Identification of p-hydroxybenzaldehyde as the ligand in the green form of old yellow enzyme

We have previously reported the isolation of old yellow enzyme complexed with a ligand of low molecular weight which imparts a distinctive charge-transfer absorption to the enzyme, making it green in color (Matthews, R. G., and Massey, V. (1969) J. Biol. Chem. 244, 1779-1789). This ligand has now been identified as p-hydroxybenzaldehyde by removal from the enzyme and characterization of its optical spectrum and subsequent mass spectral analysis. Similar compounds which are also bound to old yellow enzyme have been isolated from yeast extract and identified. These compounds give rise to complexes with old yellow enzyme which are characterized by broad long wavelength absorption bands, and they lead to inhibition of the NADPH-O2 oxidoreductase activity catalyzed by old yellow enzyme.