ISOPHENOXAZINE SYNTHASE APOENZYME FROM PYCNOPORUS COCCINEUS,

A crystalline preparation of the apoenzyme of isophenoxazine synthase 2-aminophenolO2 oxidoreductase was obtained from Pycnoporus coccincus. The holoenzyme was reconstituted by addition of riboflavin 5-phosphate and Mn2. Flavin-adenine dinucleotide did not serve as a cofactor. Conversion of o-aminophenol to 2-amino3H-isophenoxazin-3-one was stoichiometric and in the absence of Mn2 required approx. I mole of riboflavin 5phosphate acted as a catalyst. Incubation of the enzyme with o-aminophenol generated thiol groups and also made the reaction susceptible to inhibition by sulfhydryl reagents. A variety of reducing substances repressed the activity of the enzyme inhibition by ascorbic acid could be reversed by increasing the concentration of riboflavin 5-phosphate and Mn2. The reaction was also inhibited by substrate at concentrations above 0.6 mM. Author