Multiple effects of isoproterenol on horse plasma cholinesterase

Isoproterenol inhibits the hydrolysis of butyrylthiocholine by horse plasma cholinesterase, while it stimulates the hydrolysis of p-nitrophenyl butyrate. The inhibition pattern obtained for butyrylthiocholine is consistent with a dimeric model for the enzyme showing negative cooperativity. The kinetics of inhibition point to a dissociative effect of isoproterenol, superimposed on its competitive inhibitory action. The hydrolysis of p-nitrophenyl butyrate is not sensitive to changes in the subunit composition of the enzyme.