[Distribution of endogenous inhibitors of the respiratory chain in plants].
70 phosphate buffer extracts of various plant tissues of 40 species as well as of 2 bacteria were tested for the presence of endogenous inhibitors of the respiratory chain. Electron transfer particles (ETP) from beef heart mitochondria served as test object. The NADH oxidase (spectrophotometrically) and the succinate oxidase activity (manometrically) were measured. Inhibitory activities could be detected in all the plant species tested, but there were quantitative differences by orders of magnitude. The inhibitory effects were more frequent and higher in the NADH oxidase system than those in the succinate oxidase system. The highest inhibitory activities were observed with blossoms of Forsynthia intermedia, male blossoms of Corylus avellana, inflorescences of Brassica oleracea, fronds of Pteridium aquilinum and gallnuts of Quercus. The specific inhibitory activities (related to the dry mass of the extracts) suggest very efficient inhibitors having concentrations of half-inhibition in the muM-range. With 6 extracts the inhibitory activity on the NADH oxidase system was completely destroyed by boiling (Brassica oleracea, Amoracia rusticana, leaves of Digitalis purpurea, roots of Allium cepa, fruit pips of Malus domestica and mushrooms of Lactarius vellereus). The results with some plant species (Bryophyllum daigremonteanum, Allium cepa, male blossoms of Corylus avellana, Pteridium aquilinum) suggest a biological dynamics of the inhibitory activity. The inhibitor from Bryophyllum was partially characterized with regard to its mode of action. The following supposed biological functions of endogenous respiratory inhibitors of plants are discussed: 1. Involvement in the degradation of mitochondria in the course of differentiation, maturation and involution processes as well as in biologically controlled senescence processes; 2. A switch-over to the alternative mitochondrial respiratory pathway; 3. Induction and maintenance of a resting metabolism, e.g. in dormancy, by action as growth inhibitors; 4. Action as phytoncides (phytoallexines) for the defense against parasites.