QUINONES, ETC. 475 



of the cell. That is, sulfur diverts metabolism into non-productive or 

 inefficient pathways. 



Anaerobically, methylene blue prevents the reduction of sulfur by 

 spores of Neurospora sitophila (289). It is known that methylene blue 

 does not accept hydrogen directly from substrates or the pyridine nu- 

 cleotide coenzymes but from hydrogen (electron) carriers, such as the 

 flavoproteins and, possibly, cytochrome b (116). If the sulfur reduc- 

 tion process is in itself damaging to the cell, it is at this level that we 

 should look first. It may be noted that sulfur reduction is also inhib- 

 ited by 2,3-dichloro-l,4-naphthoquinone, the oxidation-reduction po- 

 tential of which is presumably close to that of methylene blue. 



Selenium and tellurium are much less toxic than sulfur, and their 

 reduction to hydrides by yeast cells is correspondingly much less rapid 

 (443). Selenium is accumulated in organic form by the mycelium of 

 Aspergillus niger in competition with sulfur, and selenate inhibition is 

 partially or wholly counteracted by reduced sulfur compounds (435). 

 These and similar data on Chlorella vulgaris (378) can be interpreted 

 in terms of competitive inhibition by a metabolite analogue. 



9. QUINONES AND OTHER UNSATURATED COMPOUNDS 



A number of actively fungistatic materials are characterized chemi- 

 cally by a system of conjugated double bonds. Of these, the most im- 

 portant are the quinonoid compounds, but the a,/?-unsaturated ketones 

 are also of interest. 



The most generally active quinonoid compounds are the halogenated 

 p-benzoquinones (266, 411) and 1,4-naphthoquinone and various de- 

 rivatives of it (228, 229, 280, 364, 413), including synthetic vitamin K 

 (2-methyl- 1,4-naphthoquinone) (15, 70, 133). The triphenylmethane 

 dyes, e.g., malachite green and crystal violet, are fungistatic and, sig- 

 nificantly, the leuco form (benzenoid) is not active against either fungi 

 (93) or bacteria (111). 



The chromophoric group of the actinomycins is quinonoid (Chapter 

 8), and the antibacterial javanicin is a naphthoquinone (Chapter 6). 

 The tropolones, some of which are produced by fungi (Chapter 6), 

 have a modified quinonoid structure (325); some of the thujaplicins 

 present in the decay-resistant heartwood of Thuja plicata show mod- 

 erate fungistatic activity (19, 344, 345). 



A few examples of quinonoid fungicides are shown in Figure 10. 



It will be recalled that many fungi possess part or all of an enzymatic 

 system which oxidizes phenols to quinones and further metabolizes the 

 quinones to black melanin pigments (Chapter 6). It follows therefore 



