Progress report. Peptide absorption in man.

In this review it is hoped to trace the evolution of our concepts of the absorption of digestion products of dietary protein. Many of the ideas current today were readily accepted by the end of the last century but were ignored by orthodox physiologists during the first half of this century-concepts of sugar absorption suffered a similar fate. Although it has now become clear that brush border hydrolysis and consequent utilization of the sugar pump is the predominant method of sugar absorption it would seem that two major mechanisms are involved in peptide absorption: on the one hand hydrolysis of peptides by brush border enzymes with subsequent uptake of the liberated amino acids by specific amino acid transport systems, and on the other hand, uptake of peptides by mechanisms independent of the specific amino acid entry mechanisms, followed by intracellular hydrolysis. Historical Background Nineteenth century physiologists believed that dietary protein was absorbed in the form of polypeptides' 2'3, a view that seemed to be confirmed when Nolf4 and Messerli5 showed that 'peptones' produced by tryptic hydrolysis of protein disappeared from the lumen of the small intestine more rapidly than equivalent amounts of free amino acids. When Cohnheim demonstrated in 19016 that 'intestinal juice' was capable of hydrolysing peptones to amino acids, some early workers suggested that protein must be hydrolysed to amino acids before absorption took place. This hypothesis began to gain ground when all known free amino acids were detected in intestinal contents obtained during protein absorption in vivo7'8'9, and when studies in vivo showed that hydrolysates of protein (consisting of amino acids) disappeared rapidly from the lumen of the small intestine10'11. Furthermore, many of the investigators at the time speculated that free amino acids passed into the portal circulation during protein absorption as the non-protein nitrogen values in peripheral and portal plasma increased during absorption of amino acids12'14, protein hydrolysates, and whole protein15'16. When only amino acids were isolated from the portal circulation during protein absorption'7, the idea that protein was completely hydrolysed to amino acids within the intestinal lumen before absorption took place became the classical view of protein absorption (eg, Verzar and MacDougall, 193618). This view was held despite later observations that intraluminal peptidase activity was insufficient to account for the absorption of 'peptone' in the form of free amino acids'9. The concept was questioned by a number of workers who claimed that pep-tides entered the …