SEMIQUINONES 



65 



Oxidant 



Semlquinone 



Reductant 



H OH 

 P yocygnin e 



The list of reversible two-step oxidation-reduction systems is now formidable 

 and, in addition to those already mentioned, includes /3-hydroxyphenazine, N- 

 methyl-/3-oxyphenazine, diphenyl benzidine sulphuric acid, phenanthraquinone-3- 

 sulphonate, lactoflavin, homogentisic acid, thiamin, lapachol, lomatiol, benzoin and 

 various coenzymes. 



It would appear that the intermediate radical in two-stage oxidations occurs 

 in so many biological oxidation-reduction reactions that this type of compound is an 

 essential factor in many dehydrogenase and oxidase systems. 



Evidence for the existence of semiquinone radicals is found not only in potentio- 

 metric and polarographic measurements but also in measurements of magnetic 

 susceptibility, since the radicals should contain an odd number of electrons and be 

 paramagnetic. In the case of phenanthraquinone-3-sulphonate reduction in alkahne 

 solution yielded a transient paramagnetic form but not in acid solution, in agreement 

 with potentiometric results. 



Pyocyanine is probably of respiratory significance, and increased oxygen 

 consumption was noted by Friedheim w^hen the pigment was added to suspensions of 

 red blood corpuscles or washed suspensions of living bacteria {B. pijocyaneus, Staphy- 

 lococcus aureus and pneumococcus, type III). Reed and Boyd (1932) also comment 

 on the respiratory significance of pyocyanine in B. pyocyaneus cultures, and its 

 effect on sea urchin eggs is described by Runnstrom (1935). 



Young (1937) describes the initial acceleration followed by inhibition produced 

 by pyocyanine on the metabolism of cerebral cortex. 



