1. The very fast pre-steady-state formation of NADH catalysed by pig M4 lactate dehydrogenase was equivalent to the enzyme-site concentration at pH values greater than 8.0 and to one-half the site concentration at pH6.8. 2. The rate of dissociation of NADH from the enzyme at pH8.0 (450s−1) in the absence of other substrates is faster than the steady-state oxidation of lactate (80s−1). The latter process is therefore controlled by a step before NADH dissociation but subsequent to the hydride transfer. 3. The oxidation of enzyme–NADH by excess of pyruvate was studied as a first-order process at pH9.0. There was no effect of NADD on this reaction and it was concluded that the ternary complex undergoes a rate-limiting change before the hydride-transfer step. 4. Some conclusions about the reactions catalysed by the M4 isoenzyme were drawn from a comparison of these results with those obtained with the H4 isoenzyme and liver alcohol dehydrogenase.

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