Digestion of Proteins - Pancreatic Phase

Digestion of Proteins - Phase 2: Pancreatic Proteases

• Trypsinogen is released from pancreatic acinar cells.
• Trypsinogen autocatalyzes to form trypsin.
• Trypsin activates other pancreatic zymogens, chymotrypsinogen, proelastase, and procarboxypeptidases.
• Chymotrypsin, elastase, and carboxypeptidases are activated to break down proteins into smaller peptides.
• Duodenal cells release cholecystokinin-pancreozymin (CCK-PZ) in response to proteins in the duodenum.
• Duodenal and pancreatic cells secrete secretin in response to acid.
• Secretin stimulates the production of bicarbonate ions (HCO3-) in pancreatic juice, which neutralizes the acid from the stomach.

Digestion of Proteins - Phase 3: Brush Border

• Brush border enzymes, endopeptidases and aminopeptidases, hydrolyze peptides into amino acids.
• Amino acids, di-, tripeptides are absorbed into epithelial cells.

Digestion of Proteins - Phase 4: Absorption

• Na+-dependent co-transport and facilitated diffusion are involved in amino acid absorption.
• Dipeptides and tripeptides are absorbed from the intestinal lumen into intestinal epithelial cells via brush border.
• Dipeptidases and tripeptidases hydrolyze di- and tripeptides into amino acids.

Protein Turnover

• Proteins are constantly synthesized and broken down - Turnover
• The turnover rate depends on the protein type.
• Proteins with regulated concentrations (e.g. hormones) or that act as signals have a high turnover rate.
• Non-growing healthy adults with stable weight: Protein synthesis and breakdown are balanced - Nitrogen (N) Balance.

Measuring Protein Turnover

• N balance can be determined by examining the intake and output of nitrogen (measured by urinary urea nitrogen)
• Tracer methods like stable and radioactive tracers are used to determine amino acid kinetics.
• Isotopically labeled tracers are used to determine the rates of amino acid release from protein breakdown and the rates of amino acid uptake for protein synthesis.

Protein in Disease and Protein Controversies

• Disease (infection, burn, surgery, trauma): Body protein is mobilized for fuel and gluconeogenesis.
• Old and chronic illness: Protein loss increases due to increased metabolic rate.
• Supplementation with specific amino acids (e.g. glutamine, arginine) may be beneficial for specific conditions.
• High protein intake in individuals with renal failure can worsen their condition.
• High protein diet for weight loss: There are controversies about the effectiveness and safety of high protein diets.
• Protein intake and bone health: High protein intake may have negative effects on bone health.

This quiz covers the enzymatic digestion of proteins during the pancreatic phase. It includes the role of trypsin and other pancreatic proteases, the action of duodenal hormones, and the involvement of brush border enzymes in the absorption of amino acids. Test your understanding of these crucial biochemical processes.