Ionic properties of an essential histidine residue in pig heart lactate dehydrogenase.

1. Pig heart lactate dehydrogenase is inhibited by addition of one equivalent of diethyl pyrocarbonate. The inhibition is due to the acylation of a unique histidine residue which is 10-fold more reactive than free histidine. No other amino acid side chains are modified. 2. The carbethoxyhistidine residue slowly decomposes and the enzyme activity reappears. 3. The essential histidine residue is only slightly protected by the presence of NADH but is completely protected when substrate and substrate analogues bind to the enzyme-NADH complex. The protection is interpreted in terms of a model in which substrates can only bind to the enzyme in which the histidine residue is protonated and is thus not available for reaction with the acylating agent. 4. The apparent pK(a) of the histidine residue in the apoenzyme is 6.8+/-0.2. In the enzyme-NADH complex it is 6.7+/-0.2. 5. Acylated enzyme binds NADH with unchanged affinity. The enzyme is inhibited because substrates and substrate analogues cannot bind at the acylated histidine residue in the enzyme-NADH complex.