Aspartate aminotransferase. The effects of ionic concentration of kinetic constants of both isoenzymes.

1. The Michaelis constants for both isoenzymes for both substrates depend strongly on ionic concentration, being approximately proportional to phosphate concentration over considerable ranges. This is probably an effect of anions only. 2. In the absence of added salt, K(m) (2-oxoglutarate) (anionic isoenzyme) is so small as to be indeterminate. 3. K(m) (l-aspartate) (anionic isoenzyme) passes through a sharp minimum at about 3.3mm-phosphate. It is not clear whether this is a specific effect of phosphate. 4. Both substrates are inhibitory at sufficiently low ionic concentrations. 5. A modified graphical procedure is described for the derivation of the kinetic constants.