Complete covalent structure of 60-kDa esterase isolated from 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced rabbit liver microsomes

The 60-kDa esterase was isolated from liver microsomes of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced rabbits and its complete amino acid sequence determined. Automated sequence analysis of intact protein, as well as characterization of the peptides obtained from enzymatic and chemical cleavages, led to the elucidation of the primary structure. The protein is a single polypeptide consisting of 539 residues and molecular weight 59,478. The active site serine is 195, and another diisopropylphospho binding site is at histidyl 441. Carbohydrate chains are attached at aspariginyl residues 61 and 363. Although 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment induces this esterase severalfold, the amino acid sequence of the induced enzyme is identical to that of the enzyme isolated from liver microsomes of untreated rabbits. The sequence of the microsomal esterase is 30% identical with the sequences of human serum cholinesterase and the acetylcholinesterase from Torpedo californica. There is also a close homology between the 60-kDa esterase and the COOH-terminal domain of bovine thyroglobulin.