During a second depletion period, urine and feces were collected and 14C excretion monitored. The urine was chromatographed on R-15 resorcinol resin and 14C was measured in all fractions. The 14C compounds in the individual fractions were shown to be different by thin layer chromatography. The amount of 14C recovered as riboflavin increased after acid hydrolysis of the urine indicating the presence of riboflavin in conjugated forms. All R-15 fractions were found to support growth in a microbiological (L. casei) assay for riboflavin. This suggested that other derivatives, in addition to riboflavin and flavin nucleotides, are biologically active. However, the conjugated compounds were apparently less active, since acid hydrolysis of the urine enhanced its growth-promoting ability of L. casei. One 14C metabolite was identified as urea, thus providing evidence for degradation of the riboflavin molecule. This study has shown that the metabolic fates of riboflavin in the rat include conjugation and extensive degradation.