タンパク質の吸収

Proteins originate from three sources: dietary food, disintegrated cells lining the gastrointestinal tract, and secreted enzymes. This mix of protein sources enters the stomach where initial breakdown begins, ensuring a continuous supply of protein for digestion and absorption throughout the small intestine.

Pepsin is the primary enzyme in the stomach that breaks down proteins into smaller fragments called polypeptides. This initial enzymatic cleavage is essential for preparing proteins for further breakdown in the small intestine by pancreatic and intestinal enzymes. The resulting polypeptides then move into the duodenum as part of chyme.

Pancreatic proteases like trypsin and chymotrypsin, along with intestinal brush border enzymes such as carboxypeptidases, work sequentially to dismantle polypeptides into tripeptides, dipeptides, and free amino acids. This coordinated enzymatic action maximizes protein breakdown for efficient absorption by enterocytes lining the small intestine.

Amino acids cross the apical membrane using sodium ion-dependent symporters through active transport. Dipeptides and tripeptides use proton-dependent symporters. Short chains are then converted to free amino acids, which exit the basolateral cell membrane via facilitated diffusion into the villi capillaries.

Short peptide chains must be converted to free amino acids to exit the basolateral cell membrane efficiently via facilitated diffusion. This conversion ensures amino acids can enter the villi capillaries and reach the liver for general circulation and metabolic use throughout the body.

During starvation, amino acids can be oxidized via the TCA cycle or converted to glucose through transamination and keto acid modification. This metabolic flexibility allows the body to use amino acids for energy production when carbohydrate and lipid stores are depleted, supporting metabolic states of the body fasting and starvation.

Protein absorption occurs primarily in the jejunum and ileum, where enterocytes lining these regions absorb the dismantled amino acids, dipeptides, and tripeptides. These segments of the small intestine are specialized for nutrient absorption and contain the necessary transport mechanisms for efficient protein uptake and delivery to circulation.