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Core Concepts: Bilirubin Metabolism
Thor Willy Ruud Hansen
NeoReviews 2010;11;e316-e322
DOI: 10.1542/neo.11-6-e316

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core concepts

Core Concepts:

Bilirubin Metabolism

Thor Willy Ruud Hansen,

Abstract

MD, PhD*

Bilirubin is formed in the reticuloendothelial system as the end product of heme
catabolism through a series of oxidation-reduction reactions. The predominant
bilirubin isomer in humans is IX-alpha (Z,Z), which, because of its lipophilic nature,
cancross phospholipid membranes. In fetal life, this characteristic permits passage of
bilirubin through the placenta into the maternal organism for excretion. Postpartum,
this same characteristic enables passage of bilirubin across the blood-brain barrier,
which is why clinicians worry about jaundice in newborns. Bilirubin is transported in
serum bound to albumin. When the bilirubin-albumincomplex reaches the liver,
bilirubin is transferred into the hepatocytes, where it is bound to ligandin. The next
step, which occurs inside the hepatocyte, is binding of bilirubin to glucuronic acid
(conjugation) through the enzyme uridine diphosphate glucuronyl transferase
(UDPGT). Both ligandin and UDPGT have very low concentrations and activities in
the fetus, but activity increases greatlyafter birth. However, during the time required
to increase these enzyme activities, bilirubin accumulates. An important factor in this
process is increased bilirubin production through the breakdown of fetal erythrocytes.
Once conjugated in the liver, bilirubin is excreted into the bile and transported
through the gut with food and further broken down, contributing to the color of
stool.Deconjugation and reabsorption of bilirubin can occur in the bowel, a process
known as enterohepatic circulation. Increased enterohepatic circulation is believed to
contribute to prolonged jaundice in some newborns and may be partially responsible
for human milk-associated jaundice. Some of the steps in bilirubin metabolism can be
influenced by drugs or feeding.

Author Disclosure
Dr Hansenhas
disclosed no financial
relationships relevant
to this article. This
commentary does not
contain a discussion
of an unapproved/
investigative use of a
commercial product/
device.

Objectives
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After completing this article, readers should be able to:

Recognize the different steps in bilirubin metabolism.
Describe how bilirubin metabolism changes from fetal topostnatal life.
Explain why bilirubin accumulates in the newborn.
Describe how the isomers of bilirubin may be relevant to its distribution in the body.
Explain why the enterohepatic circulation of bilirubin may influence the course of
neonatal jaundice.

Definition
Neonatal jaundice occurs when the concentration of bilirubin in serum (TSB) increases
to the point where the accumulation ofbilirubin in skin becomes visible to the unaided eye
in daylight conditions (or similar-quality artificial light). Such visual detection is possible
when the TSB exceeds 5 to 6 mg/dL (85 to 100 mcmol/L) and varies between observers
and lighting conditions. A distinction often is made between physiologic and nonphysiologic jaundice. This distinction is useful didactically but often not possible...