Measurement of aspartate aminotransferase isoenzymes: six procedures compared

Six procedures were evaluated for aspartate aminotransferase (EC 2.6.1.1) isoenzyme assay in human serum and tissue homogenates. Results of procedures based on immunochemical precipitation by use of antibodies directed against either the mitochondrial or (with greater precision) soluble isoenzyme correlated well with those by a differential kinetic assay involving both different pH conditions and adipate inhibition. Results with a DEAE-Sephadex ion-exchange chromatographic procedure correlated well with these techniques for specimens containing purified isoenzymes, but showed substantial positive bias for determination of the mitochondrial isoenzyme in human serum. An assay based on the differential effects of pH alone discriminated between the isoenzymes with less bias than did the chromatographic assay. Precision of the two differential pH assays was limited by significant reagent blank activity resulting from destruction of NADH at pH 6.0 or 6.2. An electrophoretic procedure in which diazonium salt is used to make oxalacetate visible was least accurate for measuring samples for which the isoenzyme composition was known.