258 



PHYSIOLOGY OF THE FUNGI 



The fungicidal action of substituted quinones may be due in part to 

 their property of reacting with free amino groups of proteins (Theis, 

 1945). Substituted naphthoquinones as antagonists of vitamin K were 

 discussed in Chap. 11. Most of the available evidence indicates that 

 the principal mechanism of quinone toxicity lies in its noncompetitive 

 inhibition of sulfhydiyl enzymes. 



It has been suggested that the mechanism of inhibition is dependent 

 upon the structure of the substituted naphthoquinones. Colwell and 

 McCall (1946) found the fungistatic and fungicidal concentrations of 

 2-methyl-l,4-naphthoquinones to be the same when Aspergillus niger 

 and an unidentified fungus were used as test organisms. Addition of 

 sodium thioglycolate or cysteine antagonized the toxic action of this 

 naphthoquinone. These authors postulate that only naphthoquinones 

 unsubstituted in position 3 react wdth sulfhydryl groups, for 2-methyl-3- 

 methoxy-l,4-naphthoquinone was not antagonized by thioglycolate or 

 cysteine. The reaction between certain naphthoquinones and sulfhydiyl- 

 containing compounds can be demonstrated in vitro. 



The amounts of various substituted 1,4-naphthoquinones required to 

 cause a 50 per cent inhibition of isolated yeast carboxylase and similar 

 reduction in the germination of Monilinia fructicola spores w^ere roughly 

 parallel (Foote et al., 1949). Carboxylase is a sulfhydryl enzyme. It is 

 probable that other sulfhydryl enzymes are also inhibited by naphtho- 

 quinones. For further information on the mechanism of quinone inhibi- 

 tion, see Geiger (1946). 



Dyes. Various dyes are fungistatic compounds. Malachite green 

 and crystal violet are used to control various fungus infections of the 

 skin. Both these dyes have a benzoquinoid structure, as is shown below: 



^ ^ 



-N(CH3)2 



(CH3)2N=' 



/ 



\/ 



^ 



CI 



Malachite green 



Leonian (1930) made a study of the toxicity of malachite green to many 

 species and strains of Phytophthora and found only three species {P. 

 hydrophila, P. melongenae, and P. sp.) able to grow in the presence of 

 1 p. p.m. of malachite green. Other species were more sensitive to this 

 dye. P. colocasiae and P. richardiae failed to grow in nutrient solutions 

 containing 1 part of malachite green in 16 million parts of medium. 

 Leonian (1932) investigated the growth-inhibiting properties of malachite 



