294
AUGUST EPPLE
Forsham, 1966). In teleosts, the activities of hexokinase and phosphatase
as well as those of the enzymes specifically involved in glycogen metabolism, pentose shunt, glycolytic ( Embden-Meyerhof ) pathway, tricarboxylic acid (Krebs) cycle, electron transport system, and transaminations
were studied.
Renold et al. (1964) detected significant hexokinase activity in homogenates of the islet tissue of the toadfish, together with 6-phosphogluconic
acid dehydrogenase, phosphoglucoisomerase, and phosphomannoisomerase activities.
By microassay, specific glucose 6-phosphatase activity was not found
in measurable amounts either in toadfish or in goosefish islets. In toadfish
islets, the nonspecific phosphatase activity (pH 6.5) was about half that
in liver, kidney, and heart, but slightly higher than in the brain. The
alkaline phosphatase activity (pH 9 ) was about twice the acid phosphatase activity (pH 5, cf. Lazarow, 1963; Lazarow et al., 1959). It is noteworthy that most of the histochemical data on glucose 6-phosphatase in
mammalian islets confirm the presence of this enzyme (cf. Grodsky and
Forsham, 1566). The location of this enzyme primarily in the endoplasmic
reticulum of the mammalian B cell led Lazarus and Barden (1965) to the
conclusion that it may be involved in insulin synthesis.
The islets of both toadfish and goosefish have a high glycogen content,
and glycogen can also be localized histochemically within the B cells.
Thus, it is surprising that very low UDPG-glycogen transglucosylase activity was found; however, a comparison with phosphorylase activity is
still lacking ( cf. Lazaroy, 1963).
Investigating the enzymes of the pentose shunt in the toadfish,
Lazarow and associates (cf. Lazarow, 1963) found that the islet tissue
has the lowest glucose 6-phosphate dehydrogenase activity of all tissues
studied except muscle. The Sphosphogluconate dehydrogenase content
of islet tissue was of the same order as glucose &phosphate dehydrogenase activity. Islet tissue also had the lowest ratio of glucose 6-phosphate to 6-phosphogluconate when compared with other organs. According to Lazarow et al. (1964b), islet would be the least likely tissue to
accumulate 6-phosphogluconate of all the toadfish tissues studied.
Two enzymes of the Embden-Meyerhof pathway were studied in the
toadfish. The aldolase activity was about 17% of that found in the muscle,
the most active tissue. The 3-phosphoglyceraldehyde dehydrogenase activity was 20% of that of the muscle. The maximal capacity of aldolase was
but a small fraction of the maximal capacity of the 3-phosphoglyceraldehyde dehydrogenase (Lazarow et aZ., 1964b).
Of the enzymes of the Krebs cycle, both succinic dehydrogenase and
AUGUST EPPLE
Forsham, 1966). In teleosts, the activities of hexokinase and phosphatase
as well as those of the enzymes specifically involved in glycogen metabolism, pentose shunt, glycolytic ( Embden-Meyerhof ) pathway, tricarboxylic acid (Krebs) cycle, electron transport system, and transaminations
were studied.
Renold et al. (1964) detected significant hexokinase activity in homogenates of the islet tissue of the toadfish, together with 6-phosphogluconic
acid dehydrogenase, phosphoglucoisomerase, and phosphomannoisomerase activities.
By microassay, specific glucose 6-phosphatase activity was not found
in measurable amounts either in toadfish or in goosefish islets. In toadfish
islets, the nonspecific phosphatase activity (pH 6.5) was about half that
in liver, kidney, and heart, but slightly higher than in the brain. The
alkaline phosphatase activity (pH 9 ) was about twice the acid phosphatase activity (pH 5, cf. Lazarow, 1963; Lazarow et al., 1959). It is noteworthy that most of the histochemical data on glucose 6-phosphatase in
mammalian islets confirm the presence of this enzyme (cf. Grodsky and
Forsham, 1566). The location of this enzyme primarily in the endoplasmic
reticulum of the mammalian B cell led Lazarus and Barden (1965) to the
conclusion that it may be involved in insulin synthesis.
The islets of both toadfish and goosefish have a high glycogen content,
and glycogen can also be localized histochemically within the B cells.
Thus, it is surprising that very low UDPG-glycogen transglucosylase activity was found; however, a comparison with phosphorylase activity is
still lacking ( cf. Lazaroy, 1963).
Investigating the enzymes of the pentose shunt in the toadfish,
Lazarow and associates (cf. Lazarow, 1963) found that the islet tissue
has the lowest glucose 6-phosphate dehydrogenase activity of all tissues
studied except muscle. The Sphosphogluconate dehydrogenase content
of islet tissue was of the same order as glucose &phosphate dehydrogenase activity. Islet tissue also had the lowest ratio of glucose 6-phosphate to 6-phosphogluconate when compared with other organs. According to Lazarow et al. (1964b), islet would be the least likely tissue to
accumulate 6-phosphogluconate of all the toadfish tissues studied.
Two enzymes of the Embden-Meyerhof pathway were studied in the
toadfish. The aldolase activity was about 17% of that found in the muscle,
the most active tissue. The 3-phosphoglyceraldehyde dehydrogenase activity was 20% of that of the muscle. The maximal capacity of aldolase was
but a small fraction of the maximal capacity of the 3-phosphoglyceraldehyde dehydrogenase (Lazarow et aZ., 1964b).
Of the enzymes of the Krebs cycle, both succinic dehydrogenase and