in individuals who were exposed at younger ages, yet the globin from 33R,
who was exposed at 1 year of age, had a low content of isoleucine.
Radiation-induced lesions may become fixed more frequently in infants
than in adults owing to a reduced opportunity for excision-replication
repair when a higher percentage of stem cells is dividing.
These
observations are consistent with findings that the induction of leukemias
is higher in prenatal and younger persons exposed to X rays® and radiation
from an atomic blast’, respectively; however, more data are needed to
establish the extent to which radiation causes a higher isoleucine
substitution frequency in the hemoglobin of infants than adults.
The elevated isoleucine substitution frequency could arise by at
least two mechanisms; one is by an increase in translational errors and
another is by radiation-induced, base substitution mutations in
erythropoietic stem cells.
If the elevated iscleucine content in
hemoglobin of exposed individuals is caused primarily by translational
errors, both the a and g chains should have a higher isoleucine content
than corresponding controls.
On the other hand, base substitution somatic
mutations should produce a higher isoleucine content in the polypeptide
chain that is the product of the mutated gene because its mRNA would now
code for isoleucine.
These alternatives may not be readily distinguishable
in cases where clones of cells with mutant o and 2 chain polypeptides
appear in the same person.
The isoleucine substitution frequencies in
the separated a and 8g chains should equal the values obtained for the
globin; this expectation was found (Table 2) but the substitution frequency
was consistently higher in the g than in the o chain both in controls and