Effect of Histidine on Thermostability of Lipase and Protease of Pseudomonas fluorescens 27

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Abstract
P. fluorescens 27 [a milk contaminant] was grown on a dialysis membrane resting on the surface of semisolid media. Thermostability of the lipase and protease between 35 and 90.degree. 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.degree. C. Both could be inactivated at lower temperatures. Protease was inactivated maximally between 45 and 55.degree. C, probably by autolysis. Lipase was inactivated maximally at 65.degree. C in peptone-yeast extract broth not in other media. The apparent cause for inactivation of lipase in peptone-yeast extract broth at 65.degree. C was the presence of histidine or histamine and MgCl2 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 stabilized to heating at 65.degree. C by dialysis of the broth, and addition of 5 mM histidine and 0.5 mM MgCl2 to the broth restored the enzyme''s sensitivity to this heat treatment.