Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Proteins
Intratissue Protein Breakdown

Alongside METABOLISM/35.html">Protein Biosynthesis, Cells constantly undergo protein degradation (proteolysis) mediated by Proteolytic Enzymes. Every cellular protein has a specific half-life—ranging from a few minutes to several weeks or more. Protein breakdown yields either Amino Acids or low-molecular-weight Peptides, which can be reused to synthesize other protein molecules or to modulate this metabolic pathway.

Intracellular proteolysis occurs primarily within specialized Organelles called Lysosomes, which contain a diverse array of hydrolytic enzymes.

Lysosomes harbor about 50 hydrolytic enzymes, including various proteinases known as cathepsins. Many of these enzymes exhibit high activity in an acidic environment (pH 5.0). Neutral and alkaline proteinases have also been identified in Skeletal Muscle, Brain, erythrocytes, and other Tissues.

Enhanced lysosomal protein degradation is observed in numerous functional and pathological metabolic alterations. For instance, various physical exercises activate lysosomal proteolysis of Proteins in skeletal Muscles, Liver, and other tissues—particularly in untrained individuals—which is associated with adaptive metabolic remodeling in these tissues. In trained organisms, physical exertion triggers significantly less breakdown of intracellular proteins. Protein degradation in lysosomes is also accelerated during starvation, as well as in conditions such as Diabetes Mellitus and rheumatoid Arthritis, ultimately leading to dystrophic states.

In addition to the lysosomal pathway, cells feature an ATP-dependent degradation mechanism for foreign and endogenous proteins, involving a specific protein (provisionally designated as "ubiquitin") whose precise role is still under investigation.

Accelerated breakdown of interstitial proteins leads to elevated levels of simple proteins (albumins and globulins) as well as specific amino acids in the Blood, which can serve as biomarkers for these processes.

The Catabolism of Conjugated Proteins involves distinct pathways for the transformation of their prosthetic groups. For example, Hemoglobin—a chromoprotein featuring a pigmented prosthetic heme group—is degraded within Cells of the reticuloendothelial system (Bone Marrow, Spleen, liver) into the protein moiety (globin) and the prosthetic group (heme). Heme loses its iron atom and is converted into Bile pigments, namely biliverdin and bilirubin. From the liver, bilirubin is excreted into the intestine via bile and partially eliminated from the body. A portion of its metabolic products is reabsorbed into the bloodstream and subsequently excreted in the urine as pigments, specifically urobilinogen and urobilin.