[Bilirubin metabolism (author's transl)]

Area of Science:

• Biochemistry
• Physiology
• Cell Biology

Background:

• Senescent red blood cells are the primary source of bilirubin production.
• Bilirubin is transported in plasma bound to albumin due to its low aqueous solubility.
• Hepatic uptake and conjugation are critical steps in bilirubin processing.

Purpose of the Study:

• To elucidate the metabolic pathway of bilirubin from red blood cell degradation to excretion.
• To understand the role of hepatic proteins and enzymes in bilirubin conjugation.
• To describe the transport mechanisms and physiological significance of bilirubin excretion.

Main Methods:

• Analysis of bilirubin production from senescent red blood cells.
• Investigation of bilirubin binding to plasma proteins and hepatic cytoplasmic proteins.
• Characterization of bilirubin conjugation by bilirubin UDP-glucuronyl transferase.
• Study of bilirubin transport across the canalicular membrane and intestinal reabsorption.

Main Results:

• 80-90% of daily bilirubin production originates from haemoglobin catabolism in senescent erythrocytes.
• Unconjugated bilirubin binds to albumin in plasma and is taken up by liver cells via non-ionic diffusion.
• Bilirubin UDP-glucuronyl transferase conjugates bilirubin to a water-soluble glucuronide in the smooth endoplasmic reticulum.
• Conjugated bilirubin is actively transported into bile canaliculi and efficiently excreted, with limited intestinal reabsorption of unconjugated forms.

Conclusions:

• The primary route of bilirubin metabolism involves erythrocyte degradation, hepatic conjugation, and biliary excretion.
• Hepatic uptake and conjugation are essential for detoxifying and eliminating bilirubin.
• The enterohepatic circulation of urobilinogen plays a minor role in overall bilirubin balance.