The butyrylcholinesterase K-variant shows similar cellular protein turnover and quaternary interaction to the wild-type enzyme

A recent study has linked the butyrylcholinesterase (BChE) K-variant and th e apolipoprotein epsilon 4 isoform to late-onset Alzheimer's disease. These findings have been controversial acid have led us to examine the differenc es between wild-type and K-variant BChE in enzyme activity, protein stabili ty, and quaternary structure, J-variant BChE (E497V/A539T) was also studied because it is associated with the K-variant mutation. The K-variant mutati on (A539T) is located in the C-terminal tetramerization domain. Wild-type, K-variant, and J-variant BChE were expressed in Chinese hamster ovary cells and purified. The purified enzymes had similar binding affinity (K-m) valu es and catalytic rates for butyrylthiocholine and benzoylcholine, In pulse- chase studies the K-variant, J-variant, and wildtype BChE were degraded rap idly within the cell, with a half-time of similar to 1.5 h. Less than 5% of the intracellular BChE was exported. The C-terminal peptide containing the K-variant mutation interacted with itself as strongly as did the wild-type peptide in the yeast two-hybrid system. Both K-variant and wild-type BChE assembled into tetramers in the presence of poly-L-proline or the proline-r ich attachment domain of the collagen tail. The native K-variant BChE in se rum showed the same proportion of tetramers as the native serum wild-type B ChE. We conclude that the K-variant BChE is similar to wild-type BChE in en zyme activity, protein turnover, and tetramer formation.