5. THE ENDOCRINE PANCREAS
295
malic dehydrogenase activities were studied in toadfish islets. The activities of both enzymes were rather low when compared with other tissue
(Lazarow and Cooperstein, 1951; Moran, 1962). Maske et d. (1956)
found insulin and succinic dehydrogenase in parallel concentrations
within the subcellular fractions of islet tissue of Pleuronectidae.
Linda11 (1962) determined the concentrations of both reduced and
oxidized forms of diphosphopyridine and triphosphopyridine nucleotide
of toadfish islet tissue. The ratio of the oxidized-to-reduced form was 0.40.
For the electron transport system of the toadfish islets, Lazarow and
co-workers obtained evidence that cytochrome c may exist predominantly
in the oxidized state. This was suggested by the observation that cytochrome oxidase activity was rather low when compared with the activities
of the other dehydrogenase systems which reduce cytochrome c
(Lazarow, 1963, 1965; Lazarow et al., 196413). These findings, together
with the high content of oxidized nicotinamide-adenine dinucleotide suggest that the redox potential of the islet tissue is high. According to
Lazarow (1954, 1965) and Lazarow et al. ( 1964b), this would be favorable for the oxidation of sulfhydryl precursors to their disulfide form during insulin synthesis. Furthermore, the high ratio of NAD+ to NADH
would make it less likely that the islet tissue inactivates alloxan by
reduction to dialuric acid.
In general, the comparison with other tissues indicates that the relative activities of the enzymes of the glycolytic, tricarboxylic, and electron
transport pathways are low, despite the high capacity of the islets to utilize glucose. As pointed out by Lazarow (1955) and Lazarow et al.
( 1964b), these enzymic activities were measured under optimal conditions in uitro; thus, the findings presented may not necessarily reflect the
relative roles of these enzymes in the intact cell.
As an intensive conversion of glucose into amino acids was found,
Hellman and Larsson ( 1962) studied the activities of glutamic-oxalacetic
transaminase, glutamicpyruvic transaminase, and in addition ornithine
carbamyltransferase in the islet tissue of Cottus quadricornis. When compared with the exocrine pancreas and liver, the islet tissue showed relatively high activities of these enzymes. The high level of ornithine
carbamyltransferase in endocrine and exocrine pancreas is a striking contrast to what would be expected from findings in mammals where the
activity of the latter enzymes in the liver was found to be 2000 times
greater than in the pancreas ( Wretlind et al., 1961 ).
It should be noted that the data presented here are not specific for the
B cells but derived from the study of the Brockmann bodies which, of
course, also contain other epithelian elements.
295
malic dehydrogenase activities were studied in toadfish islets. The activities of both enzymes were rather low when compared with other tissue
(Lazarow and Cooperstein, 1951; Moran, 1962). Maske et d. (1956)
found insulin and succinic dehydrogenase in parallel concentrations
within the subcellular fractions of islet tissue of Pleuronectidae.
Linda11 (1962) determined the concentrations of both reduced and
oxidized forms of diphosphopyridine and triphosphopyridine nucleotide
of toadfish islet tissue. The ratio of the oxidized-to-reduced form was 0.40.
For the electron transport system of the toadfish islets, Lazarow and
co-workers obtained evidence that cytochrome c may exist predominantly
in the oxidized state. This was suggested by the observation that cytochrome oxidase activity was rather low when compared with the activities
of the other dehydrogenase systems which reduce cytochrome c
(Lazarow, 1963, 1965; Lazarow et al., 196413). These findings, together
with the high content of oxidized nicotinamide-adenine dinucleotide suggest that the redox potential of the islet tissue is high. According to
Lazarow (1954, 1965) and Lazarow et al. ( 1964b), this would be favorable for the oxidation of sulfhydryl precursors to their disulfide form during insulin synthesis. Furthermore, the high ratio of NAD+ to NADH
would make it less likely that the islet tissue inactivates alloxan by
reduction to dialuric acid.
In general, the comparison with other tissues indicates that the relative activities of the enzymes of the glycolytic, tricarboxylic, and electron
transport pathways are low, despite the high capacity of the islets to utilize glucose. As pointed out by Lazarow (1955) and Lazarow et al.
( 1964b), these enzymic activities were measured under optimal conditions in uitro; thus, the findings presented may not necessarily reflect the
relative roles of these enzymes in the intact cell.
As an intensive conversion of glucose into amino acids was found,
Hellman and Larsson ( 1962) studied the activities of glutamic-oxalacetic
transaminase, glutamicpyruvic transaminase, and in addition ornithine
carbamyltransferase in the islet tissue of Cottus quadricornis. When compared with the exocrine pancreas and liver, the islet tissue showed relatively high activities of these enzymes. The high level of ornithine
carbamyltransferase in endocrine and exocrine pancreas is a striking contrast to what would be expected from findings in mammals where the
activity of the latter enzymes in the liver was found to be 2000 times
greater than in the pancreas ( Wretlind et al., 1961 ).
It should be noted that the data presented here are not specific for the
B cells but derived from the study of the Brockmann bodies which, of
course, also contain other epithelian elements.