1) One of the presumed intermediates of the gentisic acid oxidation was obtained in a crystalline form using dialyzed crude gentisic acid oxidase, free from fumarylpyruvic acid hydro-lyase, from the cells of Pseudomonas ovalis S-5.
2) From the results obtained, the crystalline intermdediate was identified as fumarylpyruvic acid, which was splitted by crude extracts into pyruvic and fumaric acids.
3) Maleylpyruvic acid, the initial product of gentisic acid oxidation, was recognized spectro-photometrically in the reaction mixture using purified enzyme. And the pathway of gentisic acid oxidative metabolism was established.
4) Colorimentric determination of fumarylpyruvic acid is also described.
The authors wish to express their sincere thanks to Honorary Prof. K. Sakaguchi for his continued guidance throughout this research.

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