108 REACTIONS INVOLVED IN TCH. 



oxidase reaction turn brown on injury, or on exposure to chloroform 

 vapour, or often when placed in absolute alcohol. The juices and 

 extracts of such plants also turn brown or reddish-brown on exposure 

 to air. 



The results of investigations made by the author 1 , considered in 

 conjunction with those obtained by other workers, led to the suggestion 

 that the direct oxidase reaction is given only when, in the plant meta- 

 bolism, certain substances are formed which can (after the death of the 

 plant) autoxidise in presence of air, and which then in the state of 

 organic peroxide form a system capable of oxidising certain artificial 

 acceptors, such as guaiacum, etc. There is evidence 2 that such a 

 substance is either pyrocatechin or some compound containing the 

 pyrocatechin nucleus. Hence, if this supposition be true, it is obvious 

 that the classification into direct and indirect oxidase, on the basis 

 of the blueing of guaiacum, etc., is a purely artificial one. The important 

 element is the peroxidase which is practically universally distributed 

 in plants. There is no direct evidence that the peroxide-peroxidase 

 system exists in the living cell, though the presence of some such system 

 is extremely probable. The formation of brown or reddish-brown 

 pigments in extracts or tissues of those plants which contain direct 

 oxidase may then be regarded as the outcome of the oxidation of a 

 mixture consisting of a peroxidase and a number of aromatic substances, 

 of which one at least is capable of acting as a peroxide by autoxidation. 



It is upon reactions of the kind just mentioned that Palladin's 

 hypothesis of ' respiration pigments ' is based. Palladin makes extracts 

 from a number of plants throughout the vegetable kingdom, and after 

 boiling to destroy any enzyme in the extracts themselves, adds peroxidase 

 solution (obtained from Horse-radish root) and hydrogen peroxide. In 

 all cases, red, reddish-brown, brown or purple pigments are produced in 

 the extracts ; these pigments, moreover, are formed most readily and in 

 greatest quantity in extracts of plants we know to contain, previous 

 to heating, a direct oxidase, i.e. an organic peroxide. From his results 

 Palladin deduces the fact that chromogens of an aromatic nature are 

 universally distributed, and may be oxidised in the presence of oxidising 

 enzymes and a peroxide (though these, let it be noted, he always adds 

 to the extract). The whole series chromogen, enzyme, peroxide and 

 pigment form a system for transferring oxygen to respirable materials, 



1 Wheldale, M., 'On the Direct Guaiacum Reaction given by Plant Extracts,' Proc. 

 R. Soc., London, 1911, LXXXIV B, pp. 121-124. 



2 Wheldale, loc. cit. 





