SUMIDA, K. D., F. FRISCH, and C. M. DONOVAN. Training suppresses hepatic lactate dehydrogenase activity without altering the isoenzyme profile.Med. Sci. Sports Exerc., Vol. 27, No. 4, pp. 507–511, 1995. A decrease in hepatic lactate dehydrogenase (LDH) activity following endurance training has been a consistent observation. In the present study, we sought to determine whether the training-induced decrease in hepatic LDH activity (pyruvate = substrate) was associated with a shift in the isoenzyme profile and/or alteration in other kinetic parameters. Animals (rats) were randomly assigned to either an endurance trained group (running 90 min at 30 m·min-1, 10% grade) or sedentary control group. Eight weeks of endurance training resulted in a significant decrease in maximal hepatic LDH activity for the forward reaction (pyruvate ± lactate), 107.3 ± 5.5 μmol·min-1·g-1, when compared with control animals, 147.3 ± 5.6 μmol·min-1·g-1‘. A similar decrease was observed for maximal LDH activity in the reverse reaction (lactate ± pyruvate), 49.8 ± 2.1 vs 66.7 ± 2.9 μmol·min-1·g-1, trained and controls, respectively. Training was also observed to decrease the Km for the reverse reaction, 5.18 ± 0.78 mM vs 6.94 ± 0.55 mM, for trained and controls, respectively. Km for the forward reaction was unaffected by training. Gel electrophoresis with densitometric evaluation revealed no shift in the isoenzyme pattern following endurance training. LDH5accounted for 89% ± 2%, whereas 6% ± 0.5% was observed in LDH4, and 4% ± 0.3% was observed in LDH3for both groups. The densitometric area was ± 34% lower from trained liver homogenates, a fractional decrease similar to that observed for maximal LDH activity. The decrease in hepatic LDH activity with endurance training appears attributable to a down regulation of enzyme content, with no significant alteration in isoenzyme distribution.