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SHORE ET AL.
tained constant in the cell-free systems employed in these studies,
incorporation is expressed as '‘C—Phe incorporated into the A.I.F.
Incorporation of '{C—Pheinto the A.I.F. in fetuses of the fed group
was reduced by irradiation (p < 0.05). Similarly, irradiation decreased
4C_Phe incorporation in the starved group (p< 0.05). Thus, the data
indicate that in all cases fetal incorporation of '“C—Phe into the A.LF.,
as measured by the cell-free system, was reduced by fetal exposure to
X rays. Incorporation in sham-irradiated animals was significantly
reduced by food and water deprivation (p < 0.05), However, incorporation in irradiated animals was not affected by food and water deprivation. It was not determined whether fetuses derived from starved irradiated animals differed nutritionally from fetuses derived from fed
irradiated animals. Thus the results indicate that, while maternal food
and water deprivation decreases incorporation of 4C_ Phe in the fetus,
irradiation causes a still further decrease in the incorporation of
'C_Phe into fetal protein.
Irradiation was acccmpanied not only by a depression in in vitro
incorporation but also by a decrease in the fetal growth rate. Thus,
expressed in terms of incorporation per whole fetus (incorporation per
gram * fetal weight), the differences were greater than those based on
incorporation per unit weight of fetal tissue.
It should be noted that the data presented in this study measure
incorporation of a labeled precursor into protein. An effort has been
made to estimate (in vivo studies) or control (in vitro studies) the
amount of label available for incorporation. However, no information
was obtained on precursor pool size or precursor specific activity.
Thus, it is not possible at this time to equate decreased incorporation
of amino acid into the A..F. with decreased protein synthesis. The
decreased growth of the irradiated fetuses suggests that the total pool
of protein in the irradiated fetuses did not increase to the same degree
that it did in the sham-irradiated fetuses during the 24-hr period following irradiation. Such a finding would be consistent with either de-
creased synthesis of protein or an increased rate of degradation of
protein with no change in the normal pattern of protein synthesis as a
consequence of irradiation,
The difference between incorporation in sham- and X-irradiated
fetuses was small though significant in the in vivo studies with starved
animals. In the studies with the cell-free system derived from shamand X-irradiated fetuses, the differences were much greater. The
latter studies examined incorporation in the post mitochondrial supernatant and thus measure cytoplasmic incorporation to the exclusion of
mitochondrial and nuclear incorporation of labeled precursor into the
A.LF, Studies currently in progress are investigating the effect of fetal
irradiation on cytoplasmic, nuclear, and mitochondrial protein synthesis.