The Covalently‐Bound Flavin of Hepatic Monoamine Oxidase: 2. Identification and Properties of Cysteinyl Riboflavin

In the previous paper in this series it was shown that a pure flavin pentapeptide isolated from hepatic monoamine oxidase contains 1 mole each of serine and tyrosine and 2 moles of glycine, as well as an additional amino acid which is covalently linked to the 8α carbon of riboflavin. This amino acid has been identified as cysteine and the linkage to the flavin as a thioether on the basis of positive chloroplatinic and negative iodine‐azide tests, performic acid oxidation or reduction with zinc, followed by acid hydrolysis, which yield cysteic acid or cysteine, respectively. The fluorescence properties of the flavin from monoamine oxidase agree with those expected for a thioether and its oxidation products. In regard to optical and electron spin resonance spectra, chemical stability, and fluorescence characteristics the flavin peptide isolated from the enzyme agrees excellently with the properties of synthetic cysteinyl 8α‐riboflavin.