Hydrophobic interactions of human milk lipase

The hydrolysis of a series of n-alkyl esters of 4-nitrobenzoic acid, and of isopropyl 4-nitrobenzoate, 4'-nitrophenyl 4-nitrobenzoate, and 4-nitrobenzoyl 1-monoglycerol, catalyzed by human milk lipase in the absence and presence of cholate stimulation, has been measured at pH 7.3, 37.5 degrees C. It has been shown that the enzyme possesses a specific alkyl binding site which is hydrophobic in nature and wide enough to accommodate two fatty acid chains lying side by side or a phenyl ring lying flat. The hydrophobic nature of this site is affected by bile salt stimulation of the enzyme. Hydrophobicity parameters have been calculated for hydrocarbon chains lying in the acyl and alkyl binding sites of human milk lipase. A mechanism is suggested for the role of bile salts in stimulating the enzyme in its activity against water soluble esters and water soluble triacylglycerols.