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Effect of Histidine on Thermostability of Lipase and Protease of Pseudomonas fluorescens 27¹

Department of Food Science and Nutrition, University of Missouri, Columbia 65211

ABSTRACT

Pseudomonas fluorescens 27 was grown on a dialysis membrane resting on the surface of semisolid media. Thermostability of the lipase and protease between 35 and 90°C was measured in phosphate buffer, a milk salts buffer, peptone-yeast extract broth, and rehydrated nonfat dry milk. Both enzymes were stable to heating for 20 min at 90°C. However, both could be inactivated at lower temperatures. Protease was inactivated maximally between 45 and 55°C, probably by autolysis. Lipase was inactivated maximally at 65°C in peptone-yeast extract broth but not in other media. The apparent cause for inactivation of lipase in peptone-yeast extract broth at 65°C was the presence of histidine or histamine and magnesium chloride in the medium. The mechanism was pH-dependent, active at pH 6.0 but not at pH 7.3, and results suggested that the imidazole group of histidine was the component that led to enzyme inactivation from heating at 65°C. The enzyme was stabilized to heating at 65°C by dialysis of the broth, and addition of 5 mM histidine and .5 mM magnesium chloride to the broth restored the enzyme's sensitivity to this heat treatment.

¹ Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 9464.

² Animal and Dairy Science Department, University of Georgia, Athens 30602.