5TSH NUTRITION
41 9
genase isoenzymes occurs at low temperatures (So-10°C) where fish
enzymes normally operate (Cowey et al., 1969). Moreover flatfish
have a single isoenzyme of lactic dehydrogenase common to both
cardiac and skeletal muscle (Lush et al., 1969).
A second pathway of glucose degradation, the pentose phosphate
cycle, provides for the oxidation of glucose-6-phosphate t o carbon
dioxide and triose phosphates; at the same time reduced nicotinamide
adenine dinucleotide phosphate (which is necessary for biosynthetic
reactions generally) is generated. This pathway is also important for
the production of pentoses in nucleic acid synthesis. The importance
of this pathway in fish has been tested mainly by comparing the rates
of evolution of radioactive carbon dioxide after intraperitoneal
injection of glu~ose-l-~~C and of glucose-6-14C. Degradation by the
pentose phosphate pathway would result in the first formed carbon
dioxide being derived wholly from the C-1 of glucose ; on the other hand
if degradation is by glycolysis and the tricarboxylic acid cycle the
rate of evolution of radioactive carbon dioxide from the C-1 or C-6
atoms of glucose would be indistinguishable. Brown (1960) concluded
from such a study on carp that glucose degradation is primarily via the
glycolysis pathway, although two pentose cycle enzymes were shown
to occur in carp liver. Similar results were obtained in studies on brook
trout, Sulvelinus fontinalis (Mitchill) (Hochachka, 1961).
The
distribution of isotope in the glucose of fish following injection of
[1-l4C] acetate likewise suggested that glucose metabolism proceeded
very largely through the Embden-Meyerhof-Parnas glycolytic pathway
(Hochachka, 1961 ; Hochachka and Hayes, 1962). It should be noted,
however, that quantitation of the different routes of glucose
metabolism using specific isotopes of glucose is fraught with
difficulties (Wood et ul., 1963).
The main pathway of complete aerobic breakdown of carbohydrate
in fish is the tricarboxylic acid cycle. This pathway has been
investigated in fish by Gumbmann and Tappel (1962a, b) and shown to
function much as in mammals. The oxidative decarboxylation of
pyruvate gives rise to acetyl coenzyme A which is oxidized t o
carbon dioxide and water in the pathway. Over the whole cycle 38
molecules of adenosine triphosphate are produced from one molecule
of glucose.
Iv. h I D S
It is probably true to say that up to the present the lipids of fish
have been more intensively studied from a chemical point of view than
have any of the other major constituents. This has led to a common
41 9
genase isoenzymes occurs at low temperatures (So-10°C) where fish
enzymes normally operate (Cowey et al., 1969). Moreover flatfish
have a single isoenzyme of lactic dehydrogenase common to both
cardiac and skeletal muscle (Lush et al., 1969).
A second pathway of glucose degradation, the pentose phosphate
cycle, provides for the oxidation of glucose-6-phosphate t o carbon
dioxide and triose phosphates; at the same time reduced nicotinamide
adenine dinucleotide phosphate (which is necessary for biosynthetic
reactions generally) is generated. This pathway is also important for
the production of pentoses in nucleic acid synthesis. The importance
of this pathway in fish has been tested mainly by comparing the rates
of evolution of radioactive carbon dioxide after intraperitoneal
injection of glu~ose-l-~~C and of glucose-6-14C. Degradation by the
pentose phosphate pathway would result in the first formed carbon
dioxide being derived wholly from the C-1 of glucose ; on the other hand
if degradation is by glycolysis and the tricarboxylic acid cycle the
rate of evolution of radioactive carbon dioxide from the C-1 or C-6
atoms of glucose would be indistinguishable. Brown (1960) concluded
from such a study on carp that glucose degradation is primarily via the
glycolysis pathway, although two pentose cycle enzymes were shown
to occur in carp liver. Similar results were obtained in studies on brook
trout, Sulvelinus fontinalis (Mitchill) (Hochachka, 1961).
The
distribution of isotope in the glucose of fish following injection of
[1-l4C] acetate likewise suggested that glucose metabolism proceeded
very largely through the Embden-Meyerhof-Parnas glycolytic pathway
(Hochachka, 1961 ; Hochachka and Hayes, 1962). It should be noted,
however, that quantitation of the different routes of glucose
metabolism using specific isotopes of glucose is fraught with
difficulties (Wood et ul., 1963).
The main pathway of complete aerobic breakdown of carbohydrate
in fish is the tricarboxylic acid cycle. This pathway has been
investigated in fish by Gumbmann and Tappel (1962a, b) and shown to
function much as in mammals. The oxidative decarboxylation of
pyruvate gives rise to acetyl coenzyme A which is oxidized t o
carbon dioxide and water in the pathway. Over the whole cycle 38
molecules of adenosine triphosphate are produced from one molecule
of glucose.
Iv. h I D S
It is probably true to say that up to the present the lipids of fish
have been more intensively studied from a chemical point of view than
have any of the other major constituents. This has led to a common