1. PENTOSE PHOSPHATE CYCLE
45
rats and insulinized diabetic rats has been adequately surveyed by Katz
(111) and no further discussion will be given here. Since fatty acid
synthesis is associated with the pentose phosphate cycle, it would seem
reasonable to assume that other conditions that give rise to fatty liver
might have an effect on the oxidation of glucose. Hence mice with a
congenital obese-hyperglycemia syndrome have microscopically obvious
fatty livers that oxidize glucose to C0 2 with a ratio of C-l: C-6 = 1.8
while lean normal littermates have livers that yield a C-l:C-6 ratio of 1.2
(308). However, the number of micrograms of glucose oxidized per
100 mg. of fat-free liver (dry weight) is, on the average, 17.7 from C-l
and 9.4 from C-6 in the obese mouse compared with normal values of
24.3 and 20.8, respectively. Therefore, the higher C-l: C-6 in obese mouse
can be accounted for by a less active TCA cycle. As in the case of many
other tissues, the addition of certain artificial electron carrier systems to
rat liver slices preferentially oxidizes glucose-l-C
14 to C
14 0 2 while only
mildly increasing the oxidation of G-6-C
14 to C
14 0 2 . Examples of these
compounds are methylene blue, phenazine methosulfate, and pyocyanine
(309). On the other hand, dinitrophenol, an uncoupler of oxidative
phosphorylation, increases both the oxidation of C-l and C-6 of glucose
although it decreases the conversion of glucose to glycogen and to fatty
acids.
12. Mammary Gland
Among the more extensively studied tissues, the lactating mammary
gland has the most active pentose phosphate cycle, this tissue being
capable of metabolizing glucose via this pathway to the extent of 40%
(111). The pioneering work on this tissue has been carried out by dock
and McLean, who showed that the specific activities of G-6-P DH and
6-PG DH (310), the levels of pyridine nucleotides (311), and the oxidation of glucose-l-C
14 (312) in the mammary gland all increase as the rat
progresses from pregnancy through the lactation and abruptly decrease
on involution of the glands. These observations suggest that the pentose
phosphate cycle has an active role in the lactating mammary gland.
[For more detailed discussion of this subject, this reviewer recommends
the reader to the widespread coverage by Glock and McLean (313) and
by Hansen and Carlson (314).]
Research on carbohydrate metabolism in mammary glands has fallen
mainly into two classes: in vitro experiments either with tissue slices
or homogenates, usually from rats, but occasionally from mice, albino
rabbits (315), and sheep (312); and in vivo experiments and perfusion
experiments with lactating cows. In the case of lactating cows, Black
et al., measuring the incorporation of C
14 from glucose-l-C
14 and from
45
rats and insulinized diabetic rats has been adequately surveyed by Katz
(111) and no further discussion will be given here. Since fatty acid
synthesis is associated with the pentose phosphate cycle, it would seem
reasonable to assume that other conditions that give rise to fatty liver
might have an effect on the oxidation of glucose. Hence mice with a
congenital obese-hyperglycemia syndrome have microscopically obvious
fatty livers that oxidize glucose to C0 2 with a ratio of C-l: C-6 = 1.8
while lean normal littermates have livers that yield a C-l:C-6 ratio of 1.2
(308). However, the number of micrograms of glucose oxidized per
100 mg. of fat-free liver (dry weight) is, on the average, 17.7 from C-l
and 9.4 from C-6 in the obese mouse compared with normal values of
24.3 and 20.8, respectively. Therefore, the higher C-l: C-6 in obese mouse
can be accounted for by a less active TCA cycle. As in the case of many
other tissues, the addition of certain artificial electron carrier systems to
rat liver slices preferentially oxidizes glucose-l-C
14 to C
14 0 2 while only
mildly increasing the oxidation of G-6-C
14 to C
14 0 2 . Examples of these
compounds are methylene blue, phenazine methosulfate, and pyocyanine
(309). On the other hand, dinitrophenol, an uncoupler of oxidative
phosphorylation, increases both the oxidation of C-l and C-6 of glucose
although it decreases the conversion of glucose to glycogen and to fatty
acids.
12. Mammary Gland
Among the more extensively studied tissues, the lactating mammary
gland has the most active pentose phosphate cycle, this tissue being
capable of metabolizing glucose via this pathway to the extent of 40%
(111). The pioneering work on this tissue has been carried out by dock
and McLean, who showed that the specific activities of G-6-P DH and
6-PG DH (310), the levels of pyridine nucleotides (311), and the oxidation of glucose-l-C
14 (312) in the mammary gland all increase as the rat
progresses from pregnancy through the lactation and abruptly decrease
on involution of the glands. These observations suggest that the pentose
phosphate cycle has an active role in the lactating mammary gland.
[For more detailed discussion of this subject, this reviewer recommends
the reader to the widespread coverage by Glock and McLean (313) and
by Hansen and Carlson (314).]
Research on carbohydrate metabolism in mammary glands has fallen
mainly into two classes: in vitro experiments either with tissue slices
or homogenates, usually from rats, but occasionally from mice, albino
rabbits (315), and sheep (312); and in vivo experiments and perfusion
experiments with lactating cows. In the case of lactating cows, Black
et al., measuring the incorporation of C
14 from glucose-l-C
14 and from