24
Ν. G. PON
6-phosphogluconate dehydrogenase activity is higher in the islets than in
the acini. On the other hand, hyperglycemic rabbits have increased levels
of glucose-6-phosphate dehydrogenase in both tissues. The islets having
greater vascularity than the acini, this suggests that the islets have
greater capacity for aerobic metabolism than the acini. The investigators
concluded from these experiments that the pentose phosphate cycle
seems to be utilized in the metabolism of large amounts of exogenously
administered glucose.
Evidence was also obtained for the operation of the pentose cycle in
the insulin-producing tumors of human pancreas composed only of beta
cells (173). Addition, in vitro, of alloxan did not alter the rate of conversion of glucose to C0 2 . The role of the pentose phosphate pathway, in
this case, may be for protein synthesis via the generation of TPNH.
Slices of beef and calf pituitaries have been examined for pentose
phosphate cycle activity (137, 174, 175). In both cases the results were
positive as determined by the use of variously labeled glucose (174,
175) as well as by enzymatic assays (137). Insulin added to the incubation mixture increased a number of parameters, but did not change
the C-l:C-6 ratio (175). Using a number of assumptions, Dumont estimated "grossly" that 25% of the C0 2 from glucose comes by way of
the hexose monophosphate shunt (174).
There is ample evidence to indicate that the pentose phosphate cycle
is operative in the thyroid glands. In Table II are listed the mammalian
TABLE II
SOURCES OF THYROIDS AND EFFECTS OF SUBSTANCES ADDED in Vitro ON THE
PENTOSE PHOSPHATE CYCLE ACTIVITY
Source
Substance added
Effect
Reference
Sheep
Synkavite, monoiodotyrosine,
Increase
(176)
diiodotyrosine
Beef
Thyroid-stimulating hormone
Increase
(177)
Rat, fed
Thyroxine
Increase
(178)
Rat, fasted
Thyroxine
No effect
(178)
Rat
Propylthiouracil, small dose
Decrease
(179)
thyroid-stimulating hormone
Increase
(179)
Dog
Acetylcholine
No effect
(180)
Calf and dog
Epinephrine
Increase
(181)
Beef, pig,
Serotonin, tryptamine,
Increase
(182)
and dog
5-methoxytryptamine
sources of the thyroid and the substances that affect the pentose phosphate activity in this tissue. Conclusions are based mainly on a measure
of the C-l:C-6 ratio of C0 2 from the oxidation of glucose labeled in the
Ν. G. PON
6-phosphogluconate dehydrogenase activity is higher in the islets than in
the acini. On the other hand, hyperglycemic rabbits have increased levels
of glucose-6-phosphate dehydrogenase in both tissues. The islets having
greater vascularity than the acini, this suggests that the islets have
greater capacity for aerobic metabolism than the acini. The investigators
concluded from these experiments that the pentose phosphate cycle
seems to be utilized in the metabolism of large amounts of exogenously
administered glucose.
Evidence was also obtained for the operation of the pentose cycle in
the insulin-producing tumors of human pancreas composed only of beta
cells (173). Addition, in vitro, of alloxan did not alter the rate of conversion of glucose to C0 2 . The role of the pentose phosphate pathway, in
this case, may be for protein synthesis via the generation of TPNH.
Slices of beef and calf pituitaries have been examined for pentose
phosphate cycle activity (137, 174, 175). In both cases the results were
positive as determined by the use of variously labeled glucose (174,
175) as well as by enzymatic assays (137). Insulin added to the incubation mixture increased a number of parameters, but did not change
the C-l:C-6 ratio (175). Using a number of assumptions, Dumont estimated "grossly" that 25% of the C0 2 from glucose comes by way of
the hexose monophosphate shunt (174).
There is ample evidence to indicate that the pentose phosphate cycle
is operative in the thyroid glands. In Table II are listed the mammalian
TABLE II
SOURCES OF THYROIDS AND EFFECTS OF SUBSTANCES ADDED in Vitro ON THE
PENTOSE PHOSPHATE CYCLE ACTIVITY
Source
Substance added
Effect
Reference
Sheep
Synkavite, monoiodotyrosine,
Increase
(176)
diiodotyrosine
Beef
Thyroid-stimulating hormone
Increase
(177)
Rat, fed
Thyroxine
Increase
(178)
Rat, fasted
Thyroxine
No effect
(178)
Rat
Propylthiouracil, small dose
Decrease
(179)
thyroid-stimulating hormone
Increase
(179)
Dog
Acetylcholine
No effect
(180)
Calf and dog
Epinephrine
Increase
(181)
Beef, pig,
Serotonin, tryptamine,
Increase
(182)
and dog
5-methoxytryptamine
sources of the thyroid and the substances that affect the pentose phosphate activity in this tissue. Conclusions are based mainly on a measure
of the C-l:C-6 ratio of C0 2 from the oxidation of glucose labeled in the