Enzymes of Poly-(beta)-Hydroxybutyrate Metabolism in Soybean and Chickpea Bacteroids

The enzymatic capacity for metabolism of poly-(beta)-hydroxybutyrate (PHB) has been examined in nitrogen-fixing symbioses of soybean (Glycine max L.) plants, which may accumulate substantial amounts of PHB, and chickpea (Cicer arietinum L.) plants, which contain little or no PHB. In the free-living state, both Bradyrhizobium japonicum CB 1809 and Rhizobium sp. (Cicer) CC 1192, which form nodules on soybean and chickpea plants, respectively, produced substantial amounts of PHB. To obtain information on why chickpea bacteroids do not accumulate PHB, the specific activities of enzymes of PHB metabolism (3-ketothiolase, acetoacetyl-coenzyme A reductase, PHB depolymerase, and 3-hydroxybutyrate dehydrogenase), the tricarboxylic acid cycle (malate dehydrogenase, citrate synthase, and isocitrate dehydrogenase), and related reactions (malic enzyme, pyruvate dehydrogenase, and glutamate:2-oxoglutarate transaminase) were compared in extracts from chickpea and soybean bacteroids and the respective free-living bacteria. Significant differences were noted between soybean and chickpea bacteroids and between the bacteroid and free-living forms of Rhizobium sp. (Cicer) CC 1192, with respect to the capacity for some of these reactions. It is suggested that a greater potential for oxidizing malate to oxaloacetate in chickpea bacteroids may be a factor that favors the utilization of acetyl-coenzyme A in the tricarboxylic acid cycle over PHB synthesis.