A method for the estimation of gastrointestinalluminal proteolysis of biologically-important dietary peptides

Abstract

Milk from several species contains a variety of biologically-important polypeptides which may function either within the developing gastrointestinal tract or at distant body sites following absorption from the gut. Because the capacity to perform such funtions may be dependent in part upon the nature of luminal digestion of the peptide, a method for estimating digestive susceptibility and for preparing generated degradation products for further study is desirable. A method is described in which a radiolabelled peptide is incubated in vitro with fluid flushed from the intestinal lumen of rats, followed by detection of degradation products soluble in acid. Using casein as substrate, rat small intestinal proteolysis was shown to be optimal at pH 7.4 in the presence of 5–10 mM CaCl 2. In contrast to previous reports of gastrointestinal deiodination in vivo, no generation of inorganic iodine occurs during the in vitro reaction with luminal fluid. Using this assay, the substantially greater digestibility of bovine casein compared to lactoferrin was demonstrated. The method is simple, reproducible, inexpensive, and requires only small amounts of luminal fluid. Application to any dietary polypeptide and gastrointestinal fluid should be possible.