[Structural organization of glutamate dehydrogenase. 2. Inactivation and dissociation of immobilized hexamer induced by urea].

The urea-induced inactivation and dissociation of catalytically active hexamer of glutamate dehydrogenase (L-glutamate-NAD(P)-oxidoreductase, EC 1.4.1.3) from bovine liver were studied using radioactive phosphopyridoxyl derivative of the enzyme immobilized on cyanogen bromide-activated Sepharose CL-4B. It is shown that at neutral pH (7.0-7.8) urea causes dissociation of glutamate dehydrogenase to directly yield catalytically inactive immobilized monomers rather than hexamer's stable fragments at the same time. At pH 8.9 or 5.6 the urea-induced is accompanied by the formation of conformationally stable immobilized dimers or trimers, respectively. The trimers are catalytically active, whereas the dimers did not exhibit any enzymatic activity. The data obtained led to suggestion that the hexamer consists of three either equivalent dimers (3 alpha 2) or of two equivalent trimers (2 alpha 3).