22
Ν. G. PON
cussed elsewhere (see Section V,E on criteria for the presence of the
pentose phosphate cycles; also cf. ref. 89). In essence, 10% of the total
glucose phosphate and the phosphogluconate dehydrogenases are located
in the particulate fractions of the brain.
Both arsenite and Synkavite, a derivative of menadione, seem to alter
the normal metabolic pattern of glucose oxidation (158). The former
inhibitor, however, not only decreases the production of C
14 0 2 from
glucose-6-C
14
, but also the oxygen uptake, so that with the increased
C
14 0 2 yield from glucose-l-C
14 the net result may be a combination of
two effects: the inhibition of the TCA cycle and the stimulation of the
pentose phosphate pathway. Synkavite, on the other hand, operates differently, by activating the oxidation of C-l by threefold without affecting
that of C-6 or the oxygen uptake. It should be mentioned that the
investigations described above (and others not mentioned) were carried
out on a number of nervous systems from many sources, among them
the rat (149), the rabbit (155), the guinea pig (158), the monkey
Macaca mulatto, (153, 159), and the cat (160).
5. Cardiac, Arterial, and Venous Tissues
Source materials for studies described below were the following:
female breeder rats with signs of arteriosclerosis (161), normal rats
(162), mongrel dogs (163), guinea pigs (164), bovine (165), humans
(166), adult and fetal pigs (167). In general, one can say that heart
muscle in the form of a homogenate (162) or as ventricle strips (163)
metabolizes glucose chiefly by glycolysis and the oxidation of pyruvate.
This applies even when the ventricle strips are electrically contracted
and are placed in a medium containing arsenate (168). To be sure,
homogenates of adult heart contain the enzymes necessary for the reduction of TPN
+ when the pentose phosphate cycle intermediates are added
(167). More specifically, Glock and McLean showed that both glucose-6phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are
present in rat cardiac muscle, although at relatively low levels (137).
Thus cardiac tissues have the potential to oxidize glucose via an alternate
pathway. In this regard, Jolly et al. concluded from experiments with
variously labeled sugars added to homogenates of fetal pig and adult pig
heart, in the presence and absence of TPN
+ , that the pentose phosphate
cycle in the soluble enzymes of the fetal heart is 4 times greater than
those of the adult heart (167). The pentose phosphate pathway in arterial tissues is operative, as is evident from a large C-l:C-6 ratio from
the respectively labeled glucose (163-165),
and from enzymological
studies (161, 166). Furthermore, aortic tissues without arteriosclerosis
have higher glucose-6-phosphate dehydrogenase activity than those with
Ν. G. PON
cussed elsewhere (see Section V,E on criteria for the presence of the
pentose phosphate cycles; also cf. ref. 89). In essence, 10% of the total
glucose phosphate and the phosphogluconate dehydrogenases are located
in the particulate fractions of the brain.
Both arsenite and Synkavite, a derivative of menadione, seem to alter
the normal metabolic pattern of glucose oxidation (158). The former
inhibitor, however, not only decreases the production of C
14 0 2 from
glucose-6-C
14
, but also the oxygen uptake, so that with the increased
C
14 0 2 yield from glucose-l-C
14 the net result may be a combination of
two effects: the inhibition of the TCA cycle and the stimulation of the
pentose phosphate pathway. Synkavite, on the other hand, operates differently, by activating the oxidation of C-l by threefold without affecting
that of C-6 or the oxygen uptake. It should be mentioned that the
investigations described above (and others not mentioned) were carried
out on a number of nervous systems from many sources, among them
the rat (149), the rabbit (155), the guinea pig (158), the monkey
Macaca mulatto, (153, 159), and the cat (160).
5. Cardiac, Arterial, and Venous Tissues
Source materials for studies described below were the following:
female breeder rats with signs of arteriosclerosis (161), normal rats
(162), mongrel dogs (163), guinea pigs (164), bovine (165), humans
(166), adult and fetal pigs (167). In general, one can say that heart
muscle in the form of a homogenate (162) or as ventricle strips (163)
metabolizes glucose chiefly by glycolysis and the oxidation of pyruvate.
This applies even when the ventricle strips are electrically contracted
and are placed in a medium containing arsenate (168). To be sure,
homogenates of adult heart contain the enzymes necessary for the reduction of TPN
+ when the pentose phosphate cycle intermediates are added
(167). More specifically, Glock and McLean showed that both glucose-6phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are
present in rat cardiac muscle, although at relatively low levels (137).
Thus cardiac tissues have the potential to oxidize glucose via an alternate
pathway. In this regard, Jolly et al. concluded from experiments with
variously labeled sugars added to homogenates of fetal pig and adult pig
heart, in the presence and absence of TPN
+ , that the pentose phosphate
cycle in the soluble enzymes of the fetal heart is 4 times greater than
those of the adult heart (167). The pentose phosphate pathway in arterial tissues is operative, as is evident from a large C-l:C-6 ratio from
the respectively labeled glucose (163-165),
and from enzymological
studies (161, 166). Furthermore, aortic tissues without arteriosclerosis
have higher glucose-6-phosphate dehydrogenase activity than those with