Evidence for a Zn²⁺-binding site in human serum butyrylcholinesterase

Purified human serum butyrylcholinesterase after treatment with either of the metal chelators EDTA or NaCN was able to bind to a Zn²⁺-chelate–Sepharose affinity column and was eluted from the column by EDTA or imidazole. Prior EDTA treatment of the enzyme was essential for binding to this affinity column. The enzyme could be labelled with ⁶⁵Zn²⁺ after EDTA treatment of the enzyme. Diethylpyrocarbonate modification of histidine residues in the EDTA-treated enzyme resulted in the abolition of both binding to the Zn²⁺-chelate–Sepharose column and labelling by ⁶⁵Zn²⁺. Stoicheiometry of ⁶⁵Zn²⁺ binding indicated approximately 0.85 mol of Zn²⁺/mol of subunit of the EDTA-treated enzyme. EDTA or NaCN treatment resulted in the loss of thermal stability of the enzyme at 37 °C which could not be reversed by Zn²⁺. Whereas the cholinesterase activity of butyrylcholinesterase was not affected by EDTA, there was significant loss of its carboxypeptidase activity in the presence of EDTA, and the loss could be reversed by added ZnCl₂. These results suggest the presence of a Zn²⁺-binding site on human serum butyrylcholinesterase and the involvement of histidine residues in the metal binding. The presence in human serum butyrylcholinesterase of a sequence HXXE…H found in many known Zn²⁺-containing enzymes supports these findings.