Metabolism in root nodules and function of leghemoglobin

大豆根瘤由侵染和未侵染日本慢生根瘤菌的细胞组成。受感染的细胞很大，充满了类细菌。据推测，结核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，两种细胞之间的氮碳代谢和氮碳化合物相互供应的关系研究甚少。在大豆发育的不同阶段，根瘤形成原生质体。可以将感染细胞、未感染细胞和皮层细胞分离，测量几种酶的活性。在感染细胞中发现黄嘌呤脱氢酶活性。在未感染的细胞中发现了尿酶活性。在未感染的细胞中，磷酸烯醇丙酮酸羧化酶（CO_2暗固定）活性较高。苹果脱氢酶活性在感染细胞和未感染细胞中均有发现，但在感染细胞中较高。酶的分布表明，未感染细胞在CO_2-dark固定过程中产生的苹果酸盐将被供应给感染细胞，而感染细胞产生的尿酸将被运送到未感染细胞，尿酸在那里转化为尿囊素。钙和硼在最佳浓度时促进固氮。体外实验表明，即使在低氧浓度下，豆血红蛋白也能提高线粒体和类细菌的呼吸作用。硝态氮在24小时内抑制固氮。抑制作用是根据硝酸盐生产亚硝酸盐进行的。在亚硝酸盐的存在下，血红蛋白转化为亚硝基血红蛋白。由硝酸盐产生的亚硝酸盐竞争性地增强了血红蛋白的o_2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.