Immunochemical properties of immunoglobulin G conjugated with lactate dehydrogenase.

Quantitative binding affinities of immunoglobulin G (IgG) for the five isoenzymes of lactate dehydrogenase (LD; EC 1.1.1.27) were determined for IgG isolated from three patients' sera that contained LD-IgG complexes. These three IgGs formed soluble complexes with four (LD 1, 2, 3, 5) of the five LD isoenzymes in two of the patients and in the third they bound with three (LD 2, 3, 4) of the five LD isoenzymes without inhibiting the enzyme activity or precipitating with the enzyme molecules. When rabbit antibody to human IgG was added to these sera, the complexes between the LD isoenzymes and the patients' isolated IgG were completely precipitated. The equilibrium constants for the respective isolated IgG complexes with LD-3 were 0.102, 2.58, and 7.49 X 10(8) L/mol. In these three cases, LD-3 evoked the strongest response from the prepared IgG, demonstrating that the site of antigen recognition of LD-linked IgG was not associated with the structure of individual H and M subunits.