Accelerator mass spectrometry for assaying irreversible covalent modification of an enzyme by acetoacetic ester

Abstract

Protein modification (sometimes known as crosslinking) often requires two or more steps to affix a small molecule irreversibly. Two-step reductive alkylation of the enzyme rabbit muscle aldolase with ethyl 3- 14C-acetoacetate and sodium cyanoborohydride attaches less radioactivity than with cyanoborohydride omitted. The 14C level incorporated into aldolase corresponds to only about 15–30 modified protein molecules per million. Accelerator mass spectrometry (AMS) provides the only technique currently available for investigating the shorter chains from CNBr-cleavage of modified aldolase. Examination of individual fragments reveals that reductive alkylation of the active site lysine in the presence of cyanoborohydride (+BH 3CN) is negligible when compared with the extent of covalent modification in the absence of cyanoborohydride (−BH 3CN). Labeling by ethyl acetoacetate cannot result from simple acetoacetylation, because dialysis with hydroxylamine does not wash it out. The amount of 14C incorporated from ethyl 3- 14C-acetoacetate without cyanoborohydride is roughly proportional to the number of tyrosine residues in each CNBr-fragment, and we surmise that ethyl acetoacetate attaches irreversibly via a reaction specific to that amino acid. Cyanoborohydride inhibits this reaction, but appears to diminish the susceptibility of the active site tyrosine (which is close to a lysine in the tertiary structure of aldolase) less than other tyrosine residues.