An automated method for measuring Rhesus monkey erythrocyte and plasma cholinesterase

An automated colorimetric method using the substrate acetylthiocholine was modified to measure red blood cell (RBC) and plasma cholinesterase (ChE) in samples from rhesus monkeys. The kinetic parameters Km and V(max) were calculated for ChE from both sources. Nonenzymatic hydrolysis of acetylthiocholine was uniform and small, and reproducibility of RBC and plasma ChE values was extremely high. Hemolyzing RBC (by freezing, or by using water or a nonionic detergent as diluents) did not affect ChE activity significantly, but hemolysis did increase nonenzymatic activity. The anticoagulants heparin and EDTA had no effect on RBC or plasma ChE measurements.A microdilution technique was devised for RBC ChE measurements using 13 microliters of RBC packed in a standard microhematocrit tube. Reproducibility was quite high, but values using this technique were significantly greater than in duplicate samples assayed using standard techniques. Since plasma ChE activity is much lower than that of RBC, the more efficient extrusion of plasma in RBC samples packed in microhematocrit tubes probably accounted for the higher values. Normal levels of plasma and RBC ChE in adult rhesus monkeys were calculated. There were no significant differences between sexes.