Enzymes of the tricarboxylic acid cycle and the malate‐aspartate shuttle in the N2‐fixing endophyte of Alnus glutinosa

The occurrence and localization of enzymes involved in energy supply and biosynthesis was studied in root nodules of Alnus glutinosa (L.) Vill. Vesicle clusters of the endophyte, Frankia sp., contain NADP‐dependent isocitrate dehydrogenase, succinate dehydrogenase, fumarase and malate dehydrogenase. The data indicate that both the endophyte and the host are capable of metabolizing carbon compounds via the tricarboxylic acid cycle. Both vesicle clusters of the endophyte and root nodule cells contain glutamate‐oxaloacetate transaminase which can function in a malate‐aspartate shuttle. This might enable transport of reducing equivalents from the host cell cytoplasm to the endophyte.