Interactions of narcotics and their antagonists with human serum esterase. VII: Further studies on the affinity of drugs for the substrate-binding site of the enzyme.

Earlier studies on the competitive inhibition of human serum esterase by morphine and related compounds were continued. Very good correlation was found between the inhibitor constants of 57 compounds and an empirical function, the "hydrophobic factor," which contains a positive contribution from the hydrophobic nature of a compound and negative contributions from its hydrophilic characteristics and from strain in its molecule. However, structural factors plainly also play a role in the affinity of each compound for the active site of the esterase because it was found necessary to include another positive term for a flexible hydrophobic group and because as a rule stereoisomers--even enantiomorphs--with identical hydrophobic factors differ in affinity; but no correlation could be found between the absolute configurations of the enantiomorphs and their affinities for the active site.