Direct immunological determination of aspartate aminotransferase isoenzymes.

Abstract We examined the suitability of a rapid immunological technique to determine the amount of aspartate aminotransferase (EC 2.6.1.1) isoenzymes in human serum or tissue extracts. Purified isoenzymes were absorbed as a monolayer to the surface of an indium metal film on glass. The enzyme retains immunological reactivity, allowing the specific binding of aspartate aminotransferase antibodies at the surface. The amount of isoenzyme in a specimen is estimated from the competition for the antibody between the free isoenzyme in the specimen and that at the surface. The surface is further incubated with goat antibodies to rabbit IgG, and the extent of antibody binding is determined by densitometry. There is no cross reactivity between the cytoplasmic and mitochondrial forms, so these two isoenzymes can be determined simultaneously. The minimum detectable concentration by this technique is about 50 micrograms of enzyme protein per liter. The within-day coefficient of variation for determination of either isoenzyme was about 20%. Our results suggest that normal and patients' sera contain considerably more immunologically active than catalytically active isoenzymes.