Macro lactate dehydrogenase: an LDH-immunoglobulin M complex that inhibits lactate dehydrogenase activity in a patient's serum.

A macro lactate dehydrogenase (LDH, EC 1.1.1.27) isoenzyme with a low total LDH activity was present in the serum of a 57-year-old woman with a drug eruption (cutaneous lesions from an allergic reaction to drug administration). The patient's LDH was shown by immunoelectrophoresis to be bound to immunoglobulin G (IgG) and M (IgM). It was found that the patient's IgM acted as an inhibitor of LDH that was specific for the M subunit. When IgM was treated with 0.1 mol/l of 2-mercaptoethanol (2-ME), the inhibiting effect of the IgM to LDH activity disappeared, while treated IgM continued to bind to the LDH molecule. The LDH activity increased approximately two-fold when the patient's serum was treated with 2-ME. LDH activity in normal human serum was inhibited and an abnormal pattern of LDH isoenzyme appeared when the patient's IgM was added to normal serum. The present case seems to be the first report of LDH-IgM complex with a marked decrease of LDH activity.