50
Ν. G. PON
guinea pig skin slices from the ear with Pj
32 in the presence of glucose
and then examined the products chromatographically. Ribose-5-phosphate, sedoheptulose-7-phosphate and sedoheptulose-l,7-diphosphate are
found among the products, indicating that the hexose monophosphate
shunt is operative in this tissue.
Burns produced on the back by applying water for 5 minutes at 50°
did not alter the epidermis G-6-P DH activity (329).
Bernstein et al. have characterized the literature on in vitro carbohydrate metabolism in cartilaginous tissues as scarce (335). This description may be applied as well to the operation of the pentose
phosphate cycle in bone tissues. In the few cases examined, there are
indications of the operation of the pentose phosphate cycle in both
cartilaginous and osseous tissues. The cartilage was obtained from male
weanling rats which develop tibial epiphyseal plates with a high density
of chondrocytes induced by a high calcium rachitogenic diet. On feeding
slices of these tibial epiphyses with glucose-l-C
14 and glucose-6-C
14
, the
oxidation rate of the former substrate is greater than that of the latter
by a factor of 1.9 (335). In vitro addition of insulin increases the formation of C0 2 from both C-l and C-6 to a ratio of 2.5 whereas growth
hormone has no effect. The recovery of the specifically labeled carbon
into the tissue polysaccharides does not exhibit the selective loss of the
C-l, and in fact more label from the C-l was incorporated into the
polysaccharides than that from C-6. Growth hormone, on the other hand,
gave an increase in polysaccharide synthesis.
In the epiphyseal-metaphyseal bone slices of rabbits, lactate arises
entirely from glucose aerobically. Most of this lactate is formed via the
EMP pathway with only a small contribution, ca. 15%, via the pentose
phosphate cycle (336). Parathyroid, which induces bone demineralization, administered in vivo to weanling rabbits, causes an elevated level
of serum calcium. These treated animals yield epiphyseal-metaphyseal
bone slices which give enhanced C0 2 formation from glucose with relatively more C0 2 from C-6 than from C-l (337). The data suggest that
the activity of the TCA cycle is activated, a suggestion which finds additional support in the ability of fluoroacetate to abolish completely the
effect of the parathyroid extract on C0 2 production. During all these
changes, the pentose phosphate cycle activity remains constant.
15. Tumors
Animal tumors are characterized by a high rate of lactate production
both aerobically and anaerobically (338). This is in contrast with many
normal adult tissues in which the anaerobic rate averages less than onequarter that of the tumors, whereas the aerobic rate of the normal tissue
Ν. G. PON
guinea pig skin slices from the ear with Pj
32 in the presence of glucose
and then examined the products chromatographically. Ribose-5-phosphate, sedoheptulose-7-phosphate and sedoheptulose-l,7-diphosphate are
found among the products, indicating that the hexose monophosphate
shunt is operative in this tissue.
Burns produced on the back by applying water for 5 minutes at 50°
did not alter the epidermis G-6-P DH activity (329).
Bernstein et al. have characterized the literature on in vitro carbohydrate metabolism in cartilaginous tissues as scarce (335). This description may be applied as well to the operation of the pentose
phosphate cycle in bone tissues. In the few cases examined, there are
indications of the operation of the pentose phosphate cycle in both
cartilaginous and osseous tissues. The cartilage was obtained from male
weanling rats which develop tibial epiphyseal plates with a high density
of chondrocytes induced by a high calcium rachitogenic diet. On feeding
slices of these tibial epiphyses with glucose-l-C
14 and glucose-6-C
14
, the
oxidation rate of the former substrate is greater than that of the latter
by a factor of 1.9 (335). In vitro addition of insulin increases the formation of C0 2 from both C-l and C-6 to a ratio of 2.5 whereas growth
hormone has no effect. The recovery of the specifically labeled carbon
into the tissue polysaccharides does not exhibit the selective loss of the
C-l, and in fact more label from the C-l was incorporated into the
polysaccharides than that from C-6. Growth hormone, on the other hand,
gave an increase in polysaccharide synthesis.
In the epiphyseal-metaphyseal bone slices of rabbits, lactate arises
entirely from glucose aerobically. Most of this lactate is formed via the
EMP pathway with only a small contribution, ca. 15%, via the pentose
phosphate cycle (336). Parathyroid, which induces bone demineralization, administered in vivo to weanling rabbits, causes an elevated level
of serum calcium. These treated animals yield epiphyseal-metaphyseal
bone slices which give enhanced C0 2 formation from glucose with relatively more C0 2 from C-6 than from C-l (337). The data suggest that
the activity of the TCA cycle is activated, a suggestion which finds additional support in the ability of fluoroacetate to abolish completely the
effect of the parathyroid extract on C0 2 production. During all these
changes, the pentose phosphate cycle activity remains constant.
15. Tumors
Animal tumors are characterized by a high rate of lactate production
both aerobically and anaerobically (338). This is in contrast with many
normal adult tissues in which the anaerobic rate averages less than onequarter that of the tumors, whereas the aerobic rate of the normal tissue