In order for the intestinal mucosa to absorb dietary proteins they have to be digested into single amino acids or very short peptides of a length of not more than four amino acids. In order to study the efficiency of the digestive endopeptidases to digest folded proteins we have analyzed several target proteins under different conditions, native proteins, heat denatured and acid treated. The three pancreatic serine proteases, trypsin, chymotrypsin, and pancreatic elastase, were found to be remarkable inefficient in cleaving native folded proteins whereas pepsin, which acts at a very low pH (pH 1.2) was much more efficient, possibly due to the denaturing conditions and thereby better accessibility to internal cleavage sites at the low pH. Heat treatment improved the cleavage considerably by all three pancreatic enzymes, but acid treatment followed by return to neutral pH did not have any major effect. Cleavage at the low pH when the protein is in a denatured state, is apparently very efficient. This indicates that pepsin is the prime enzyme cleaving the properly folded native proteins and that the pancreatic enzymes primarily are involved in generating single amino acids or very short peptides for efficient uptake by the intestinal mucosa.

中文翻译：

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消化酶胃蛋白酶、胰蛋白酶、胰凝乳蛋白酶和胰弹性蛋白酶切割紧密折叠的蛋白质的效率存在显着差异

为了让肠粘膜吸收膳食蛋白质，它们必须被消化成单个氨基酸或长度不超过四个氨基酸的非常短的肽。为了研究消化内肽酶消化折叠蛋白的效率，我们分析了不同条件下的几种靶蛋白、天然蛋白、热变性和酸处理。发现三种胰丝氨酸蛋白酶，胰蛋白酶、胰凝乳蛋白酶和胰弹性蛋白酶在切割天然折叠蛋白方面效率非常低，而在极低 pH 值 (pH 1.2) 下起作用的胃蛋白酶效率更高，这可能是由于变性条件所致从而在低 pH 值下更好地接近内部切割位点。热处理显着改善了所有三种胰酶的裂解，但酸处理后恢复到中性 pH 值没有任何重大影响。当蛋白质处于变性状态时，在低 pH 下切割显然是非常有效的。这表明胃蛋白酶是切割正确折叠的天然蛋白质的主要酶，并且胰酶主要参与生成单个氨基酸或非常短的肽，以便肠粘膜有效吸收。