SUMMARYThe effects of certain variables on the thermal stability of spinach catalase were investigated with model systems. The rate of thermal inactivation of spinach catalase was accelerated as heating temperature was increased. Inactivation kinetics were not first‐order with solutions of the purified enzyme between 50 and 60°C and with spinach extracts at 55°C, hut became first‐order with the latter preparation as the temperature was increased to 65°C. The presence of a heat‐labile catalase inhibitor is postulated. Catalase was more thermostable in spinch extracts than in solutions of the purified enzyme. Purified spinach catalase at 55°C was more thermostable in solutions at pH 5.5 and 7.0 than at pH 9.0. The thermostahility of purified spinach catalase was not influenced by the enzyme concentration or by the presence of 1.5 NaCl, 3.6 sucrose, or 3.6 starch in pH 7.0 solutio
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