bilirubiiniprosessin

Bilirubiiniprosessi, also known as bilirubin metabolism, refers to the complex biochemical pathway responsible for the breakdown and excretion of bilirubin, a yellowish pigment that is a byproduct of heme catabolism. Heme is primarily derived from the breakdown of red blood cells, specifically from hemoglobin.

The process begins when old or damaged red blood cells are removed from circulation by macrophages, mainly in the spleen, liver, and bone marrow. Inside these macrophages, the heme molecule is broken down. First, heme oxygenase cleaves the porphyrin ring of heme, producing biliverdin, a green pigment, and carbon monoxide. Biliverdin is then reduced by biliverdin reductase to form bilirubin.

This initially formed bilirubin is unconjugated (or indirect) bilirubin. It is lipid-soluble and not water-soluble, which means it cannot be directly excreted by the kidneys. Therefore, unconjugated bilirubin binds to albumin, a protein in the blood, for transport to the liver.

In the liver, unconjugated bilirubin is taken up by hepatocytes. Within these liver cells, it undergoes conjugation with glucuronic acid, a process catalyzed by the enzyme UDP-glucuronosyltransferase. This forms conjugated (or direct) bilirubin, which is water-soluble and less toxic.

Conjugated bilirubin is then actively transported into the bile. From the bile, it moves into the small intestine. In the intestine, gut bacteria further metabolize conjugated bilirubin into urobilinogen. A portion of urobilinogen is oxidized to urobilin, which is responsible for the brown color of feces. Another portion of urobilinogen is reabsorbed into the bloodstream and excreted by the kidneys as urobilin, giving urine its characteristic yellow color.