Guo M R, Fox P F, Flynn A, Kindstedt P S
Food Chemistry, University College, Cork, Ireland.
J Dairy Sci. 1995 Nov;78(11):2336-44. doi: 10.3168/jds.S0022-0302(95)76860-6.
The susceptibility of beta-LG and sodium caseinate to proteolysis by pepsin and trypsin was investigated using SDS or urea-PAGE. The effects were studied of heat, urea, and 2-mercaptoethanol on proteolysis. Native beta-LG was resistant to hydrolysis by pepsin or trypsin because of its compact globular structure. Heat treatment of beta-LG solutions at 90 to 100 degrees C for 5 or 10 min caused changes in the structure or conformation of the protein that rendered it accessible to pepsin and enhanced the extent of proteolysis by trypsin. The susceptibility of beta-LG to proteolysis by pepsin was markedly increased in the presence of urea (3 to 6 M), and the effect was reversible after removal of urea by dialysis. Proteolysis by trypsin was also increased by the presence of 2% 2-mercaptoethanol. Sodium caseinate was very accessible to pepsin without pretreatment and was extensively hydrolyzed at pH 1 to 5 in the presence of 5 M urea (which prevented the protein from precipitation in the isoelectric region); optimal pH was about 2. The activity of pepsin on sodium caseinate at pH 2 was not significantly affected by urea concentration up to about 8 M. The results indicated that the changes in conformation and structure of beta-LG that were induced by heating, reduction, or urea rendered the protein susceptible to peptic hydrolysis.
使用SDS或尿素-PAGE研究了β-乳球蛋白和酪蛋白酸钠对胃蛋白酶和胰蛋白酶蛋白水解的敏感性。研究了加热、尿素和2-巯基乙醇对蛋白水解的影响。天然β-乳球蛋白由于其紧密的球状结构而对胃蛋白酶或胰蛋白酶的水解具有抗性。在90至100℃下对β-乳球蛋白溶液进行5或10分钟的热处理会导致蛋白质结构或构象发生变化，使其易于被胃蛋白酶作用，并增强了胰蛋白酶的蛋白水解程度。在存在尿素（3至6M）的情况下，β-乳球蛋白对胃蛋白酶蛋白水解的敏感性显著增加，通过透析去除尿素后，这种作用是可逆的。2%的2-巯基乙醇的存在也会增加胰蛋白酶的蛋白水解作用。酪蛋白酸钠未经预处理就很容易被胃蛋白酶作用，并且在pH值为1至5且存在5M尿素（防止蛋白质在等电区域沉淀）的情况下会被广泛水解；最佳pH值约为2。在pH值为2时，胃蛋白酶对酪蛋白酸钠的活性在尿素浓度高达约8M时不受显著影响。结果表明，加热、还原或尿素诱导的β-乳球蛋白构象和结构变化使该蛋白质易于发生胃蛋白酶水解。
