Metabolism in root nodules and function of leghemoglobin

大豆根瘤由感染和未感染慢生根瘤菌的细胞组成。被感染的细胞很大，充满了类杆菌。据推测，根瘤中的固氮作用是通过感染细胞和未感染细胞之间的相互作用进行的。然而，对两种细胞之间的氮碳代谢和氮碳化合物的相互供应关系的研究很少。感染细胞、未感染细胞和皮层细胞可以被分离出来，用于几种酶活性的测定。在感染细胞中发现黄嘌呤脱氢酶活性。在未感染细胞中发现尿酸酶活性。未感染细胞的磷酸烯醇式丙酮酸羧化酶(CO2暗固定)活性较高。感染细胞和未感染细胞均有苹果酸脱氢酶活性，但感染细胞中苹果酸脱氢酶活性较高。酶分布表明，未感染细胞在CO_2暗固定过程中产生的苹果酸将被输送到未感染细胞，尿酸在未感染细胞中转化为尿囊素。钙和硼在最佳浓度时可促进固氮。体外实验表明，即使在低氧浓度下，豆红蛋白也能提高线粒体和类杆菌的呼吸，硝酸盐供应在24小时内抑制固氮。抑制作用的进行与硝酸盐产生亚硝酸盐的过程一致。在亚硝酸盐存在下，豆红蛋白转化为亚硝酰豆红蛋白。从硝酸盐产生的亚硝酸盐竞争性地干扰了豆类血红蛋白的氧结合。

Soybean nodules consist of cells infected and uninfected with Bradyrhizobium japonicum. The infected cells are large and filled with bacteroids. Nitrogen fixation in nodules supposedly proceeds with the interaction between infected and unifected cells. However, the relation of nitrogen-carbon metabolism and mutual supply of nitrogen- and carbon- compounds between both cells are little studied.Protoplasts were prepared from nodules in several stages of soybean development. Infected cells, uninfected cells and cortex cells could be separated for the measurements of several enzyme activities. Xanthine dehydrogenase activity was found in infected cells. Uricase activity was found in uninfected cells. Phosphoenolpyruvate carboxylase (CO_2 dark fixation) activity was high in uninfected cells. Malic dehydrogenase activity was found in both infected and uninfected cells, but higher in infected cells.The enzyme distribution indicates that the malate produced in CO_2-dark fixation of uninfected cells will be supplied to the infected cells and the uric acid produced in infected cells will be transported to the uninfected cells, where uric acid is converted to allantoin.Calcium and boron stimulated nitrogen fixation when they were in optimum concentrations. The in vitro experiments showed that leghemoglobin elevated the respiration of mitochondria and bacteroids even under low oxygen concentrations.Nitrate supply inhibited nitrogen fixation within 24 hours. The inhibition proceeded in accordance with the production of nitrite from nitrate. Leghemoglobin was converted to nitrosyl-leghemoglobin in the presence of nitrite. The O_2-binding of leghemoglobin was competitively inhigited by nitrite produced from nitrate.