Typically, LDH means Lactate Dehydrogenase. It is defined as a tetramer having four subunits. Maybe, the subunits either H or M polypeptide chains.These four subunits together further form five isoenzymes when they combine in different proportions. They are H4 (which is written as HHHH), H3M1 (as HHHM), H2M2 (as HHMM), H1M3 (as HMMM), and M4 (as MMMM).