Studies on ‘usual’ and ‘atypical’ serum cholinesterase using α-naphthyl acetate as substrate

Summary Estimates of the Michaelis constants for usual5 and ‘atypical’ serum cholinesterase with α-naphthyl acetate as substrate were obtained. Expressed as m-moles α-naphthyl acetate per litre the values were: ‘usual’ enzyme 0-76 + 0-056, ‘atypical’ enzyme 0-58 + 0-057. The inhibition of the activity of the two enzymes by dibucaine, RO 2-0683, and fluoride was also examined using α-naphthyl acetate as substrate. It was found that, in the appropriate concentration range, the ‘usual’ enzyme was inhibited to a much greater degree than the ‘atypical’ enzyme by dibucaine and RO 2-0683. With fluoride, however, the difference, though significant, was much less marked. The findings are compared with similar data obtained when choline esters were used as substrates for the enzymes, and are considered in terms of the hypothesis that a structural difference at the ‘anionic’ site is the cause of the difference in the properties of the two enzymes.