Studies in Bile Salt Solutions .XIII. Hydrophobic Substrate Effects on the Esterase Activity of Bile-Salt-Stimulated Human-Milk Lipase. Hydrolysis of 4-Nitrophenyl Alkanoates and Alkyl 4-Nitrobenzoates

Abstract

The pseudo-first-order rate constants of hydrolysis of a series of 4-nitrophenyl alkanoates and a series of n-alkyl esters of 4-nitrobenzoic acid and of 4-nitrophenyl hexahydrobenzoate and cyclohexyl 4-nitrobenzoate, catalysed by bile-salt-stimulated human milk lipase in the absence and presence (2 mmol dm-3) of sodium cholate/cholic acid and sodium taurocholate , have been measured at pH 7.3, 310.5 K. It has been shown that the enzyme possesses a specific esterase acyl binding site which almost completely excludes the binding therein of a cyclohexyl group. There is also present a specific alkyl binding site which can fully accommodate a cyclohexyl ring. Both binding sites are hydrophobic in nature, but although the hydrophobic nature of the alkyl binding site is affected by bile-salt stimulation, that of the acyl site is not. Hydrophobicity parameters have been calculated for hydrocarbon chains lying in the acyl and alkyl binding positions of bile-salt-stimulated human milk lipase.