Digestive Enzymes and Gastrointestinal Hormones

The chemical breakdown of food into absorbable molecules is carried out by a series of digestive enzymes, each secreted at a particular point along the alimentary canal and each specific to one class of food substrate, and the rate and timing of these enzyme-rich secretions is itself coordinated by a small number of gastrointestinal (GI) hormones that respond to the actual arrival and composition of food. This section brings together, region by region, the principal enzymes covered individually in the digestive-gland and macronutrient sections of this chapter, and then takes up the three GI hormones named in the WBCHSE syllabus.

Digestion begins in the buccal cavity, where salivary amylase (ptyalin), secreted in saliva, acts on cooked or boiled starch, breaking it down partially to maltose; because this enzyme requires a near-neutral pH, its action is halted once food reaches the strongly acidic environment of the stomach. In the stomach itself, the chief action is proteolytic: pepsin (formed from the inactive precursor pepsinogen by the action of hydrochloric acid) breaks dietary proteins down into shorter polypeptide fragments called proteoses and peptones, while a small amount of gastric lipase acts on any already-emulsified fat (such as that in milk) to release some fatty acids and glycerol, though this is a minor contribution to overall fat digestion. In the duodenum, pancreatic juice supplies the most important battery of digestive enzymes in the body: trypsin and chymotrypsin (each activated from an inactive zymogen) continue protein digestion to smaller peptides, carboxypeptidase begins releasing free amino acids from the ends of these peptide chains, pancreatic amylase continues starch digestion (resuming what salivary amylase had begun and the stomach had interrupted) to maltose, and pancreatic lipase, greatly assisted by the emulsifying action of bile salts, hydrolyses triglycerides into free fatty acids and monoglycerides. Finally, at the brush border of the small intestine's absorptive epithelium, a further set of enzymes completes digestion at the very site of absorption: the disaccharidases maltase, sucrase and lactase split maltose, sucrose and lactose respectively into their constituent monosaccharides (glucose, fructose and galactose), while brush-border peptidases split the last small peptides into free amino acids ready for absorption. …