The second enzyme in the heme pathway, ALAD, is very sensitive to
inhibition by heavy metals, for example, lead. Which of the following
test results would distinguish lead-based poisoning from acute inter-
A. Decreased serum and urinary ALA and PBG
B. Increased serum and urinary ALA and PBG
C. Increased serum and urinary ALA, decreased serum and urinary
D. Decreased serum and urinary ALA, increased serum and urinary
A nsw ers
C. Given the patient’s symptoms, neurologic
cutaneous, the main
porphyrias to consider are hereditary coproporphyria and variegate
porphyria. Both HCP and VP show elevated ALA and PBG during
acute attacks, so urine and serum tests for these molecules would not
distinguish. Similarly both disorders show coproporphyrin in stool.
In this case, only a spectrofluorometric assay to distinguish between
coproporphyrin and protoporphyrin in plasma would be sufficient.
C. Variegate porphyria patients present with the same, but generally
milder, symptoms as AIP. The emission wavelength at 626 nm is
characteristic of protoporphyrin IX; the compound is responsible for
the skin sensitivity not observed in AIP. PCT can also be ruled out
because the characteristic fluorescence emission for uroporphyrin is
615 nm. Homozygous deletion of PPO is unlikely because the
patient’s symptoms were triggered by stress in the form of caloric
restriction. Patients with homozygous deficiency in the heme synthe-
sis enzymes show early onset and more severe symptoms.
C. An increase in urine/serum ALA without concomitant increase in
PBG indicates disrupted activity of ALA dehydratase. ALA alone can
cause the same neurovisceral symptoms as ALA and PBG together
cause in AIP. ALAD-deficiency porphyria is an extremely rare auto-
somal recessive disorder.