62
EDWARD C. CANTINO AND JAMES S. LOVETT
Let us turn, therefore, to the second question regarding the need for
continued generation of sufficient reducing power for reductive carboxylation of ketoglutarate to isocitrate. We may ask: during exponential
growth, does the RS cell generate as much reducing power as the OC
cell? We do not know. But, if we assume that it does, then is the reducing
power (presumably siphoned off in large measure by OC cells via terminal
oxidation) conserved and used by the RS cell which is now respiring at
one-tenth the rate of the OC cell? Certainly, some of it could be conserved, for the exponential growth-rate is reduced only by about onehalf (Cantino and Lovett, 1960). While this comparison does not provide
proof, it is consistent with the thought that some of the available
reducing power is shunted away from routes which lead to terminal
oxidase systems, and towards alternate paths which terminate in the
reductive carboxylation of ketoglutarate. Two direct observations bear
upon this point.
The differential rate of exponential synthesis (i.e., relative to the
exponential rate of growth) of a TPN-specific glucose-6-phosphate
dehydrogenase is maintained in high gear in a developing RS cell ; that
is, this differential rate is the same as that found in an OC cell (compare
Goldstein and Cantino, 1962, with Lovett and Cantino, 1960b). Furthermore, as one would expect from the comparative biochemistry of other
organisms, this enzyme system can be coupled to the isocitric dehydrogenase via TPN to drive reductive carboxylation of ketoglutarate to
isocitrate in extracts of B. emersonii (Cantino and Horenstein, 1959).
Thus, (a) the rapid rate of exponential synthesis of both isocitric
dehydrogenase and glucose-6-phosphate dehydrogenase during exponential growth of an RS cell, combined with (b) a greatly reduced rate of—
and the early cessation of—synthesis of ketoglutaric dehydrogenase,
suggest that an important source of reducing power lies in the early,
TPN-dependent stages of the hexose monophosphate pathway. Turnover studies with synchronized, single generations in the stage of
exponential growth should provide the necessary direct evidence for this
suggestion (Fig.6).
A second likely source (although not an absolute requirement; see
Section III, E, 2) of reducing power may reside in the pathway which
leads to synthesis of melanin. This pigment, which impregnates the
chitinous RS wall, does not begin to appear immediately; its apparent
synthesis—at least its accumulation in detectable amounts—does not
start until about two-thirds of exponential growth has gone by (Lovett
and Cantino, 1960b). If it is assumed that synthesis of the tyrosinase
formed by Blastocladiella
(Cantino and Horenstein, 1955) follows the
same course, then a second source of reduced TPN is made available ;
this enzyme system, in cell-free extracts of B. emersonii, can also be
EDWARD C. CANTINO AND JAMES S. LOVETT
Let us turn, therefore, to the second question regarding the need for
continued generation of sufficient reducing power for reductive carboxylation of ketoglutarate to isocitrate. We may ask: during exponential
growth, does the RS cell generate as much reducing power as the OC
cell? We do not know. But, if we assume that it does, then is the reducing
power (presumably siphoned off in large measure by OC cells via terminal
oxidation) conserved and used by the RS cell which is now respiring at
one-tenth the rate of the OC cell? Certainly, some of it could be conserved, for the exponential growth-rate is reduced only by about onehalf (Cantino and Lovett, 1960). While this comparison does not provide
proof, it is consistent with the thought that some of the available
reducing power is shunted away from routes which lead to terminal
oxidase systems, and towards alternate paths which terminate in the
reductive carboxylation of ketoglutarate. Two direct observations bear
upon this point.
The differential rate of exponential synthesis (i.e., relative to the
exponential rate of growth) of a TPN-specific glucose-6-phosphate
dehydrogenase is maintained in high gear in a developing RS cell ; that
is, this differential rate is the same as that found in an OC cell (compare
Goldstein and Cantino, 1962, with Lovett and Cantino, 1960b). Furthermore, as one would expect from the comparative biochemistry of other
organisms, this enzyme system can be coupled to the isocitric dehydrogenase via TPN to drive reductive carboxylation of ketoglutarate to
isocitrate in extracts of B. emersonii (Cantino and Horenstein, 1959).
Thus, (a) the rapid rate of exponential synthesis of both isocitric
dehydrogenase and glucose-6-phosphate dehydrogenase during exponential growth of an RS cell, combined with (b) a greatly reduced rate of—
and the early cessation of—synthesis of ketoglutaric dehydrogenase,
suggest that an important source of reducing power lies in the early,
TPN-dependent stages of the hexose monophosphate pathway. Turnover studies with synchronized, single generations in the stage of
exponential growth should provide the necessary direct evidence for this
suggestion (Fig.6).
A second likely source (although not an absolute requirement; see
Section III, E, 2) of reducing power may reside in the pathway which
leads to synthesis of melanin. This pigment, which impregnates the
chitinous RS wall, does not begin to appear immediately; its apparent
synthesis—at least its accumulation in detectable amounts—does not
start until about two-thirds of exponential growth has gone by (Lovett
and Cantino, 1960b). If it is assumed that synthesis of the tyrosinase
formed by Blastocladiella
(Cantino and Horenstein, 1955) follows the
same course, then a second source of reduced TPN is made available ;
this enzyme system, in cell-free extracts of B. emersonii, can also be