Liver Acetyl Coenzyme A Carboxylase

• Published in: The Journal of biological chemistry 1968-08, Vol.243 (16), p.4227-4235
• Main Authors: Gregolin, C, Ryder, E, Lane, M D
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)93247-3

Acetyl coenzyme A carboxylase has been isolated from chicken liver and purified more than 1000-fold in good yield. It is an unusually stable protein exhibiting negligible loss of enzymatic activity during 1 week of storage (1 to 5 mg of protein per ml) at room temperature in potassium phosphate buffer, pH 7. The pure enzyme catalyzes the carboxylation of 8.8 µmoles of acetyl-CoA (or 7.0 µmoles of propionyl-CoA) per min per mg of refractometrically determined protein at its pH optimum of 7.5 at 37°. The carboxylase-catalyzed carboxylation of acetyl-CoA is activated 15- to 16-fold by dl -isocitrate or citrate and 5-fold by malonate; tricarballylate is essentially inactive. Arrhenius plots for acetyl-CoA carboxylation in the presence or absence of isocitrate are biphasic, having a transition point at about 21°. Below this temperature, where there is very little activator effect, the temperature coefficient (Q 10 ) in the presence of isocitrate is 8.0. Above the transition point, where isocitrate activation is evident, the temperature coefficient (Q 10 ) in the presence of activator is 2.0 and in its absence is a negative value. At pH 7.5 and 37°, the carboxylase catalyzes the over-all reverse reaction (decarboxylation), ATP- 32 P i exchange, malonyl-CoA- 14 C-acetyl-CoA exchange, and ADP-, P i -, and Mg 2+ -independent malonyl-CoA decarboxylation reactions at 14%, 6.2%, 33%, and 1.2%, respectively, of the rate of the over-all forward (carboxylation) reaction. All of the above mentioned reactions exhibit essentially the same tri- and dicarboxylic acid activation patterns. Evidence is presented which indicates that isocitrate activation of the carboxylase is associated with an increased maximum velocity.