Marked suppression of LDH activity by the sugar chain of LDH-binding immunoglobulin G in the serum of a patient with Myocardial infarction

We encountered a very rare case of cardiac failure in which serum-negligible LDH activity was detected. The patient was a 58-year-old male with a history of myocardial infarction. In this case, LDH was bound to the lambda type of immunoglobulin G; and the activity of both subunit's of LDH, i. e., LDH-H (B) and LDH-M (A), were suppressed. As we reported formerly, this suppression of LDH activity is considered to be attributable to the binding of immunoglobulin G. In our previous study, we analyzed the structure of the suger chain of the binding immunoglobulin G (IgG) after treatment with various enzymes, and found that the binding capacity of the IgG in the patient's serum to LDH isozymes was changed by treatment with neuraminidase. In the present study, we removed the sialic acid residue from the IgG in the patient's serum by means of neuraminidase digestion, and thereafter attempted to ligate the resultant asialo-immunoglobulin G to LDH isozymes from normal volunteers in order to clarify the mechanism of the suppression of LDH isozymes. We found that the antigenicity of the IgG in the patient's serum was reduced by removing its sialic acid residue. These findings demonstrate that the sialic acid residue of the sugar chain of the IgG in the patient's serum was directly associated with the suppression of LDH activity by the enzyme-binding immunoglobulin observed in this case of cardiac failure with a history of myocardial infarction.