Mitochondrial aspartate aminotransferase linked to immunoglobulin G of the kappa-lambda type: report of a case

Macromolecular aspartate aminotransferase (L-aspartate: 2-oxoglutarate aminotransferase EC 2.6.1.1, AST) was found in the serum of a patient with benign hypertension. The serum total AST and mitochondrial AST (mAST) activities were proportionately higher. The abnormal AST was found to be a macromolecular complex composed of mAST and immunoglobulin G of the kappa-lambda type. The dissociated IgG from the complex was shown to combine with human and rat mAST, but not with cytosolic AST of both species. Molecular mass of the macromolecular AST was estimated to be 360,000 Da. These results indicate that the complex may consist of one IgG molecule associated with two mAST molecules. By the method of papain digestion the binding site of immunoglobulin in the complex appeared to be located in the Fab portion of the IgG molecule. This finding strongly suggests that the AST-immunoglobulin complex is a specific antigen-antibody complex.