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

IV. Porphyrias
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to localize the metabolic lesion in the erythropoietic tissue.
They also reported that the fluorescence was seen only
in morphologically abnormal nucleated erythrocytes that
contained abnormal nuclear inclusions. Similar nuclear
abnormalities were observed in bovine CEP bone marrow
( Watson et al. , 1959 ). Schmid et al. (1955) concluded that
there were two populations of erythrocytes, one normal
and one containing free porphyrins.
The presence of two populations of erythrocytes was
reported in humans, but this was attributed to the intermittent
hemolytic crises that occurred ( Gray et al. , 1950 ).
Runge and Watson (1969) , who were studying fluorescing
bovine CEP bone marrow cells after bleeding, also concluded
that there was only a single population of erythrocytes
in bovine CEP.
The hematology of newborn CEP calves also has striking
differences from that of older CEP calves and cows
( Kaneko and Mills, 1970 ). There is an intense erythrogenic
response in the neonatal CEP calf, which persists
for the first 3 weeks of life. Nucleated erythrocyte counts
during the first 24 h of life ranged from 5000 to 63,500 μ l.
Reticulocyte counts were lower than expected (6.4%) and
increased to a peak of only 12.5% at 4 days of age. The
persistent reticulocytosis is thought to be due to a delay
in maturation of the reticulocytes ( Rudolph and Kaneko,
1971 ; Smith and Kaneko, 1966 ). This delay in maturation
of the reticulocyte, which is proportional to the degree of
anemia, is now a well-established phenomenon during the
reticulocyte response to a blood loss or hemolytic anemia.
In essence, this delay represents the increase in survival
time of the reticulocyte beyond its normal 1-day survival
time. This increased survival time is the now commonly
used maturation correction factor (MCF) for estimating the
reticulocyte production index (RPI) when evaluating the
response to an anemia.
d . Mechanism of the Anemia
A responsive hemolytic anemia is a well-established occurrence
in CEP. Erythrocyte porphyrins are high in CEP, and
if these erythrocytes with high porphyrin concentrations
were more susceptible to destruction, a shortening of their
life span would be expected. Erythrocyte life span is shortened
in bovine ( Kaneko, 1963 ) and in human CEP ( Gray
et al. , 1950 ). There is general agreement that this shortening
of life span is associated with the hemolytic process,
but the mechanism of the hemolysis remains obscure. It
has been shown that erythrocyte survival in bovine CEP
is inversely correlated with erythrocyte coproporphyrin
concentration ( Kaneko et al. , 1971 ). The shortest erythrocyte
survival time of 27 days (normal 150 days) was
associated with the highest erythrocyte coproporphyrin
concentration. The porphyrins through their lipid solubility
are presumed to damage the erythrocyte membrane
leading to the hemolysis. In vivo 59 Fe metabolic studies
were completely compatible with a hemolytic type of
anemia, and ineffective erythropoiesis (i.e., bone marrow
hemolysis) was also demonstrated ( Kaneko, 1963 ; Kaneko
and Mattheeuws, 1966 ). Plasma iron turnover and transfer
rates, erythrocyte iron uptake, and organ uptakes were
increased as expected in a hemolytic process.
The mechanism of cell damage has also been studied
in reticulocytes and in nucleated erythrocytes. A biochemical
defect in the bovine CEP reticulocyte in vitro was
expressed as an increase in porphyrin synthesis, a marked
decrease in heme synthesis and a delay in the maturation
time of the reticulocyte ( Smith and Kaneko, 1966 ). The T 1/2
for the maturation of the reticulocyte was 50 h compared to
a normal of 3 to 10 h. This delay in reticulocyte maturation
is thought to be the direct result of the defect in heme synthesis,
because the rate of heme synthesis controls the rate
of maturation of the reticulocyte ( Schulman, 1968 ). This
means that there is an increase in the reticulocyte survival
time inversely proportional to the degree of anemia.
A similar delay in the maturation of the metarubricyte
to the reticulocyte was observed in the bone marrow cells
of CEP cows ( Rudolph and Kaneko, 1971 ), but there was
no effect on the earlier stages of nucleated erythrocytes.
Therefore, the more mature erythrocytic cells are the cells
most affected by the high porphyrin content. This is not
surprising because heme and hemoglobin synthesis are
most active in the later stages of erythrocytic cell development.
Ultimately, the accumulation of porphyrins in these
cells, whether in bone marrow or in blood, induces hemolysis.
Upon exposure of surface capillaries to sunlight,
photohemolysis of the type observed in erythropoietic protoporphyria
( Harber et al. , 1964 ) would further aggravate
the hemolysis.
This hemolytic mechanism might also explain the striking
erythrogenic response seen in the neonatal porphyric
calf. Because most of the porphyrins are within the fetal
erythrocytes and these would not normally cross the placenta,
the porphyrin containing erythrocytes would accumulate
in the fetus and a profound hemolysis would occur
in utero. This hemolysis in turn would induce a marked
erythrogenic response in the fetus, and this is observed at
birth. At birth, porphyrins are high, but they fall to their
steady state level in about 3 weeks in CEP calves. This is
comparable to the rate of clearance of 14 C-porphyrin into
urine, which fell to 0.1% of the initial concentration in 3
weeks. Furthermore, 3 weeks is also the time at which the
erythrogenic response is stabilized at a steady state level in
the neonatal calf ( Kaneko and Mills, 1970 ).
In summary, as a result of the heme synthetic defect in
erythropoietic porphyria, there is excess porphyrin accumulation
in the mature and developing erythrocytes, which
induces their hemolysis in the circulation or in the bone
marrow. There is a corresponding shortening of erythrocyte