Is the peptidase activity of highly purified human plasma cholinesterase due to a specific cholinesterase isoenzyme or a contaminating dipeptidylaminopeptidase?

The peptidase site of human plasma cholinesterase (butyrylcholinesterase) is distinct from its esteratic site. We found that the number of peptidase sites on an enzyme highly purified from pooled plasma is less than 0.1, as compared with 4 esteratic sites, per tetramer. However, the subunits which carry the peptidase sites are electrophoretically indistinguishable from esteratic subunits. The atypical-silent enzyme (Ea1Es1) had a much higher absolute peptidase activity when substance P was used as the substrate, and we found that the number of peptidase and esteratic sites of this enzyme was roughly the same. This suggests that the mutated esteratic site of the silent possesses a peptidase activity. The esteratic site of the usual allozyme (Eu1Eu1) has no peptidase activity towards substance P. However, a small proportion of peptidase subunits are present in all preparations of enzymes purified from the plasmas of homozygote individuals. The peptidase activity of butyrylcholinesterase might therefore correspond to a specific isoenzyme produced by an epigenetic mechanism or produced by a gene distinct from genes E1 and E2 encoding for cholinesterase subunits. However, the possibility that highly purified cholinesterase contains traces of a dipeptidylaminopeptidase cannot be completely ruled out.