The Dissociation into Subunits of Aspartate Aminotransferase from Pig Heart Muscle

Ultracentrifugation studies show that, at concentrations of about 0.1 mM the aspartate aminotransferase of pig heart muscle does not dissociate into subunits either in 6 M guanidine hydrochloride, pH 7.5, or 8 M urea solution, pH 7.5. On prolonged treatment (8 weeks, 0°) with urea, however, complete dissociation occurs and the measured value of the molecular weight drops to approx. 40,000. The protein so obtained is enzymically inactive and migrates on electrophoresis in starch gel at pH 8.0 as a single, fast‐moving band. Gel filtration experiments show that the holoenzyme (pyridoxal form) partly dissociates into two subunits at a concentration of about 3 nM. The specific activity of the enzyme does not change in this region; the monomer and dimer have, therefore, the same catalytic activity per mole of cofactor. The apoenzyme is unstable at high dilution and its dissociation has not been directly demonstrated by gel filtration. However, indirect evidence obtained from a study of the kinetics of its recombination with cofactor suggests that dissociation occurs at a concentration of about 50 nM.