Training-induced alterations in lactate dehydrogenase reaction kinetics inrats: A re-examination

The kinetics and the isozyme composition of lactate dehydrogenase (LDH) wer e measured in rat plantaris muscle during a 26 week endurance-training prog ram. Alterations in the LDH isozyme pattern were detectable after 6 weeks a s the percentage of the M-4 isozyme was reduced from 89 to 76 % and the tot al percentage of M subunits compared with H subunits declined by 8 %. At 16 weeks, M-4 accounted for only 62 % of the total. The replacement of M with H subunits continued when training was prolonged as M-4 represented only 5 2 % of the total isozymes at 26 weeks. Conversely, training for 6 and 16 we eks produced no changes in either V-max or K-m. At 26 weeks, these values d eclined for both the forward (pyruvate to lactate) and backward reactions. The rate constants for both reactions were also reduced. These data suggest that changes in LDH isozyme pattern do nor contribute significantly to the enhancement of lactate oxidation that may occur after training. They also suggest that the Functional significance of alterations in LDH structure an d/or Function are best determined from analysis of the overall reaction kin etics as opposed to individual characteristics such as the isozyme pattern, K-m or V-max.