it became evident that the nucleoside, riboflavin, does not itself represent the original active group of the flavoprotein; it must be considered a breakdown CH a I N NH. N N H~ C "/\/ ""-C- O H3~( V " C=O 3 II I 1 - 3 H ~ " ff . . . . . . . . . . . / NH H3CJJ"N)"C/~m N?' C II II o o lumichrome lumiflavin product of a naturally occurring flavin nucleotide. Riboflavin-5'-phosphate (FMN, for flavin mononucleotide; see formula) was indicated by THEORELL as the original active group. In 1938, WARBURG and CHRISTIAN (1938 [7]-[10]) / OH ,CH CHOH- CHOH- CHOH-CH -P" 0 2- 20 N ,N OR H~A/ V " C-O 3 II I 1 - 3 H ~/"W,7'''C/NH II o Riboflavin-5'-phosphoric acid could bring excellent evidence for the idea that the co-enzyme of another flavo­ protein, n. amino acid oxidase, isflavin-adenine-dinucleotide or FAD (see formula). This was supported by ABRAHAM (1939), who identified both FMN and adenosine­ 5'-phosphate as hydrolysis products of FAD. The discovery of the coenzyme­ role of FAD has induced WARBURGand coworkers to engage in some speculation concerning the possibility that FMN is itself an artifact, produced through a breakdown of FAD. This aspect will be discussed later, in connection with recent work done on OYE in THEORELL'S laboratory. o NH 2 N CH3 .