Metabolism - Transport
Bilirubin metabolism
Transport
Bilirubin released into the circulation by the reticuloendothelial cells is rapidly bound to albumin. 1 gram of albumin can bind 8.3 mg of bilirubin at the primary binding site; therefore a patient with 3.5 gm/dL of albumin can potentially bind 29 mg/dL of bilirubin. However, in the neonate, decreased albumin levels and the presence of substances competing with bilirubin for the same binding sites can reduce the binding capacity. This bilirubin-albumin binding capacity is of theoretic importance in defining the risk of bilirubin encephalopathy. However its measurement is technically difficult, its interpretation controversial and its clinical usefulness severely limited. Bilirubin exists in 4 different forms in serum:- unconjugated bilirubin reversibly bound to albumin, which makes up the major portion of unconjugated bilirubin in serum.
- a tiny fraction of unconjugated bilirubin NOT bound to albumin, the "free" bilirubin; this unbound, lipid-soluble fraction is capable of crossing membranes and is potentially neurotoxic.
- conjugated bilirubin, water-soluble and easily excreted in both urine and bile.
- conjugated bilirubin covalently bound to albumin, the so-called "delta" bilirubin; this fraction is virtually absent in the first 2 weeks of life, but accounts for a significant portion of the total bilirubin in patients with cholestatic jaundice.
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