1. PENTOSE PHOSPHATE CYCLE
49
phate (328). The dehydrogenases as well as the isomerase activities seem
to be least in this tissue as compared to all others examined. Apparently,
few, if any, isotopic methods have been applied to the study of glucose
metabolism in skeletal muscles.
14. Skin and Bones
The presence of G-6-P DH and 6-PG DH has been demonstrated
in human skin by Hershey and his associates (329-331).
In particular,
in the three layers of the sole, the epidermal cells contain the highest
level of 6-PG DH, the dermis the intermediate, and the keratin the
lowest (329). This pattern is also followed by G-6-P DH, but in this
study, the dermis was further subdivided into superficial and deep
dermis regions, in which case the deep dermis has the lowest activity,
even lower than that of the keratin G-6-P DH (331). A more thorough
study of the enzymatic activity of G-6-P DH in the various strata and
skin appendages revealed an interesting distribution pattern. The sebaceous glands have the highest activity, some 5.5-fold greater activity than
that in the epidermis. The following skin layers and appendages have
G-6-P DH in the order of decreasing activity: sebaceous glands, hair
follicle, epidermis, sweat glands, superficial dermis, and deep dermis.
Roughly the same distribution pattern of 6-PG DH activity was observed
in the skin (330). That the sebaceous gland should have such a high
level of dehydrogenases is not too surprising in view of the fact that
lipid synthesis occurs in this tissue and thus TPNH is required. In fact,
isocitrate dehydrogenase, another TPNH-generating system, gives the
same picture of localization as those of G-6-P DH and 6-PG DH.
Measurements of the C
14 0 2 production from glucose-l-C
14 by young
rat skin, indicated that there is a preferential oxidation of the glucose
carbon atom 1 (332). The C-l:C-6 of the C0 2 formed, however, varied
with time; at the end of 1 hour this ratio is ca. 4.7 whereas at greater
than 3 hours it is ca. 2. The authors concluded that, qualitatively, the
pentose phosphate cycle occurs in the young rat skin. Freinkel, while
working with postmortem human skin slices from the scalp and abdomen,
estimated that a major portion of the assimilated glucose which is oxidized to C0 2 and of the C0 2 formed is via the pentose phosphate cycle
(333). Moreover, methylene blue can elevate the C-l:C-6 of C0 2 of
either epidermal cells or dermis (with sebaceous glands) from a normal
of about 2 to 8. With all the shortcomings of the isotopic techniques
and the demonstration of the presence of enzymes in the skin, there is
yet to be found a proof-positive method actually to demonstrate the
operation of the pentose phosphate cycle in the skin. An attempt in this
direction was carried out by Yardley and Godfrey (334). They incubated
49
phate (328). The dehydrogenases as well as the isomerase activities seem
to be least in this tissue as compared to all others examined. Apparently,
few, if any, isotopic methods have been applied to the study of glucose
metabolism in skeletal muscles.
14. Skin and Bones
The presence of G-6-P DH and 6-PG DH has been demonstrated
in human skin by Hershey and his associates (329-331).
In particular,
in the three layers of the sole, the epidermal cells contain the highest
level of 6-PG DH, the dermis the intermediate, and the keratin the
lowest (329). This pattern is also followed by G-6-P DH, but in this
study, the dermis was further subdivided into superficial and deep
dermis regions, in which case the deep dermis has the lowest activity,
even lower than that of the keratin G-6-P DH (331). A more thorough
study of the enzymatic activity of G-6-P DH in the various strata and
skin appendages revealed an interesting distribution pattern. The sebaceous glands have the highest activity, some 5.5-fold greater activity than
that in the epidermis. The following skin layers and appendages have
G-6-P DH in the order of decreasing activity: sebaceous glands, hair
follicle, epidermis, sweat glands, superficial dermis, and deep dermis.
Roughly the same distribution pattern of 6-PG DH activity was observed
in the skin (330). That the sebaceous gland should have such a high
level of dehydrogenases is not too surprising in view of the fact that
lipid synthesis occurs in this tissue and thus TPNH is required. In fact,
isocitrate dehydrogenase, another TPNH-generating system, gives the
same picture of localization as those of G-6-P DH and 6-PG DH.
Measurements of the C
14 0 2 production from glucose-l-C
14 by young
rat skin, indicated that there is a preferential oxidation of the glucose
carbon atom 1 (332). The C-l:C-6 of the C0 2 formed, however, varied
with time; at the end of 1 hour this ratio is ca. 4.7 whereas at greater
than 3 hours it is ca. 2. The authors concluded that, qualitatively, the
pentose phosphate cycle occurs in the young rat skin. Freinkel, while
working with postmortem human skin slices from the scalp and abdomen,
estimated that a major portion of the assimilated glucose which is oxidized to C0 2 and of the C0 2 formed is via the pentose phosphate cycle
(333). Moreover, methylene blue can elevate the C-l:C-6 of C0 2 of
either epidermal cells or dermis (with sebaceous glands) from a normal
of about 2 to 8. With all the shortcomings of the isotopic techniques
and the demonstration of the presence of enzymes in the skin, there is
yet to be found a proof-positive method actually to demonstrate the
operation of the pentose phosphate cycle in the skin. An attempt in this
direction was carried out by Yardley and Godfrey (334). They incubated