Utilization of riboflavin homologues by D-amino acid oxidase and xanthine oxidase.

D-Amino acid oxidase and xanthine oxidase, two enzymes possessing ionically bound flavin coenzymes have been studied with their flavin coenzymes derived from either 7-ethyl-8-methyl-flavin or 7-methyl-8-ethyl-flavin, vitamin-like homologues of riboflavin. 7-Ethyl-8-methyl-flavin caused a significant reduction of both D-amino acid oxidase and xanthine oxidase in the liver, but not in the kidney. 7-Methyl-8-ethyl-flavin caused a significant reduction of D-amino acid oxidase in both the liver and kidney, a significant reduction of xanthine oxidase in the liver, but a large and significant increase of the latter enzyme in the kidney. An improved procedure for the assay of xanthine oxidase has been described.