Immunoassay of enzymes--an overview

Immunochemical quantitation of isoenzymes of aspartate aminotransferase and lactate dehydrogenase

The applicability of immunochemical techniques to the determination of aspartate aminotransferase (AspAT, EC 2.6.1.1) and lactate dehydrogenase (LDH, EC 1.1.1.27) isoenzymes in human serum are reviewed. In the case of AspAT, the human enzymes of mitochondrial (m-AspAT) and cytosolic (s-AspAT) origin were purified to homogeneity from liver and erythrocytes respectively and used to prepare isoenzyme-specific anti-sera in rabbits. Immunoprecipitation and immunoinhibition assays using partially purified antibodies or monovalent Fab fragments were found to provide better accuracy and precision than column chromatographic, electrophoretic, or differential kinetic techniques. A variety of immunochemical techniques were examined for the determination of enzyme protein including radioimmunoassay, turbidimetric procedures, and an assay using the indium slide technique. In the last, purified isoenzyme was absorbed as a monolayer to the surface of an indium metal film upon glass. The enzyme retains immunological reactivity, allowing the specific binding of antibody at the surface. The minimum detectable concentration by this technique is greater than 50 micrograms/L of enzyme protein; results suggest that normal and patient sera contain considerably more immunologically reactive s- and m-AspAT than catalytically active enzyme.