Gastric digestion of protein through pancreozyme action optimizes intestinal forms for absorption, mucin formation and villus integrity

Abstract

Recovery of dietary protein proceeds through two phases of digestion before suitable forms exist for absorption. The first phase engages the gastric system where low pH weakens overall structures allowing pepsin to disrupt hydrophobic bonding and enhance aqueous compatibility. Secondly, trypsin, chymotrypsin and elastase proteolysis in conjunction with carboxypeptidases A and B cooperate to form a mixture of free amino acids and peptides that progressively arise in the small intestinal lumen. Free amino acids are dominated by the aromatic, aliphatic and basic ones while resulting peptides largely involve the nonessentials.Motility convectively transfers digestion products to the unstirred water layer of the upper villus where filtration limits entry to low molecular weight solutes that can be further digested by underlying enzymes to optimize membrane absorption. Mucin oligosaccharides are credited for creating a microenvironment having reduced pH’s ∼5.5–6.0 that favors enzymes finalizing digestion as well as absorption of two type peptides. This pH is speculated to optimize peptide forms having either a zwitterion-like charge for proton gradient transfer or be non-dissociated and passively diffused. A high frequency of peptides having either glycine or proline can be rationalized as providing particularly favorable electronic terms for peptide absorption by either approach while being non-competitive.The upper villus has first access to absorbed products where goblet cells assure continuance of mucin and continuity of the unstirred water layer. Glutamine is their dominant nutrient for synthesis of mucin oligosaccharides while also providing glutamic acid for either formation of its associated protein or is consumed for energy. Dietary cystine and threonine are frequently limiting, and their assurance is necessary for the mucin core while ready availability of glycine and/or serine together with proline also foster mucin formation. Unused absorbed products descend the villus in venules adjacent to the surface and provide nutrition as well as information about the lumen for adaptation of replacement cells. Sustaining the villus with absorbed nutrients takes priority for continuation of operational efficiency before their entry into the portal system and subsequent body use.