Abstract
Pancreatic lipase (glycerol-ester hydrolase, EC 3.1.1.3) is inhibited by emulsions of diethyl p-nitrophenyl phosphate (E600) or by solutions of this organophosphate in the presence of bile salts. The inhibited enzyme contained a single phosphoryl radical per mole. This radical was shown to be bound to a serine residue in the sequence Leu-Ser-Gly-His. Therefore, lipase is a serine-histidine enzyme like ordinary carboxylesterases and a number of proteases.However, the specific properties of lipase were confirmed when it was observed that the enzyme was not inhibited by E600 solutions in the absence of bile salts. Inhibition in the presence of bile salts is assumed to be caused by inclusion of E600 into micelles of these compounds.Moreover, in contrast with other serine enzymes, lipase is not inhibited by diisopropylfluorophosphate (DFP) even in the presence of bile salts. It is also not inhibited by sulfonyl halides unless the nucleophilicity of the sulfur atom is enhanced by an electrophilic substituant. These results are in perfect agreement with the observations already made with the substrates of lipase.In addition, concentrated DFP solutions were found to react with a single, non-essential tyrosine residue in native lipase. This residue is located in the sequence: Thr-Asn-Gln-Asn-Glx-Asx(Asx, Glx)-Tyr-Glx-Leu.
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