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Extracellular proteases from three psychrotrophic strains of Pseudomonas fluorescens were purified and characterized. The cultivation temperature decisively influenced the excretion of protease into the medium. Highest enzyme production occurred at 10°C (one strain) or 20°C (two strains). The purified proteases showed a molecular weight between 47kDa and 51kDa, the pI values ranged from 7.8 to 7.4. All three proteases were characterized by several closely focused bands. Sensitivity to metalchelating agents indicates that P. fluorescens proteases are metalloproteases (EC 3.4.24). The pH optimum for azocaseinolytic activity was at pH 6-7, the temperature optimum was at 40-45°C. The apparent energies of activation were 36.9-38.0kJ mol-1. The proteases were thermolabile. By adding calcium ion an increasing thermostability was observed. SDS, urea and several metal ions inhibited proteolytic activities to different extents. Casein was the preferred substrate for the proteases. With casein concentrations up to 0.4-0.75% a Michaelis-Menten-type kinetics was observed, at higher casein concentrations a substrate inhibiton occurred. None of the proteases showed aminopeptidase activity.
Margesin et al. (Wed,) studied this question.