Ν ON-FILAMENTOUS AQUATIC
FUNGI
67
then supplied from other sources, perhaps from glucose-6-phosphate
dehydrogenase via pentose (Weimberg and Doudoroff, 1955; Levin and
Racker, 1959) to keep the system going. With formation and turnover
of succinic acid at this site having reached a standstill, repression of
isocitritase is released; and, as its synthesis begins, it commences to
draw upon the pool of isocitrate. The flow from ketoglutarate, through
isocitrate, to succinate and glyoxylate now increases ; at this second site,
glyoxylate is metabolized via the constitutive glycine-alanine transaminase, while succinate is drawn off. The nature of the 'sink' for
succinate is, of course, unknown. It may be a structural one or, alternatively, perhaps a chemical derivative; from what we know of the
metabolism of RS cells, the i^-succinyl glutamic acid reported to be
involved in bacterial spore formation (Aubert et al., 1961 ) is an attractive
possibility.
In the main, this scheme is hypothetical. But as before, future work
should show to what extent it truly represents the early stages
through which the bicarbonate trigger mechanism operates in B.
emersonii.
Two other major questions are brought forcefully to mind as new
enzymes are described in almost countless numbers from month to
month: (a) are there likely alternate routes, not as yet tested or
considered, that ketoglutarate, isocitrate, and succinate may take in
addition to those believed to be the main ones ; (b) are there other loci at
which CO 2 and/or bicarbonate might enter into the metabolic machinery
of Blastocladiella and through which morphogenesis might be affected if
not effected? The answer is, of course, yesf ; but, the question is much
too broad to deal with in this discussion. Suffice it to say for now that
some alternate pathways known to occur in other organisms seem to be
ruled out with certainty, some are neither in nor out for sure, and some
have simply not been tested.
At the speed with which enzyme taxonomy is expanding, there is no
choice but to be judiciously selective in searching for, and then examining
in detail, enzymes or other factors of some potential consequence in
morphogenesis. But, once the more important systems have been
t Condensations via malic synthetase (see Kornberg, 1959, for review), the reaction
between ketoglutarate and glyoxylate to yield a-keto-0-hydroxyadipic acid and C0 2
(Franke and Jilge, 1961), the conversion of succinate and the α-carbon of glycine to yield
aminolevulinic acid (Shemin, 1956), the irreversible glyoxylate-glutamate transaminase
of Pitts et al. (1961) which yield ketoglutarate and glycine, the oxidation of glycine via
glyoxylate carboligase to yield glyoxylate, glycerate, and pyruvic acid (e.g. Dagley et al. 9
1961), the control of the tricarboxylic acid cycle by possible condensation of glyoxylate
with oxaloacetate to yield oxalomalate (Ruffo, 1961 ), not to mention the many possibilities
for C0 2 fixations via C lt C 2 , C 3 and C 5 compounds (see review by Quayle, 1961, and Walker,
1962)—all of these, and no doubt many more to come—will have to be considered as the
work on Blastocladiella proceeds.
FUNGI
67
then supplied from other sources, perhaps from glucose-6-phosphate
dehydrogenase via pentose (Weimberg and Doudoroff, 1955; Levin and
Racker, 1959) to keep the system going. With formation and turnover
of succinic acid at this site having reached a standstill, repression of
isocitritase is released; and, as its synthesis begins, it commences to
draw upon the pool of isocitrate. The flow from ketoglutarate, through
isocitrate, to succinate and glyoxylate now increases ; at this second site,
glyoxylate is metabolized via the constitutive glycine-alanine transaminase, while succinate is drawn off. The nature of the 'sink' for
succinate is, of course, unknown. It may be a structural one or, alternatively, perhaps a chemical derivative; from what we know of the
metabolism of RS cells, the i^-succinyl glutamic acid reported to be
involved in bacterial spore formation (Aubert et al., 1961 ) is an attractive
possibility.
In the main, this scheme is hypothetical. But as before, future work
should show to what extent it truly represents the early stages
through which the bicarbonate trigger mechanism operates in B.
emersonii.
Two other major questions are brought forcefully to mind as new
enzymes are described in almost countless numbers from month to
month: (a) are there likely alternate routes, not as yet tested or
considered, that ketoglutarate, isocitrate, and succinate may take in
addition to those believed to be the main ones ; (b) are there other loci at
which CO 2 and/or bicarbonate might enter into the metabolic machinery
of Blastocladiella and through which morphogenesis might be affected if
not effected? The answer is, of course, yesf ; but, the question is much
too broad to deal with in this discussion. Suffice it to say for now that
some alternate pathways known to occur in other organisms seem to be
ruled out with certainty, some are neither in nor out for sure, and some
have simply not been tested.
At the speed with which enzyme taxonomy is expanding, there is no
choice but to be judiciously selective in searching for, and then examining
in detail, enzymes or other factors of some potential consequence in
morphogenesis. But, once the more important systems have been
t Condensations via malic synthetase (see Kornberg, 1959, for review), the reaction
between ketoglutarate and glyoxylate to yield a-keto-0-hydroxyadipic acid and C0 2
(Franke and Jilge, 1961), the conversion of succinate and the α-carbon of glycine to yield
aminolevulinic acid (Shemin, 1956), the irreversible glyoxylate-glutamate transaminase
of Pitts et al. (1961) which yield ketoglutarate and glycine, the oxidation of glycine via
glyoxylate carboligase to yield glyoxylate, glycerate, and pyruvic acid (e.g. Dagley et al. 9
1961), the control of the tricarboxylic acid cycle by possible condensation of glyoxylate
with oxaloacetate to yield oxalomalate (Ruffo, 1961 ), not to mention the many possibilities
for C0 2 fixations via C lt C 2 , C 3 and C 5 compounds (see review by Quayle, 1961, and Walker,
1962)—all of these, and no doubt many more to come—will have to be considered as the
work on Blastocladiella proceeds.