Susceptibility to trypsinolysis of esterified milk proteins

Methyl-, ethyl- and propyl-esters of beta -lactoglobulin. alpha -lactalbumi n and beta -casein were prepared and then hydrolyzed with trypsin in variou s conditions. Resulting hydrolysates were analysed by SDS electrophoresis a nd RP-HPLC. The degree of hydrolysis of esterified samples was generally lo wer than those of the non-modified proteins. The highest degrees of hydroly sis were obtained at pH 7-8 with native and esterified protein samples. bet a -Lactoglobulin propyl ester and beta -casein methyl ester yielded the low est degrees of hydrolysis. Ethyl- and propyl-esters of beta -casein showed high resistance towards tryptic attack, even after 20 h of hydrolysis. SDS electrophoretic patterns of tryptic hydrolysates of native proteins showed bands corresponding to low molecular weights. Tryptic hydrolysates of ester ified proteins showed bands with higher sizes. RP-HPLC profiles of tryptic hydrolysates of esterified samples showed peaks with longer elution times t han those obtained with native proteins, indicating the presence of more hy drophobic peptide populations. A peptic pre-treatment improved tryptic acti on on esterified proteins. It resulted in a better resolution of RP-HPLC pr ofiles and in a complete disappearance of the protein after 20 h tryptic hy drolysis. (C) 2001 Elsevier Science B.V. All rights reserved.