RESPIRATION AND FERMENTATION IN THE CARROT, DAUCUS CAROTA I. RESPIRATION

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ALTHOUGH THERE has been a great deal of research in recent years dealing with the nature of respiration and fermentation in yeast and in animal tissues, higher plants have not been intensively investigated in this regard. It has been the purpose of this study to investigate the nature of respiration and fermentation in the carrot, Daucus carota, and to extend our knowledge concerning the relationship between these two processes. Part I is a report of the experiments on respiration, while Part II, to be published shortly, presents the experiments on fermentation and its relationship to respiration. Three distinct oxidases involved in plant respiration are now known-cytochrome oxidase, catechol oxidase, and a group of oxidases known as the flavine or yellow enzymes. Cytochrome oxidase (Keilin, 1929) also known as indophenol oxidase or phaeohemin oxidase (Warburg, 1927), is found in yeast and is widely distributed in the animal kingdom. It is inactivated by cyanide, azide, and carbon monoxide, the carbon monoxide inhibition being light reversible. Catechol oxidase (Batellei and Stern, 1912; Keilin and Mann, 1938; Kubowitz, 1937), occuring primarily in plants, is inactivated by the same three poisons, but the carbon monoxide inactivation is not light reversible. The flavine enzymes (Theorell, 1937), the first of which was isolated from yeast only seven years ago (Warburg and Christian, 1932), are cyanide-insensitive oxidases, whose prosthetic group is a phosphate ester of vitamin B2 (riboflavine). In this paper we present evidence for the operation of two different oxidases in the carrot plant. One of these is quite similar to or identical with the cytochrome oxidase and accounts for the major part of the respiration of the carrot root and young leaves. The other, insensitive to poisoning by cyanide, azide, and carbon monoxide, accounts for a fraction of the respiration of roots and young leaves and the entire respiration of mature leaves. AIATERIALS AND METHODS.-Warburg (1928) clearly stated the simple assumption involved in the use of reversible enzyme poisons such as cyanide and carbon monoxide in the study of cell metabolism. A poison of known chemical behavior is assumed to react inside the intact cell with enzymes which contain chemical groups of the same type with which this poison reacts in vitro. Removing the poison or otherwise breaking up the enzyme-poison compound allows the metabolism to return to its original unpoisoned state. By this method we may study respiration and fermentation in the cell under relatively normal conditions. Carrots of the horticultural variety Nantes Coreless, grown in the university greenhouse, were used for these experiments except where otherwise designated. Some preliminary experiments were made