Lactate dehydrogenase

Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells (animals, plants, and prokaryotes). It catalyzes the conversion of lactate to pyruvic acid and back
This is an important function because lactic acid builds up in muscle tissue. LDH works to prevent muscle fatigue and failure in various ways. 
Each LDH molecule is made of four subunits. There are somewhat different types of LDH in different tissues. The main type in the lungs has two M subunits and two H subunits (2H2M). The M and H subunits are coded by two different genes. The M subunit is coded by LDHA on human chromosome 11, and the H subunit is coded by LDHB on chromosome 12.