Lactate dehydrogenase (LDH) is a reversible enzyme, catalysing the transition between pyruvate and lactate. The favoured direction of transfer can be altered by the composition of LDH. LDH is made up of 4 H and M subunits. H subunits are better at converting lactate to pyruvate, while M subunits favour converting pyruvate to lactate.
Endurance training increases the proportion of H subunits, reducing the loss of lactate from cells. This allows oxidative phosphorylation to release more energy, which is beneficial during long-duration physical activity.