CLINICAL CASES

181

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

glucose 1-phosphate.

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