534 ANAEROBIC RESPIRATION 



Regardless of the precise mechanism by which oxidases operate it is evident 

 that these enzyme systems can accomplish oxidations at the expense of atmos- 

 pheric oxygen. While peroxidases appear to be of universal occurrence in 

 plants, oxygenases are of more limited distribution. Obviously only those 

 plant tissues which contain both will exhibit oxidase activity. 



A plant tissue can be tested for the presence of an oxidase system by 

 dropping a small portion of the macerated tissue into an alcoholic solution 

 of gum guaiacum. Development of a blue color, due to the oxidation of 

 guaiacol, indicates the presence of oxidase. The test is the sanie as that 

 employed for peroxidases except that hydrogen peroxide is not added. 



Two types of oxidases are generally recognized. The phejiolases seem 

 to be the more widely distributed of the two. Enzymes of this type are 

 found in many of the higher plants, in certain fungi, and in some of the 

 invertebrate animals. Each of the phenolases can catalyze the oxidation 

 of one or more phenol compounds. Among these are such substances as the 

 cresols, toluidins, pyrogallol, phloroglucinol, resorcinol, phenolphthalein, hy- 

 droquinone, and guaiacol. The first enzyme of this type to be discovered 

 was found in the juice of the lac tree {Rhus vernicifera). This enzyme was 

 called laccase, and the phenolases as a group were formerly called laccases. 



When the fruiting bodies of some species of fungi are exposed to the 

 air they produce a blue pigment, while those of other species turn at first 

 pink or red and later black. The bluing of fungous tissues is due to the 

 effect of phenolases. The pink and red colorations result, however, from the 

 operation of another oxidase known as tyrosinase. This enzyme acts on 

 tyrosine, an amino acid containing a phenol group, and other chemically related 

 substances, oxidizing them through several intermediate stages to the black pig- 

 ment melanin. Intermediate steps in this oxidation process exhibit a series 

 of colors ranging from pink to red to violet, and finally to black. Tyrosine 

 is not oxidized by the phenolases. This enzyme occurs not only in the fungi 

 but in the tissues of many of the higher plants, and in some animals as well. 



4. Dehydrogenases. — Many biological oxidations and reductions involve 

 intermolecular transfers of hydrogen rather than of oxygen. Enzymes which 

 catalyze such transfers of hydrogen are called dehydrogenases, oxido-reductases, 

 or reductases. Apparently there are a number of different enzymes of this 

 general type and considered as a group they are widely and probably univer- 

 sally distributed in living organisms. 



The action of a typical dehydrogenase can be observed by dropping a 

 block of fresh potato tuber tissue into a dilute solution (about O.025 per 

 cent) of methylene blue under such conditions that oxygen does not have free 

 access to the tissue or solution. The methylene blue gradually becomes 



