CHARACTERISTICS OF ASPARTATE-AMINOTRANSFERASE BINDING IMMUNOGLOBULIN DETERMINED BY THE ISOTOPE METHOD

A woman who had no known underlying diseases showed a persistent eleva tion (about 300 U/L) of serum aspartate aminotransferase (AST) without other abnormal laboratory findings. Cellolose gel electrophoresis sho wed that the AST activity in the patient had an atypical band with slo wer mobility than normal AST. When the sera from the patient and from a patient with acute hepatitis were mixed, the atypical band increased in density and the band of normal size AST disappeared. When the seru m was fractionated on Sephadex G-200 gel filtration medium, almost all AST activity was found between the void volume and the gamma-globulin fraction. However, the AST activity in this fraction was not retained on dissociation into small AST by acid treatment. This suggests the l oss of enzyme activity in dissociated small AST. The patient's serum w as then incubated with iodine 125-labeled porcine AST; when this was f ractionated on gel filtration medium, the main radioactivity was elute d in the void volume fraction. The binding activity for I-125-porcine AST was found in the gamma-globulin fraction obtained by gel filtratio n. The affinity constant of I-125- porcine AST binding to the gamma-gl obulin fraction was 1.0 x 10(-8) mol/L by Scatchard analysis. The bind ing gamma-globulin appeared to be (polyclonal) IgG, and the binding si te was located in F(ab')(2) and Fab fragments. The IgG could be bound with both human and porcine AST but not with chick AST. Thus the IgG a ppears specific for AST of mammalian species. These experimental resul ts suggest that the prolonged elevation of serum AST in our patient ma y result from the presence of an AST/IgG complex, and with excess AST- binding IgG in circulation.