Catalytic and immunochemical properties of homomeric and heteromeric combinations of aldolase subunits.

The catalytic and immunochemical properties of homomerit and heteromeric rabbit fructose diphosphate aldolase tetramers are described. Heteromers composed of either A and C (A&!, A&, AC) or B and C (B&Z, B&, BCJ subunits have maximal velocities and Michaelis constants indistinguishable from those of an equivalent mixture of homomers. These results indicate the subunits act independently and also imply the existence of four catalytically active sites in the aldolase molecule (one active site per subunit). Specific noncross-reaction antibodies prepared against the homomeric aldolases A*, B4, and C4 react with heteromers containing one or more subunits to which the antibodies had been directed. This interaction results in essentially complete inhibition of the catalytic activity of the heteromer. The amount of antibody required is inversely proportional to the number of antigenic subunits in the heteromer. These results suggest that (a) interaction with antibody may alter subunit interactions that are required for catalytic activity, and (b) antigenic sites may be modified in homomeric and heteromeric combination.