An enzyme found in all tissues, especially liver and muscle. Five different isoenzymes are known, from LDH1 (cardiac muscle and brain tissue) to LDH5 (liver, muscle tissue and saliva). Frequently measured following myocardial infarction, serum LDH begins to increase 12 to 24 hours after the infarction and reaches a peak on the second or third day, returning to normal within 10 days. Also useful in the diagnosis of pulmonary embolism in which LDH levels uniquely increase without any increase in other MI markers.