Electron Transport Mechanisms in Tobacco Roots. I. Studies on the Cytochrome and Related Systems.

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Abstract
A study was conducted to determine the properties and the relative significance of the cytochrome system in the roots of tobacco plants. By use of a direct spectroscopic method with particulates and spectrophotometric studies with digitonin extracts of root particulates, absorption bands were demonstrated that are characteristic of cytochrome oxidase and cytochromes b or c or perhaps both. An evaluation of the cytochrome c oxidative capacity of extracts provided evidence that roots contain sufficient activity of this enzyme to account for the respiratory oxygen uptake by intact roots. The Michaelis constant for the cytochrome oxidase-oxygen-complex using digitonin extracts of roots was estimated to be 4.5 x 10-6 [image]. The comparable Km for particulates without digitonin was estimated to be 5 x 10-6 [image]. These values are approximately one-third that estimated for the phenol-oxidase oxygen complex in tobacco root extracts. Succinic-cytochrome c reductase activity, inhibited by Antimycin A and SN 5949 (2-hydroxy-3-(2-methyloctyl)-l, 4-naphtho-quinone), was demonstrated in particulates from tobacco roots. Also the activity of a reduced pyridine nucleotide cytochrome c reductase was demonstrated in particles and it was shown that the cytochrome c reductase capacity of this enzyme was sufficient to account for the respiratory oxygen uptake of roots. Tobacco root extracts failed to oxidize DPNH at an appreciable rate unless some electron acceptor such as cytochrome c, menadione, or chlorogenic acid was added to reaction mixtures.