Complexes of immunoglobulins A and G with aspartate aminotransferase isoenzymes in serum.

We report the presence of complexes between aspartate aminotransferase (AST, EC 2.6.1.1) and immunoglobulin (Ig) in the serum of a patient suffering from lung cancer with metastasis to the liver. After fractionation of the serum by gel filtration, AST-Ig complexes (AST-IgA, AST-IgG) were demonstrated by counterimmunoelectrophoresis. Dissociating the complexes and recombining them with purified isoenzyme fractions, s-AST (cytoplasmic) and m-AST (mitochondrial), revealed that only s-AST binds to IgG, whereas IgA binds to both s-AST and m-AST. Although the association of AST with IgG has been reported, to our knowledge this is the first finding of both AST-IgA and AST-IgG complexes in a patient's serum. Serum AST-IgG complexes have been demonstrated in both healthy and diseased individuals; in the latter category, as reported here and by others, the liver is implicated.