Absorption, Metabolism, and Excretion of 8α-(Amino Acid)Riboflavins in the Rat

Abstract

Although most flavin exists as coenzymes noncovalently associated with flavoproteins from which riboflavin is released during digestion, a significant fraction is covalently bound FAD, which can be enzymatically released as 8α-(peptidyl)riboflavins. The fate of S-cysteinyl- and N(3)-histidyl-riboflavins as occur in mitochondrial monoamine oxidase and succinate dehydrogenase, respectively, has been determined with appropriate [¹⁴C]derivatives administered orally and intraperitoneally to rats. The amino acid-flavins appeared in those tissues that concentrate riboflavin. There was some enterohepatic circulation of ¹⁴C from 8α-[S-(N-acetyl)-L-cysteinyl]-D-[2-¹⁴C]riboflavin. At physiological levels, the compounds did not adequately replace or significantly antagonize the vitamin in deficient or normal rats. Excretion of both N-acetylcysteinyl- and histidyl-riboflavins and their metabolites was competitive with riboflavin; urinary loss was influenced by the amount of each administered, either separately or together with the vitamin. Considerable portions of both 8α-(amino acid)riboflavins were apparently decarboxylated to catabolites that have retained the α-amino and α-N-acetyl groups of the original histidyl- and N-acetylcysteinyl-riboflavins, respectively. In addition, the thioether linkage of the N-acetylcysteinyl flavin and catabolites was partially oxidized to sulfoxides. Overall, it appears that 8α-(peptidyl)flavins were largely unavailable as sources of riboflavin, although they occur in foods and during turnover of certain flavoproteins. They were, however, digested, absorbed, partially metabolized, and excreted. Hence, they should be asserted in the total economy of vitamin B₂.