Inhibition of Penicillium duponti carboxylesterase by the fungicides captan and folpet

Abstract

Captan, folpet, and perchloromethylmercaptan were effective inhibitors of Penicillium duponti p-nitrophenylpropionate esterase activity (I 50 = 0.5 – 2 μ M) whereas α-naphthyl acetate esterase activity was not affected by the presence of these compounds. Captan and folpet are both equally effective at pH 7.3 and 8.3. The ionic composition of the medium had strong effects on the degree of inhibition produced by all inhibitors but did not alter esterase activity. Neither succinamide nor phthalimide caused inhibition of the p-nitrophenylpropionate esterase activity: The trichloromethylmercaptan portion of these fungicides appears to be responsible for the observed inhibition. The rapidity of captan and folpet inhibition of esterase activity (complete in < 1 min) compared to the rates of spontaneous decomposition ( t 1 2 > 1 min ) and the insensitivity of captan and folpet inhibition to hydrogen ion concentration suggest that generation of spontaneous decomposition products is not required for inhibition. The results are consistent with a mechanism in which the entire fungicide molecule binds to the protein followed by enzyme-promoted reactions of captan and folpet which result in loss of esterase activity.