Topics of the Lecture

• Overview of liver function, which encapsulates vital roles including detoxification, metabolism, and storage.

• The role of the liver in nutrition as it processes nutrients absorbed from the digestive tract.

• Unique metabolic pathways in the liver that enable it to perform specialized biochemical functions, pivotal for overall health.

• Biotransformation processes that modify chemical compounds for elimination.

• Metabolism of bilirubin, crucial for preventing toxicity from hemolysis of red blood cells.

• Methods to measure liver function through various laboratory tests and clinical assessment.

The Role of the Liver in Carbohydrate Metabolism

• The concentration of glucose in the blood is maintained at a constant level, specifically in the range of $4-6 ext{ mM}$. This homeostasis is achieved through the precise regulation of glucose-producing and glucose-utilizing pathways in the liver, ensuring a steady energy supply.

• Sources of glucose production in the liver include:

  1. Glycogenolysis - This is the enzymatic process through which glycogen (stored glucose) is broken down into glucose molecules, primarily occurring during fasting or physical exercise to maintain blood sugar levels.

  2. Gluconeogenesis - This metabolic pathway synthesizes glucose from non-carbohydrate precursors, such as amino acids and glycerol, particularly crucial during prolonged fasting.

  3. Fructose and galactose metabolism - The liver plays a significant role in converting fructose and galactose into intermediates like glucose-6-phosphate, enabling their entry into glycolysis or gluconeogenesis.

  4. Cori cycle - This cycle, vital during exercise, involves the conversion of lactate produced in muscles back into glucose in the liver via gluconeogenesis, thereby recycling nutrients.

  5. Alanine cycle - This process involves the conversion of alanine (derived from protein degradation) back into glucose, ensuring a continuous supply of energy, while the nitrogen is converted into urea for excretion.

Cori Cycle and Alanine Cycle

Cori Cycle:

• Lactate produced from anaerobic glycolysis in muscle cells is transported to the liver.

• In the liver, lactate undergoes gluconeogenesis to be converted back into glucose.

• After its synthesis, glucose is released into the bloodstream, where it is utilized by tissues, thus maintaining energy homeostasis.

Alanine Cycle:

• This cycle involves the degradation of proteins, where the amino groups from amino acids are transferred to pyruvate, forming alanine.

• Alanine is then transported to the liver where the liver deaminates it, converting the carbon skeleton into glucose and processing the nitrogen into urea for elimination from the body.

The Role of the Liver in Lipid Metabolism

• The liver serves as the primary site for the synthesis of fatty acids, neutral fats, ketone bodies, and cholesterol, all pivotal for maintaining energy balance and cellular structure.

Absorptive State:

• During this metabolic state, following food intake, the liver converts excess glucose into fatty acids through the formation of acetyl-CoA, facilitating energy storage.

• The liver retrieves fatty acids from lipids supplied by chylomicrons (fat transporters) that come from the intestine post-meal.

• These fatty acids are subsequently converted into neutral fats and phospholipids, which contributes to the formation of very-low-density lipoproteins (VLDL), responsible for transporting lipids throughout the body.

Postresorptive State:

• In this phase, when fasting, adipose tissue releases fatty acids into the bloodstream, providing an alternative energy source.

• The liver takes these fatty acids and performs oxidative degradation to produce acetyl-CoA.

• Acetyl-CoA can then be converted into ketone bodies, serving as an essential energy source, particularly for tissues such as the brain during prolonged fasting.

The Role of the Liver in Amino Acid Metabolism

• The liver is the primary site for amino acid degradation, resulting in the production and release of ammonia, a potentially toxic by-product.

Ammonia:

• Ammonia is highly toxic to cells, especially nerve cells, and can lead to severe neurological dysfunction if allowed to accumulate.

• Therefore, ammonia must be quickly inactivated and excreted from the body, primarily happening through the formation of urea, a less toxic compound.

Transamination Reaction:

• This vital biochemical reaction involves the transfer of amino groups between amino acids and keto acids, crucial for amino acid metabolism:

  • From alanine to pyruvate (Ala to Pyr):

    • $ext{Ala} + ext{α-ketoglutarate} \rightarrow ext{Glu} + ext{2-OG}$

  • From aspartate to oxaloacetate (Asp to OAA):

    • $ext{Asp} + ext{α-ketoglutarate} \rightarrow ext{OAA} + ext{Glu}$

Patient Case Study

• A patient exhibits decreased urea production accompanied by increased ammonia concentration in the blood, a clinical situation indicating impaired liver function regarding its ability to remove amino nitrogen from the body effectively, potentially leading to hepatic encephalopathy.