1. PENTOSE PHOSPHATE CYCLE
17
phosphate with a large unlabeled pool and via transketolase exchange
reactions (10).
VI. Chordates
A. MAMMALS
Almost half of all the studies on the pentose phosphate cycle have
been carried out on mammalian systems. An excellent survey of the occurrence of this pathway in mammal tissues has been written by Katz
(111). Several problems arise, however, because of the complexity of a
living system. For example, we are faced with a study of metabolic
pathways in a biological organization which ranges from the most complicated form, the intact animal, to the tissues, to the cellular level, to
the organelles, and ultimately to the simplest form, the enzymes (112).
Even with these classifications clearly outlined, there remain the factors
that may influence the activity of the pentose phosphate cycle. Some
examples of these modifying factors are the physiological and pathological conditions of the animal, its growth stage, the ionic environment in
which tissues are suspended, diet, hormonal regulation, genetic effects,
and many others.
1. Intact Animal
Research of this type has been conducted mainly on humans, rats,
mice, and lactating cows. Labeled substrates were administered to the
animal, and the pattern of labeling in the product was thus determined.
Thus in the study of the pentose phosphate cycle in humans, in one case,
radioactive acetate was injected and the distribution of label in blood
glucose was measured (113). In two other cases, variously labeled
glucose was administered and the labeling pattern of the ribose in the
urine (114) as well as the recovery of C
14 0 2 (115) were determined.
With rats the method was essentially similar (116-118);
however, in
some instances (with mice also) the incorporation of the tracer in liver
glycogen was used as a measure of the operation of pentose cycle (119,
120). Labeled alanine isolated from the proteins of internal organs of
rats, such as heart, lungs, spleen, reproductive organs, and kidney, also
served as an index of pentose phosphate cycle activity (121). In vivo
studies in cows have been concentrated on the incorporation of label in
the constituents of the milk (122), and the respired C0 2 (123). In
general the different investigators concluded that glucose is metabolized
mainly, i.e., greater than 90%, via the Embden-Meyerhof-Parnas sequence; that gluconate is catabolized by the oxidative portion of the
pentose phosphate cycle, and that ribose is formed mostly by the
17
phosphate with a large unlabeled pool and via transketolase exchange
reactions (10).
VI. Chordates
A. MAMMALS
Almost half of all the studies on the pentose phosphate cycle have
been carried out on mammalian systems. An excellent survey of the occurrence of this pathway in mammal tissues has been written by Katz
(111). Several problems arise, however, because of the complexity of a
living system. For example, we are faced with a study of metabolic
pathways in a biological organization which ranges from the most complicated form, the intact animal, to the tissues, to the cellular level, to
the organelles, and ultimately to the simplest form, the enzymes (112).
Even with these classifications clearly outlined, there remain the factors
that may influence the activity of the pentose phosphate cycle. Some
examples of these modifying factors are the physiological and pathological conditions of the animal, its growth stage, the ionic environment in
which tissues are suspended, diet, hormonal regulation, genetic effects,
and many others.
1. Intact Animal
Research of this type has been conducted mainly on humans, rats,
mice, and lactating cows. Labeled substrates were administered to the
animal, and the pattern of labeling in the product was thus determined.
Thus in the study of the pentose phosphate cycle in humans, in one case,
radioactive acetate was injected and the distribution of label in blood
glucose was measured (113). In two other cases, variously labeled
glucose was administered and the labeling pattern of the ribose in the
urine (114) as well as the recovery of C
14 0 2 (115) were determined.
With rats the method was essentially similar (116-118);
however, in
some instances (with mice also) the incorporation of the tracer in liver
glycogen was used as a measure of the operation of pentose cycle (119,
120). Labeled alanine isolated from the proteins of internal organs of
rats, such as heart, lungs, spleen, reproductive organs, and kidney, also
served as an index of pentose phosphate cycle activity (121). In vivo
studies in cows have been concentrated on the incorporation of label in
the constituents of the milk (122), and the respired C0 2 (123). In
general the different investigators concluded that glucose is metabolized
mainly, i.e., greater than 90%, via the Embden-Meyerhof-Parnas sequence; that gluconate is catabolized by the oxidative portion of the
pentose phosphate cycle, and that ribose is formed mostly by the