We study two structurally distinct forms of phosphoenolpyruvate carboxykinase found in either the cytosol (PEPCK-C) or mitochondria (PEPCK-M). Although the two forms are 40% nonidentical in amino acid makeup, they are similar in structure and catalysis. Hormones (e.g., glucagon, insulin) and diet influence transcription of the PEPCK-C gene, but not the PEPCK-M gene. We use site-directed mutagenesis of cDNAs for both isozymes in order to determine effects of specific amino acids on catalytic function. We also study the influence of structure on protein and mRNA stability of these PEPCK isozymes. The short half-life and hormonal control of PEPCK-C suggests that there are labile regions in either the mRNA or protein.