| Abstract: | Exposed thiol groups of rabbit muscle aldolase A were modified by 5,5'-dithiobis(2-nitrobenzoic) acid with concomittant loss of enzyme activity. When 5-thio-2-nitrobenzoate residues bound to enzyme SH groups were replaced by small and uncharged cyanide residues the enzyme activity was restored by more than 50%. The removal of a bulky C-terminal tyrosine residue from the active site of aldolase A resulted in enzyme which was inhibited by 5,5'-dithiobis(2-nitrobenzoic) acid only by 50% and its activity was nearly unchanged after modification of its thiol groups with cyanide. The results obtained show directly that rabbit muscle aldolase A does not possess functional cysteine residues and that the inactivation of the enzyme caused by sulfhydryl group modification reported previously can be attributed most likely to steric hindrance of a catalytic site by modifying agents. |
