Purification and kinetic properties of ATP: citrate oxaloacetate lyase from rat brain.

Abstract— A method for a partial purification of ATP:citrate oxaloacetate lyase from rat brain is described. The Lineweaver–Burk plots of velocity vs citrate concentration are biphasic in the presence of fixed concentrations of MgCl₂. Therefore two values of K_m, corresponding to low and high concentrations of citrate, can be determined. When MgCl₂ is added in equimolar concentrations with citrate, a monophasic plot with one K_m of 0.13 mm is obtained. The K_m value for MgATP^2- was independent of citrate concentration, being equal to 0.40–0.43 mm. The K_m for CoA was 0.0007 mm. ADP and P_i are competitive inhibitors with respect to ATP. K_i for MgADP⁻ is equal to 0.13 mm. dl -isocitrate and cis-aconitate are partially competitive inhibitors with respect to citrate with K_i values of 5.8 and 4.8 mm, respectively. α-Ketoglutarate and pyruvate are noncompetitive inhibitors with respect to ATP and citrate, with K_i values equal to 9 and 45 mm, respectively. The physiological significance of these effectors for the regulation of citrate lyase activity in brain is discussed.