Three aspartate aminotransferase isoenzymes were identified from extracts of carrot (Daucus carota L.) cell suspension cultures. These isoenzymes were separated by DEAE chromatography and were analyzed on native gradient polyacrylamide gels. The relative molecular weights of the isoenzymes were 111,000 +/- 5000, 105,000 +/- 5000, and 94,000 +/- 4000 daltons; they were designated forms I, II, and III, respectively. Form I, the predominant form, has been purified to apparent homogeneity (>300-fold) using immunoaffinity chromatography with rabbit anti-pig AAT antibodies. Form I has a subunit size of 43,000 M(r), as determined on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Isoelectric focusing (IEF)-PAGE has resolved three bands at a pl of approximately 5.2. Form I may be composed of subunits of similar molecular weight and different charges, and the three bands with AAT activity on the IEF-PAGE gel are a combination of hetero- and homodimers. Form I has a broad pH optimum of 7.5 to 10.0. K(m) values of 23.6, 2.8, 0.05, and 0.22 millimolar were obtained for glutamate, aspartate, oxaloacetate, and alpha-ketoglutarate, respectively. The mode of action is a ping-pong-bi-bi mechanism.