The esterase-like activity of serum albumin may be due to cholinesterase contamination

Purpose. The "esterase-like activity" of human serum albumin (HSA) is descr ibed in the literature, but a contamination of commercially available HSA p reparations by plasma cholinesterase is conceivable in some cases. The purp ose of the present work was to examine this hypothesis. Methods. The hydrolytic activity of HSA and its inhibition by physostigmine were measured fluorimetrically by monitoring the hydrolysis of the ester s ubstrate moxisylyte. Affinity chromatography was used to separate cholinest erase and HSA. The cholinesterase activity in the eluted fractions was asse ssed using Ellman's reagent and butyrylthiocholine as substrate. Results. A significant variation in the esterase-like activity of different albumin batches was observed. This activity was strongly inhibited by phys ostigmine, a well-known inhibitor of cholinesterase. Affinity chromatograph y led to a complete separation between HSA and the esterase activity, which was found exclusively in the cholinesterase fraction. Conclusions. The apparent esterase-like activity of HSA toward moxisylyte a nd butyrylthiocholine was due to a contamination by cholinesterase. With th ese substrates, HSA showed a total lack of esterase-like activity.