Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 8 Porphyrins and the Porphyrias
for lead exposure. Erythrocyte ALA-D is strongly inhibited
by lead, and as a result, ALA rises in plasma and is
excreted in the urine. Measurement of ALA is difficult,
and the results lack sensitivity for low-level lead exposure;
therefore, instead of its substrate ALA, erythrocyte ALA-D
is more commonly assayed. Farant and Wigfield (1982) ,
using the ratio of ALA-D activities assayed at two different
pH’s, demonstrated ALA-D inhibition at blood lead
concentrations of 10 to 15 μ g/dl (0.50 to 0.70 μ mol/l). They
found this to be a highly sensitive and reliable index of the
blood lead concentration.
Coproporphyrin also rises in plasma and is excreted
in urine, and like ALA, it is also difficult to measure and
lacks sensitivity. Hence, this method is not used as an
index of lead poisoning.
FER-Ch is the second major enzyme that is strongly
inhibited by lead, and as a result, PROTO IX accumulates
in the erythrocytes. This PROTO IX is a zinc complexed
PROTO IX instead of the “ free ” PROTO IX. Piomelli et al .
(1982) found that erythrocyte zinc PROTO IX increased
when blood lead concentrations were at 15 to 18 μ g/dl
(0.75 to 0.85 μ mol/l).
It is not quite as sensitive an index of blood lead concentration
as ALA-D but is well below the diagnostic criteria
for lead poisoning. George and Duncan (1981) found
marked elevations in erythrocyte PROTO IX in experimental
lead poisoning in calves. Modern fluorometers
specifically designed to measure porphyrins have greatly
simplified the assay. For these reasons, the current test of
choice to monitor lead exposure is the blood zinc PROTO
IX concentration.
The final diagnosis of lead poisoning ultimately rests
on the measurement of blood lead concentration, and this
is best done using flame atomic absorption spectrophotometry.
In children, a blood lead concentration of 30 μ g/dl
(1.45 μ mol/l) is currently regarded as normal, but it has been
shown that zinc PROTO IX rises at blood lead levels of half
that amount ( Piomelli et al. , 1982 ). It is clear that the heme
synthetic pathway is affected at blood lead concentrations
well below that considered normal. Zook et al . (1970)
reported a normal range for blood lead in the dog of 10 to
50 μ g/dl (0.48 to 2.41 μ mol/l) and considered a blood lead
concentration of 60 μ g/dl (2.90 μ mol/l) diagnostic of lead
poisoning. In the domestic rabbit, the blood lead concentration
is reported to be 2 to 27 μ g/dl (0.10 to 1.30 μ mol/l)
( Gerken and Swartout, 1986 ). Therefore, lead concentrations
of 30 μ g/dl (1.45 μ mol/l) are considered diagnostic
of lead poisoning in the dog and in all animals.
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