Subcellular distribution of aspartate aminotransferase isoenzymes in chicken heart: quantitative study

1. The content of the two aspartate aminotransferase isoenzymes in isolated mitochondria and in the cytosolic fraction from chicken heart was determined by radioimmunoassays. 2. The cationic isoenzyme was found to be associated with the mitochondrial fraction; its content measured in the cytosolic fraction was within the range of that of contaminating mitochondrial marker enzymes. 3. The anionic isoenzyme was found exclusively in the cytosolic fraction, in mitochondria a content of less than 0.05% of the total was measured. 4. Thus, in birds, the anionic and the cationic isoenzyme of aspartate aminotransferase show the same strict intracellular heterotopism as found previously in mammals.