[en] The authors have shown previously that the activity of the pyruvate dehydrogenase complex (PDC) increased several fold during differentiation of 3T3-L1 preadipocytes into adipocytes. The increase in total activity correlated with coordinated increases in the relative rates of synthesis (RRS) of α and β subunits of the pyruvate dehydrogenase component. They now report investigations of the changes in the activity of E₃, another component of PDC, in 3T3-L1 cells during differentiation initiated by exposing cells to insulin, dexamethasone and 1-methyl-3-isobutylxanthine for 48 h followed by continued exposure to insulin alone. The specific activity (SA) of E₃ increased 3 to 4-fold during the differentiation. The amount of immunoprecipitable E₃ protein similarly increased in the differentiated cells. Immunotitrations of E₃ indicated that the SA of E₃ remained the same in both cell types. Pulse labeling of cells with ³⁵S-methionine revealed a 3.5-fold increase in the RRS of E₃ in adipocytes compared to preadipocytes. The apparent degradation rates of E₃ were not significantly different in the two cells types. E₃ mRNA content, measured using an E₃ cDNA clone, was also increased during the differentiation. The increase in the RRS of E₃ is the primary determinant for increased enzyme protein in adipocytes