1. Series of colorimetric dynamic assays allowed the study of the inhibition of cholinesterases by F(-) ions in vitro, by using, as sources of enzyme, whole human blood, human serum, homogenized rat brain and two preparations of red blood cells (human and bovine) whose enzymic purity was ascertained.
2. The first evidence of inhibition of human serum pseudocholinesterase by fluoride was noticed at 15-25 µm-fluoride. Ten times as much fluoride was needed to start inhibition of acetylcholinesterase of the red blood cells.
3. The action of fluoride on the enzymic reaction was immediate. The reversibility of the inhibition was shown by dialysis and dilution.
4. Kinetic measurements showed that the inhibition under study was not dependent on the substrate concentration and was of the uncompetitive type, similar to that observed in the presence of a heavy metal (cadmium).
5. The activity of serum cholinesterase did not change in the absence of Mg(2+) and Ca(2+) ions. Fluoride was shown to inhibit the enzyme in the absence of these ions as well as of phosphate.
6. Fluoride could inhibit cholinesterases in the presence of three different substrates and had no action on the non-enzymic hydrolysis.
7. It is thought that the halide is bound reversibly to the enzyme molecule, with the probable exclusion of the active site, but no firm conclusion could be reached on this point.