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。受感染的细胞很大，充满了拟杆菌。根瘤中的固氮作用被认为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，对大豆根瘤和根瘤细胞之间氮碳代谢和氮碳化合物相互供应的关系研究甚少。感染细胞、未感染细胞和皮层细胞可分离用于几种酶活性的测量。黄嘌呤脱氢酶的活性被发现在感染的细胞。在未感染的细胞中发现尿酸酶活性。在未感染的细胞中，磷酸烯醇式丙酮酸羧化酶（CO_2暗固定）活性较高。苹果酸脱氢酶活性在感染和未感染细胞中均存在，但在感染细胞中较高，酶的分布表明未感染细胞CO_2暗固定产生的苹果酸将被供应给感染细胞，而感染细胞产生的尿酸将被运输到未感染细胞，在此尿酸转化为尿囊素。钙和硼在最适浓度时促进固氮。体外实验表明，豆血红蛋白即使在低氧浓度下也能促进线粒体和类菌体的呼吸作用，硝酸盐供应在24小时内抑制固氮作用。这种抑制作用与硝酸盐生成亚硝酸盐的过程一致。在亚硝酸盐存在下，豆血红蛋白转化为亚硝基豆血红蛋白。由硝酸盐生成的亚硝酸盐竞争性地抑制豆血红蛋白的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.