224 
CHARLES O. APPLEMAN 
incapable of being directly oxidized in vitro by the oxidases; therefore, 
the situation remains nearly as perplexing as ever. 
The very ingenious hypothesis elaborated by Palladin^^ and his 
co-workers to explain the mechanism of respiration comes the nearest 
of anything yet offered to overcome the chief difficulties encountered 
in ascribing to the oxidases a function in respiration. It must be 
remembered, however, that this explanation is still in the hypo- 
thetical stage. 
There are certain substances in probably all plant and animal cells 
which have the power to decompose hydrogen peroxide with the evo- 
lution of molecular oxygen. Loew^thought that this action on hydrogen 
peroxide is due to a special enzyme to which he gave the name catalase. 
When one considers the abundance and wide distribution of catalase 
in plant and animal tissues, it is natural to suppose that it plays some 
important role in metabolism. But so far its function has not been 
definitely established. Much theory, based upon rather scanty 
experimental data, has attempted directly or indirectly to connect 
catalase activity with the oxidative forces of the cell. The work of 
Lesser^o seems to furnish the most conclusive evidence in this direction. 
He made a large number of catalase determinations in different small 
animals and in different organs and tissues of the same animal and 
concluded that catalase is connected with physiological oxidations. 
Although a strict interpretation of his results does not show a casual 
relation between catalase and cell oxidation, it does show a remark- 
able correlation. Zieger^^ also made a study of the catalase content 
of nearly all groups of animals except the protozoa. He did not 
succeed in establishing a relationship between intensity of respiration 
and catalase activity, but he did show that there is some relation 
between catalase content and metabolism. He brought out this 
fact by the study of the catalase content in organs which are very 
active chemically, as the liver and kidney. The evidence from the 
plant side, for such a relationship, is indirect and inconclusive. KohP 
indirectly connects catalase activity with respiration by his claim 
that it functions in alcoholic fermentation. 
In conformity with the literature both the oxidases and catalase 
have been spoken of as enzymes although in the light of our present 
knowledge their place in the category of enzymes is extremely doubt- 
ful. It is questionable whether they are even definite chemical 
substances. It may be more correct to speak of oxidase and catalase 
