STUDIES ON CRYSTALLINE LACTATE DEHYDROGENASE FROM CARDIAC AND SKELETAL MUSCLE OF PLAICE (PLEURONECTES PLATESSA) WITH PARTICULAR REFERENCE TO TEMPERATURE

1. 1. The isolation of the single lactate dehydrogenase (l-lactate: NAD+ oxido-reductase, EC 1.1.1.27) from both the cardiac and skeletal muscle of plaice is described. 2. 2. These enzymes appear to be homogeneous and identical to one another, as judged by ultracentrifugal analysis and starch-gel electrophoresis. The two enzymes are indistinguishable on the basis of their reactivity with coenzyme analogues, and they are similar in amino acid composition. 3. 3. There is marked substrate (pyruvate) inhibition of the plaice cardiac/skeletal muscle lactate dehydrogenase measured at 10°, and this is similar in extent to the pyruvate inhibition of beef H4 lactate dehydrogenase (LDH-1) measured at 35°. Beef H4 lactate dehydrogenase acting at 10° exhibits a greater degree of pyruvate inhibition than does plaice muscle lactate dehydrogenase at this temperature. Beef M4 lactate dehydrogenase (LDH-5) shows little pyruvate inhibition at 35°. 4. 4. The marked product (l-lactate) inhibition of plaice muscle lactate dehydrogenase at 10° is compared with that of beef H4 lactate dehydrogenase. 5. 5. The findings on substrate and product inhibition of plaice muscle lactate dehydrogenase at low temperature are discussed in the context of lactate dehydrogenase isoenzyme function. © 1969.