Branching enzyme: 1,4-a-Glucan branching enzyme; an enzyme that
removes an oligosaccharide of about seven glucosyl residues from the
nonreducing end of a glycogen chain and transfers it to another chain,
creating an a(1^6) glycosidic bond.
Debranching enzyme: A bifunctional enzyme that catalyzes two reactions
in the degradation of glycogen. It transfers a trisaccharide from the
nonreducing end of a four glucosyl residue branch of the glycogen mol-
ecule to the nonreducing end of the same or adjacent glycogen molecule
(oligo-1,4-1,4-glucantransferase activity). It also hydrolyzes the
a(1^6) linkage of the remaining glucosyl residue of the branch, releas-
ing free glucose (amylo-1,6-glucosidase activity).
Glycogen: The storage form of glucose in tissues. It is a large polysaccha-
ride composed of glucose residues in primarily a(1^4) glycosidic link-
ages with some a(1^6) branch points.
Glycogenesis: The synthesis of glycogen from glucose 1-phosphate.
Glycogenolysis: The breakdown of glycogen to glucose 1-phosphate (and
some small amount of free glucose).
Glycogen phosphorylase: The enzyme that causes the release of glucose
1-phosphate from glycogen. It accomplishes this by catalyzing a phos-
phorolysis of glucosyl residues from glycogen; that is, it breaks the
a(1^4) glycosidic bonds by adding inorganic phosphate and releasing
Glycogen synthase: The enzyme that causes the addition of glucosyl
residues to a growing glycogen molecule using UDP-glucose and releas-
ing inorganic pyrophosphate.
D ISC U SSIO N
Acute fatty liver of pregnancy (AFLP) is a rare (occurring in 1 in 7000 to
16,000 deliveries) but potentially fatal disease that typically develops during
the third trimester. Patients most commonly present with clinical and labora-
tory evidence of acute hepatic failure with decreased hepatic metabolic activ-
ity. Hypoglycemia, nausea and vomiting, jaundice, general malaise, elevated
blood pressure, disseminated intravascular coagulation, hemorrhage, infec-
tion, and encephalopathy are the most common clinical findings. The etiology
of the syndrome is not clear, although recent reports have linked some cases
of AFLP with a fetal inborn error in fatty acid metabolism. For the major-
ity of cases in which an inborn error in the fetus does not appear to play a role,
the cause of the disease is unknown.
One of the primary functions of the liver is to maintain blood glucose
levels. When blood glucose levels are high following a meal, the liver takes in
glucose via the high capacity, insulin-insensitive glucose transporter GLUT 2
and converts it into glycogen for storage, metabolizes it to pyruvate by glycol-
ysis, or uses it to produce NADPH and pentoses for biosynthetic processes via
the pentose phosphate pathway. When blood glucose levels drop after fasting or