Expansion
Pepsin in the stomach, pancreatic proteases in the duodenum, brush border peptidases last
Expansion
- Stomach: pepsin, activated from pepsinogen by acid, works best at pH 2. It is not essential; gastrectomy does not cause protein malnutrition
- Duodenum: enterokinase (enteropeptidase) on the brush border converts trypsinogen to trypsin, which then activates chymotrypsinogen, procarboxypeptidase, proelastase and more trypsinogen, an autocatalytic cascade
- Brush border and cytosolic peptidases complete digestion
Absorption
- Free amino acids by several sodium-dependent carriers, grouped by side chain
- Di- and tripeptides by the PepT1 transporter, which is actually the more efficient route, and are then hydrolysed inside the cell
Two illustrative disorders: Hartnup disease affects neutral amino acid transport, causing a pellagra-like picture from tryptophan loss; cystinuria affects dibasic amino acid transport in both gut and kidney, causing cystine stones. Both spare peptide-bound amino acids, which is why nutrition is largely preserved.