[Reversible dissociation of malate dehydrogenase from plants]

It was demonstrated that 0.2 M citric acid (pH 2.5) inactivates highly-purified malate dehydrogenase from tea leaves; the degree of inactivation depends on temperature and time of incubation. The enzyme activity is restored by certain inorganic salts, the degree of reactivation being dependent on pH, ionic strengths of salts and duration of enzyme incubation with both inactivating and reactivating agents. Urea and guanidine hydrochloride also have a reversibly inactivating effect on the enzyme. The degree of inactivation depends on their concentration and incubation time. In the latter case reactivation of enzyme is achieved by dialysis or 20-40-fold dilution of the enzyme preparation. A kinetic study demonstrated that inactivation of enzyme by the above-mentioned agents is due to the enzyme dissociation into 4 catalytically inactive subunits with molecular weights of 17 500 +/- 1000, which under certain conditions are capable of reassociating into an active molecule of enzyme with completely restored native conformation.