The Number, Location, and Reactivity of the Cysteine Residues of Sturgeon Muscle Aldolase

Abstract

Sturgeon muscle aldolase contains six cysteine residues per subunit. These residues appear to occur in homologous positions to six of the eight cysteine residues of rabbit muscle aldolase. Three of the six residues can react with either iodoacetic acid or 5,5′-dithiobis-(2-nitrobenzoic acid) in the absence of denaturing agents. Reaction of three residues with iodoacetic acid inactivates the enzyme. The presence of substrate protects one of these residues and in this case no activity loss is observed. However, the results indicate that the effects on activity are due to the addition of the modifying group rather than to a loss of sulfhydryl groups. No residue corresponding to the previously demonstrated substrate-protected cysteine of rabbit aldolase was located in the sturgeon enzyme, which demonstrated that this residue was not essential to aldolase activity. It therefore appears unlikely that cysteine residues have a direct or an auxiliary catalytic role in aldolase activity.