Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Digestion, Nutrient Transport, and Metabolic Interrelationships
There are also five metabolic pathways for amino acids

Amino Acids absorbed in the intestine and subsequently delivered to the Liver also undergo several Major Metabolic Pathways (Fig. 24-10).

a. Transport to other Tissues

Amino acids from the liver can enter the Circulatory system and be distributed to other Organs, where they serve as building blocks for tissue METABOLISM/35.html">Protein Biosynthesis (Ch. 29).

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Fig. 24-10. Pathways of Amino acid metabolism in the liver.

b. Biosynthesis of liver and Plasma Proteins

Liver proteins undergo continuous turnover, characterized by a remarkably High Metabolic Rate with a mean half-life of only a few days. Furthermore, the liver is the primary site of synthesis for most plasma proteins.

c. Deamination and degradation

Amino acids not utilized for protein biosynthesis in the liver or other organs undergo deamination and degradation to yield acetyl-CoA and Citric Acid Cycle intermediates (Sec. 22.21). The latter can be converted into glucose and Glycogen via Gluconeogenesis (Sec. 20.1). Acetyl-CoA is either oxidized in The Citric Acid Cycle to generate energy stored as ATP or converted into Lipids, which, as described above, are stored as reserves. Ammonia released during Amino Acid Breakdown is converted into the excretory product urea via The Urea Cycle, which takes place in the liver (Sec. 19.15).

d. Participation in the glucose-Alanine cycle

The liver also participates in the metabolism of amino acids intermittently released from peripheral tissues. Several hours after each meal, alanine is transported from the Muscles to the liver, where it undergoes deamination, and the resulting Pyruvate is converted into Blood glucose via gluconeogenesis (Sec. 19.12). The glucose returns to Skeletal Muscle to replenish its glycogen stores. One of the Functions of this cyclic process, known as the glucose-alanine cycle, is to buffer fluctuations in blood glucose levels between meals. Immediately following the Digestion AND ABSORPTION of dietary CARBOHYDRATES, as well as The conversion of a portion of liver glycogen into glucose, sufficient glucose enters the bloodstream. However, during the postabsorptive period preceding the next meal, partial degradation of Muscle Proteins yields amino acids, which transfer their amino groups to the glycolytic product pyruvate via Transamination to form alanine. Thus, both pyruvate and NH₃ are delivered to the liver in the form of alanine. In the liver, alanine is deaminated, the resulting pyruvate is converted into glucose for release into the blood, and NH₃ is incorporated into urea and excreted. The resulting amino acid deficit in muscles is subsequently replenished after a meal by the absorption of dietary amino acids.

e. Conversion into NUCLEOTIDES and other products

Amino acids serve as precursors in the Biosynthesis of Purine and pyrimidine nucleotide bases (Sec. 22.13), as well as in the synthesis of specialized substances such as Porphyrins (Sec. 22.10), Hormones, and other nitrogenous compounds. These PATHWAYS OF AMINO acid metabolism are summarized in Fig. 24-10.