Demonstration of Cytochrome Oxidase in Onion Root Tips.

The effect of carbon monoxide on the oxygen consumption of apical segments of the onion root has been determined employing conventional Warburg manometry. A CO/O₂ ratio of 19 shows inhibition averaging about 37% as compared to values obtained in a control atmosphere of nitrogen and oxygen (N/O₂=19). This inhibition is photo-reversible. A CO/O₂ ratio of 1 does not show inhibition of respiration of freshly excised root tips, nor does the addition of glucose to freshly excised root tips raise the rate of respiration. If root tips are excised and then starved for 15 hours, an atmosphere of one half carbon monoxide and one half oxygen strongly inhibits the oxygen consumption both in the presence and absence of added substrate. The addition of cytochrome c. along with an agent which can directly reduce it (ascorbate), to homogenates of onion root tips resulted in marked stimulation of oxygen consumption, which was demonstrated to he photo-reversibly inhibited by carbon monoxide. These results are discussed. and it is concluded that cytochrome oxidase is present in the onion root tip, in amounts probably capable of mediating most of the terminal hydrogen transfer. No demonstration was made as to whether cytochrome oxidase functions as the terminal oxidase in the normal respiration of this tissue. [ABSTRACT FROM AUTHOR]