[Studies on a serum factor inhibiting lactate dehydrogenase (LDH) activity in a patient's serum]

We found extremely low activity of serum lactate dehydrogenase (LDH, EC. 1.1.1.27) in a 70-year-old female patient. The decrease of LDH activity was observed when the normal serum was incubated with the patient's serum. Inhibition rate of LDH activity by the patient's serum was higher at 4 degrees C than at 37 degrees C. The patient's serum inhibited both subunits of LDH, and inhibited more strongly the M-subunit than the H-subunit LDH isoenzymes. IgG (lambda type) in the patient's serum was found to be responsible for the inhibition of LDH activity. The mechanism why IgG inhibiting LDH activity developed in the patient's serum remain undetermined.