Senescence in French-bean nodules: Occurrence of different proteolytic activities

A decline in nitrogenase activity (C2H2 reduction) of nodules of Phaseolus vulgaris L. cv. Contander was correlated with a decrease in their soluble protein including leghe-moglobin. Concomitantly, two distinct proteolytic activities against leghemoglobin with acidic and alkaline pH optima were detected. The corresponding proteases were purified about 30-fold by ammonium sulfate precipitation, gel filtration and hydroxy-apatite chromatography. Both the acidic (pH optimum 3.5) and the alkaline (pH optimum 8.0) proteases were thiol enzymes. They were characteristic of senescing nodules, whereas only an acidic serine protease was present in functional nodules.