Amino Acid Absorption

Overview – Amino Acid Absorption

• Nitrogen balance 
  • positive
    • amino acid (AA) intake > excretion
    • examples
      • growth
      • pregnancy
  • negative
    • AA excretion > intake
    • examples
      • kwashiorkor
      • starvation
      • infection
• AA absorption in intestine
  • proteins degraded by trypsin and pepsin
    • enteropeptidase cleaves trypsinogen (inactive) to trypsin (active)
  • absorbed as AA’s across the gut lumen with specific transporter for similar AA’s
    • e.g., transporter for basic AA’s, transporter for large neutral AA’s
  • deficiencies
    • lack of absorption with pancreatitis due to ↓ in degradative enzyme production
    • Hartnup disease
      • defect in large neutral AA transporter in the intestine and renal proximal tubule cells
      • result is a tryptophan deficiency
      • presents similar to pellagra
• AA absorption in kidney
  • AA’s that are filtered from the glomerulus can be actively reabsorbed in the proximal convoluted tubules with similar transporters as the gut
  • deficiencies
    • cystinuria
      • genetic defect in transporter for cysteine, ornithine, lysine, arginine 
      • AR
      • presentation
        • cystine staghorn calculi
          • note: cystine = 2x cysteine attached by a disulfide bridge
          • cystine kidney stones result from high concentrations in urine
      • treatment
• alkalinization of the urine with acetazolamide

Introduction

Amino acid absorption is the process by which dietary proteins are broken down into individual amino acids, which can then be transported across the intestinal wall and into the bloodstream for use by the body. Amino acids are the building blocks of proteins and are essential for a wide range of physiological processes, including the synthesis of new proteins, neurotransmitters, and hormones.

The process of amino acid absorption begins in the stomach, where dietary proteins are broken down into smaller peptides by digestive enzymes such as pepsin. These peptides are then further broken down into individual amino acids by enzymes in the small intestine, such as pancreatic proteases and peptidases.

Types

There are several types of amino acid absorption that occur in the body:

1. Active transport: Active transport is the primary mechanism for amino acid absorption in the small intestine. It requires energy in the form of ATP and involves specific transporter proteins located on the surface of intestinal cells. These transporter proteins actively transport amino acids across the intestinal wall and into the bloodstream, against a concentration gradient.
2. Facilitated diffusion: Facilitated diffusion is another mechanism for amino acid absorption that occurs in the small intestine. This process does not require energy and involves transporter proteins that allow amino acids to passively diffuse across the intestinal wall and into the bloodstream, down a concentration gradient.
3. Sodium-dependent transport: Sodium-dependent transport is a type of active transport that involves the co-transport of amino acids and sodium ions across the intestinal wall. Sodium-dependent transport is particularly important for the absorption of certain amino acids, such as proline and glycine, which cannot be transported efficiently by other mechanisms.
4. Peptide transport: Peptide transport is a type of active transport that involves the absorption of dipeptides and tripeptides, which are small chains of amino acids. These peptides are broken down into individual amino acids once they are transported into the intestinal cells.
5. Transcellular transport: Transcellular transport involves the transport of amino acids across the intestinal cells, rather than between cells. This process is particularly important for the absorption of certain amino acids, such as branched-chain amino acids, which are metabolized by the intestinal cells before being transported into the bloodstream.

Overall, the different types of amino acid absorption are important for ensuring that a wide range of dietary proteins can be broken down and absorbed efficiently by the body. The mechanisms involved in amino acid absorption are complex and involve a variety of transporter proteins and metabolic pathways.

Function

The function of amino acid absorption is to provide the body with the essential building blocks for protein synthesis and other physiological processes. Amino acids are required for the synthesis of new proteins, including structural proteins such as collagen and elastin, enzymes, hormones, and neurotransmitters.

The body requires a constant supply of amino acids to maintain normal physiological function, and adequate absorption of dietary amino acids is essential for meeting this requirement. Inadequate amino acid absorption can lead to a range of health problems, including protein malnutrition, impaired growth and development, and impaired immune function.

In addition to their role in protein synthesis, amino acids are also involved in a range of other physiological processes, including:

1. Energy metabolism: Amino acids can be used as a source of energy by the body. They can be broken down and used to produce ATP, the primary energy currency of the body.
2. Neurotransmitter synthesis: Amino acids such as glutamate and glycine are used as neurotransmitters in the brain and nervous system. The synthesis of these neurotransmitters requires an adequate supply of dietary amino acids.
3. Hormone synthesis: Amino acids are also required for the synthesis of hormones such as insulin, which regulates blood glucose levels, and growth hormone, which is involved in growth and development.

Overall, amino acid absorption is a vital process that plays a key role in maintaining normal physiological function. A balanced and varied diet that includes a range of protein sources can help to ensure that the body receives an adequate supply of dietary amino acids.

Complications

There are several complications associated with amino acid absorption, which can lead to health problems if left untreated. Some of the common complications include:

1. Malabsorption syndrome: Malabsorption syndrome is a condition in which the body is unable to absorb nutrients, including amino acids, from food properly. This can be caused by a variety of factors, including damage to the intestinal lining, pancreatic insufficiency, and certain medications. Malabsorption syndrome can lead to protein malnutrition, which can cause weight loss, fatigue, and impaired immune function.
2. Celiac disease: Celiac disease is an autoimmune disorder that causes damage to the intestinal lining in response to gluten, a protein found in wheat, barley, and rye. This damage can lead to impaired absorption of amino acids and other nutrients, leading to malnutrition and other health problems.
3. Short bowel syndrome: Short bowel syndrome is a condition in which a significant portion of the small intestine is removed surgically. This can lead to impaired absorption of amino acids and other nutrients, as the small intestine is responsible for the majority of nutrient absorption in the digestive system.
4. Inflammatory bowel disease (IBD): Inflammatory bowel disease, including Crohn’s disease and ulcerative colitis, can cause inflammation and damage to the intestinal lining, leading to impaired absorption of amino acids and other nutrients.
5. Genetic disorders: There are several genetic disorders that can affect amino acid absorption, including cystinuria and Hartnup disease. These disorders can cause impaired absorption of specific amino acids, leading to health problems such as kidney stones and skin rashes.

Overall, amino acid absorption complications can lead to a range of health problems if left untreated. Treatment typically involves identifying and addressing the underlying cause of the absorption problem, as well as ensuring an adequate supply of dietary amino acids through a balanced and varied diet or supplementation.

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