Comparative temperature-stability properties of malate dehydrogenases from some thermophilic fungi

Temperature-activity and temperature-stability relationships of malate dehydrogenases from eight thermophilic fungi were determined. Temperature optima for maximum activity of the enzymes varied between 50 degrees and 60 degrees and the Arrhenius plots were linear between 5 degrees and 50 degrees. 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 degrees revealed heat lability of the enzyme from most of the thermophilic fungi. The t1/2's (min) were less than 10 for the enzymes from P. duponti, S. thermophile and Thermoascus aurantiacus; less than 30 for those from Chaetomium thermophile var. coprophile, H. lanuginosa and C. thermophile var. dissitum; and greater than 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.