COMPARATIVE TEMPERATURE-STABILITY PROPERTIES OF MALATE DEHYDROGENASES FROM SOME THERMOPHILIC FUNGI*

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Abstract
Temperature-activity and temperature-stability relationships of malate dehydrogenases from 8 thermophilic fungi were determined. Temperature optima for maximum activity of the enzymes varied between 50-60.degree. C and the Arrhenius plots were linear between 5-50.degree. C. The energies of activation ranged from 2.1 kcal/mol for the enzyme from Sporotrichum thermophile to 9.1 kcal/mol for that from Penicillium duponti. Heat inactivation kinetics at 50.degree. C revealed heat lability of the enzyme from most of the thermophilic fungi. The t1/2 [half-life] (min) were < 10 for the enzymes from P. duponti, S. thermophile and Thermoascus aurantiacus; < 30 for those from Chaetomium thermophile var. coprophile, Humicola lanuginosa and C. thermophile var. dissitum; and > 30 for those from Mucor pusillus and H. insolens. Salts of Na+, K+ and NH4+, and citrate protected the enzymes from H. lanuginosa, C. thermophile var. dissitum and M. pusillus against heat inactivation.