Comparative Studies of Liver and Muscle Aldolase. Ii. Immunochemical and Chromatographic Differentiation.

Crystalline fructose diphosphate aldolase preparations obtained from rabbit muscle and bovine liver differ in catalytic efficiency (fructose diphosphate cleavage activity of muscle aldolase is about 10 times that of the liver enzyme) and substrate specificity (the fructose diphosphate to fructose l-phosphate activity ratio is 50 for muscle and 1 for liver aldolase) (2). In spite of these distinctive catalytic properties, the two protein molecules have similar features; for example, their molecular weight and number of sulfhydryl groups (3-6). Treatment of the muscle enzyme with carboxypeptidase yields a product with altered substrate specificity and catalytic activity superficially resembling the liver enzyme (7). The catalytic properties of partially carboxypeptidase-degraded muscle aldolase are even more reminiscent of those of the liver enzyme. Furthermore, muscle aldolase has 3 carboxyl-terminal tyrosine residues (7) but the bovine liver aldolase contains only 1 to 2 tyrosine residues released by carboxypeptidase (8). There are several possible explanations of the contrasting molecular properties of these two enzymes: first, the discrepancies may reflect species rather than organ differences; second, the liver enzyme may represent a specifically degraded form of the muscle enzyme; third, the enzymes may be distinct proteins. In the present study these possibilities are resolved through definition of the catalytic and immunochemical properties as well as by physical separation of the aldolase activity obtained from various tissues. It is demonstrated that mammalian systems contain at least two distinct enzymatic entities exhibiting aldolase activity, one exhibiting a high selectivity for fructose diphosphate and a high turnover number, and the other, a broader substrate specificity and a lower catalytic activity.