Effects of pH and temperature on the wild-type and a mutant form of Neurospora glutamate dehydrogenase

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Abstract
We confirm the observation of Btirk (1965) that Neurospora crassa NADP-linked glutamate dehydrogenase normally exists in an inactive form below pH 7.0 and in a fully active form above pH 8.0 in either tris or orthophosphate buffer. At pH 7.4 the enzyme is about half activated at 25[degree]. The variety of the enzyme produced by the mutant am21 shows a similar behavior except that the transition is shifted about one pH unit in the alkaline direction. The am21 enzyme has previously been reported to be activated by brief warming to 30[degree] in phosphate buffer at pH 8.0. The wild-type enzyme shows a similar effect at pH 7.0. In tris buffer this effect is much less pronounced. The am21 enzyme is extremely unstable at 47[degree] at pH 7.0; its stability is somewhat greater at lower pH, and is markedly increased by increasing the pH in the range 7.0-8.7. The wild - type enzyme also shows an indication of a stability minimum at pH 7.0, but a temperature of 60[degree] is needed for a measurable rate of inactivation. The inactive form of the enzyme is much more subject to thermal irreversible denaturation that is the active form.