AROMATIC COMPOUNDS 175 



The enzymes responsible for the oxidation of phenols are the poly- 

 phenol oxidases, of which two have been separated: 



1. Tyrosinase, which oxidizes both monophenols (tyrosine, p-cresol, 

 etc.) and polyphenols (catechol, pyrogallol, etc.). Oxidation of tyro- 

 sine leads, by a complex series of enzymatic and non-enzymatic re- 

 actions, to the formation of black pigments, the melanins (Chapter 

 8). 



2. Laccase, which oxidizes polyphenols but not monophenols. In 

 contrast to tyrosinase, laccase is sensitive to carbon monoxide and 

 has a somewhat different substrate range. 



Both enzymes are copper proteins. The characteristic reaction, 

 often used for assay, is the oxidation of catechol to o-benzoquinone: 



+ ±0 2 -> +H 2 (13) 



Catechol o-Benzoquinone 



Although other polyphenol oxidases probably occur in fungi (108, 

 286), they have not been characterized. Most assays do not, indeed, 

 discriminate between laccase and tyrosinase. 



Interest in the polyphenol oxidases has been stimulated by the 

 Bavendamm reaction (35) of the wood-rotting fungi: on an agar 

 medium containing gallic or tannic acid the fungi causing white rots 

 of wood and capable of utilizing lignin form a colored zone. The 

 brown rot fungi in general do not form the zone. Several surveys of 

 the wood-rotting fungi have established that the correlation between 

 zone production and type of rot is high, although not perfect (86, 160, 

 397). The usual substrates do not distinguish between tyrosinase and 

 laccase, but work with specific substrates and on the distribution of 

 the enzymes between cells and medium indicates that extracellular 

 laccase, rather than tyrosinase, is responsible for the Bavendamm re- 

 action (274, 335, 343, 344). 



Extracellular polyphenol oxidases are formed also by other basi- 

 diomycetes, including parasitic and mycorrhizal species and both 

 coprophilic and litter-inhabiting saprophytes (340). Fungi other than 

 basidiomycetes have been less well studied; tyrosinase is found in 

 Glomerella cingulata (500) and Neurospora crassa (210, 287) and — 

 judged by color reactions — in Streptomyces scabies (282) and in Sclero- 

 tinia fructicola and Macrosporium sarcinaeforme (445). 



