The reaction of aldolase with 2-methylmaleic anhydride.

1. The reaction of rabbit muscle aldolase with 2-methylmaleic anhydride is described. All the protein amino groups can be reversibly blocked. 2. As the reaction proceeds, the enzyme activity decreases until, at about 50% citraconylation of amino groups, the enzyme is completely inhibited. At this stage, little or no dissociation of the enzyme tetramer is observed and 75% of the activity is recoverable on unblocking the amino groups. 3. At 80% blocking, the enzyme is completely dissociated but little enzymic activity is recoverable after unblocking. Inability to recover activity after citraconylation and unblocking correlates with the onset of dissociation of the citraconyl-aldolase seen on ultracentrifugation. 4. The only irreversible modification of the enzyme primary structure detectable after the citraconylation and unblocking reactions is the partial loss of thiol groups. It is probable that this is responsible for the inability to reform active enzyme from the citraconylated subunit. 5. Other reversible side reactions of maleic anhydride and citraconic anhydride that may occur with proteins are discussed.