Oxaloacetate forms in several ways in nature. A principal route is upon oxidation of -malate, catalyzed by malate dehydrogenase, in the citric acid cycle. Malate is also oxidized by succinate dehydrogenase in a slow reaction with the initial product being enol-oxaloacetate. <br> 
It also arises from the condensation of pyruvate with carbonic acid, driven by the hydrolysis of ATP:
:CHC(O)CO + HCO  +  ATP   →   OCCHC(O)CO  +  ADP  +  Pi
Occurring in the mesophyll of plants, this process proceeds via phosphoenolpyruvate, catalysed by phosphoenolpyruvate carboxylase.  <br>Oxaloacetate can also arise from trans- or de- amination of aspartic acid.