58
Ν. G. PON
in these two classes of animals. Certain nutritional factors induce alterations in the hepatic enzymatic levels which are essentially similar to those
effects found in mammalian systems. Hence a chicken which has been
starved for 7 days loses 100% of its hepatic G-6-P DH and about 50% of its
6-PG DH. On refeeding the chicken, one finds that both of its hepatic
dehydrogenase levels markedly increase, G-6-P DH to 370% of the normal
fed chicken and 6-PG DH only to 83% of normal. These results are in
contrast to those of the rat liver system, where both dehydrogenases are
enhanced above normal. Pathological conditions may often regulate the
pentose phosphate activity in some manner. For instance, 48 hours after
the inoculation of canary pox virus into a chick embryo, a rise in the rate
of oxidation of the C-l of glucose in the chorioallantoic membrane occurs
(376). Viral infection apparently increases TPN-linked lactate dehydrogenase activity, thereby increasing the shunt activity, but neither the
level of G-6-P DH nor the level of 6-PG DH is affected. Tumors grown
in the chorioallantoic membrane by injection of Rous sarcoma virus into
embryonated eggs of White Leghorn chicks have G-6-P DH and 6-PG
DH activities 111% and 149%, respectively, of the value for the normal
chorioallantoic membrane (377). Isotope studies showed that these
tumors utilize glucose via the shunt mechanism to the extent of only 4%
or less in wing web and in breast muscle (380) as well as in the chorioallantoic membrane (381). The G-6-P DH activity in the chick embryo
fluctuates considerably as a function of incubation time so that any correlation is made difficult (378). The rate of utilization of ribose-5-phosphate by embryonic liver homogenates and slices seems to decrease
steadily from 8 days of incubation until 20 days. The rate of metabolism
of ribose phosphate by the embryonic liver slices on the 19th day of
incubation is nearly identical to that by the adult chicken liver and
amounts to approximately one-third the rate in the 12-day-old embryonic
liver. The G-6-P DH activity in the skin plus feather primordia of the
appendages varies from 1.9 units per milligram protein in the 12-day-old
chicken embryo to 3.1 units per milligram protein on the 16th day of
incubation, and thereafter decreases sharply to 0.9 units per milligram
protein at hatching (379).
Deoxyribose synthesis occurs in the chicken embryo homogenate at
the nucleotide level from the corresponding ribose compound (382).
This seems also to be the case in the formation of deoxyribose in the
embryonic chicken cartilage which has been cultivated in vitro on a
chemically defined medium (383). The nucleotide ribose, on the other
hand, arises via the action of the transketolase-transaldolase pathways
plus the direct oxidative pathway on glucose.
Ν. G. PON
in these two classes of animals. Certain nutritional factors induce alterations in the hepatic enzymatic levels which are essentially similar to those
effects found in mammalian systems. Hence a chicken which has been
starved for 7 days loses 100% of its hepatic G-6-P DH and about 50% of its
6-PG DH. On refeeding the chicken, one finds that both of its hepatic
dehydrogenase levels markedly increase, G-6-P DH to 370% of the normal
fed chicken and 6-PG DH only to 83% of normal. These results are in
contrast to those of the rat liver system, where both dehydrogenases are
enhanced above normal. Pathological conditions may often regulate the
pentose phosphate activity in some manner. For instance, 48 hours after
the inoculation of canary pox virus into a chick embryo, a rise in the rate
of oxidation of the C-l of glucose in the chorioallantoic membrane occurs
(376). Viral infection apparently increases TPN-linked lactate dehydrogenase activity, thereby increasing the shunt activity, but neither the
level of G-6-P DH nor the level of 6-PG DH is affected. Tumors grown
in the chorioallantoic membrane by injection of Rous sarcoma virus into
embryonated eggs of White Leghorn chicks have G-6-P DH and 6-PG
DH activities 111% and 149%, respectively, of the value for the normal
chorioallantoic membrane (377). Isotope studies showed that these
tumors utilize glucose via the shunt mechanism to the extent of only 4%
or less in wing web and in breast muscle (380) as well as in the chorioallantoic membrane (381). The G-6-P DH activity in the chick embryo
fluctuates considerably as a function of incubation time so that any correlation is made difficult (378). The rate of utilization of ribose-5-phosphate by embryonic liver homogenates and slices seems to decrease
steadily from 8 days of incubation until 20 days. The rate of metabolism
of ribose phosphate by the embryonic liver slices on the 19th day of
incubation is nearly identical to that by the adult chicken liver and
amounts to approximately one-third the rate in the 12-day-old embryonic
liver. The G-6-P DH activity in the skin plus feather primordia of the
appendages varies from 1.9 units per milligram protein in the 12-day-old
chicken embryo to 3.1 units per milligram protein on the 16th day of
incubation, and thereafter decreases sharply to 0.9 units per milligram
protein at hatching (379).
Deoxyribose synthesis occurs in the chicken embryo homogenate at
the nucleotide level from the corresponding ribose compound (382).
This seems also to be the case in the formation of deoxyribose in the
embryonic chicken cartilage which has been cultivated in vitro on a
chemically defined medium (383). The nucleotide ribose, on the other
hand, arises via the action of the transketolase-transaldolase pathways
plus the direct oxidative pathway on glucose.