[Mechanism of interaction between milk xanthine oxidase and p-chloromercuribenzoate. Properties of the purified enzyme].

Two types of complexes are formed during the interaction of xanthine oxidase with p-chloromercurybenzoate (pCMB). The reversible inactive complex (presumably of absorption nature) is formed practically instantaneously and competitively with regard to the substrate (Ki=6,2 . 10(-8) M) in 0,05 M phosphate buffer (pH 7,8, 25 degrees) and does not involve the fast-reacting SH-groups of the enzyme. Reactivation of xanthine oxidase is observed during prolonged incubation of the inactive complex at 0 degrees; it is associated with the interaction between pCMB and the fact-reacting SH-groups. This interaction results in a dissociation of the inactive complex. The blocking of the slow-reacting SH-groups is accompanied by an irreversible loss of the xanthine oxidase activity. The enzyme modification by blocking of 10 fast-reacting SH-groups does not involve the Fe-S clusters, but results in local changes in the enzyme conformation. This is manifested in a 2-fold increase of Km and the rate constants of proteolysis of the modified xanthine oxidase as compared to the native enzyme. The rate constants of proteolysis by trypsin for the native and modified enzymes in 0,05 M phosphate buffer (pH 7,8; 37 degrees) are 3,7 . 10(-3) min-1 and 7,0 . 10(-3) min-1, respectively; those for chymotrypsin in the same buffer (30 degrees) are 1,5 . 10(-2) min-1 and 6,0 . 10(-2) min-1, respectively.