1. The reduction of TTC in the presence of embryo homogenates of
Phaseolus vulgaris was investigated by the colorimetrical determination of the amounts of formazan formed by the reaction under varied experimental conditions, and the results were compared with those obtained with the tissue slices (cf. previous papers).
2. The rate of TTC reduction was shown to be dependent on the presence of succinic acid in the reaction medium, and it was concluded that this reaction was due to the activity of succinic dehydrogenase in the homogenate.
3. The rate of TTC reduction was fairly constant for considerable reaction period, and the activity of the succinic dehydrogenase system was conveniently measured by determining the initial rate of formazan production.
4. In harmony with the above conjecture, the poisons known to inhibit the action of succinic dehydrogenase (malonate, oxaloacetate, pyrophosphate, ferricyanide, arsenite, monoiodoacetate) all inhibited the TTC reduction in the presence of sodium succinate.
5. The inhibitory action of potassium cyanide on the TTC reduction was demonstrated with homogenates as well as with slices of bean embryo. Sodium azide had no apparent effect on the reduction.
6. The seemingly contradictory results of other investigators concerning the reduction of TTC and especially the problem of participation of the succinic dehydrogenase on the process were discussed.
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