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Abstract. The biotin carboxylase component of acetyl CoA carboxylase has been purified approximately 2000 times from Escherichia coli. This protein, which catalyzes the carboxylation of free d-biotin, is free of the biotin-containing carboxyl carrier protein, is homogeneous by polyacrylamide gel electrophoresis and analytical ultracentrifugation, and has been crystallized. Biotin carboxylase, with a molecular weight of approximately 100,000, is composed of two 50,000-dalton subunits. The catalytic capacity of biotin carboxylase is markedly enhanced by ethanol (11 times at 15% v/v), and certain other organic solvents; this may mimic an effector-mediated response. The kinetic effect is exclusively on the maximal velocity of the reaction. Activation by ethanol is reversible and not accompanied by aggregation or disaggregation of the enzyme.Acetyl CoA carboxylase, a key enzyme in the biosynthesis of fatty acids and other acetogenins, catalyzes the ATP-dependent formation of malonyl CoA from acetyl CoA and bicarbonate (Reaction 3). After the recognition that enzymebound biotin was involved in this reaction,1 investigations with a number of acyl CoA carboxylases2 clarified the role of biotin and showed that the overall process could be partitioned into the following half-reactions: Me2t+