Lactate acid dehydrogenase was irradiated in the presence of its substrates and the coenzymes. The coenzymes afforded protection against the inactivation by irradiation over the entire pH range studied. Both pyruvate and lactate afforded specific protection to the enzyme at the pH optima for their binding. Studies of the four isozymes present in the LDH preparation indicated that the irradiation caused no major structural changes. However, there was an indication of slight configurational changes. These data support the concept of specific substrate protection. Furthermore, the data support the hypothesis that the substrate can bind to LDH independently of the coenzyme and that this binding is strongly pH-dependent.

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