230
GEORGE G. LATIES
lating systems, namely, adenylic acid or its equivalent, and Mg+
+ . The
studies cited in Table 2 collectively affirm this point. Table 3 in turn
offers some indication of the phosphorylative activity of mitochondria
obtained from a diversity of plant organs.
Mitochondrial preparations from plant materials seem up to now to
depend less rigorously upon the addition of exogenous phosphate-transferring systems than do carefully prepared mitochondrial suspensions
from animal tissues (Potter et αι., 1953). The reasons for this are
several. Mitochondria from various plant tissues contain an active hexokinase (Saltman, 1953). The more carefully the mitochondria are prepared the greater the fraction of total hexokinase which is bound to the
particles. Since mitochondria are normally suspended in sucrose, a
combination of intramitochondrial hexokinase and invertase (see Millerd and Bonner, 1953) provides a complete phosphate-transferring system in the presence of adenylate and Mg+ + . In such a system catalytic
quantities of adenylate bring about the phosphorylation of appreciable
quantities of glucose to glucose-6-phosphate. Plant mitochondrial preparations also frequently display appreciable ATPase activity (Bonner and
Millerd, 1953). It remains a moot question as to whether there is any
ATPase activity in vivo in plant as well as in animal tissues.
When the oxidative steps in such a series as comprises the tricarboxylic
acid cycle share in common one or more transphosphorylating enzymes
or coenzymes, the respiratory pattern observed upon addition of a given
substrate may differ qualitatively, according to whether the transphosphorylating system limits the respiration rate or not. This conclusion was
reached by Laties (1953b) on the basis of experiments involving an inhibition by malonate of the oxidation of malate or citrate. When cauliflower-bud mitochondria, in the absence of exogenous hexokinase, were
given high concentrations of malate or citrate (0.03-0.05 M), together
with catalytic quantities of adenylate, there was no inhibition of the
respiratory activity by malonate, which is known to inhibit succinic dehydrogenase competitively. When substrate concentrations were lowered
(0.01 M), however, an inhibition by malonate was demonstrable. It was
established that the failure of malonate to inhibit at high substrate
levels was attributable not to the extensive formation of succinate from
the initial substrate, as might be expected, but rather to the absence
of succinate formation, and hence to the lack of participation of succinic
dehydrogenase in the over-all respiration. The observations were thereupon explained in terms of a competition among the oxidative components of the cycle for a common phosphorylative intermediate. That
component of the cycle for which a high concentration of substrate was
supplied was thought to preempt the common intermediate. As a conse-
GEORGE G. LATIES
lating systems, namely, adenylic acid or its equivalent, and Mg+
+ . The
studies cited in Table 2 collectively affirm this point. Table 3 in turn
offers some indication of the phosphorylative activity of mitochondria
obtained from a diversity of plant organs.
Mitochondrial preparations from plant materials seem up to now to
depend less rigorously upon the addition of exogenous phosphate-transferring systems than do carefully prepared mitochondrial suspensions
from animal tissues (Potter et αι., 1953). The reasons for this are
several. Mitochondria from various plant tissues contain an active hexokinase (Saltman, 1953). The more carefully the mitochondria are prepared the greater the fraction of total hexokinase which is bound to the
particles. Since mitochondria are normally suspended in sucrose, a
combination of intramitochondrial hexokinase and invertase (see Millerd and Bonner, 1953) provides a complete phosphate-transferring system in the presence of adenylate and Mg+ + . In such a system catalytic
quantities of adenylate bring about the phosphorylation of appreciable
quantities of glucose to glucose-6-phosphate. Plant mitochondrial preparations also frequently display appreciable ATPase activity (Bonner and
Millerd, 1953). It remains a moot question as to whether there is any
ATPase activity in vivo in plant as well as in animal tissues.
When the oxidative steps in such a series as comprises the tricarboxylic
acid cycle share in common one or more transphosphorylating enzymes
or coenzymes, the respiratory pattern observed upon addition of a given
substrate may differ qualitatively, according to whether the transphosphorylating system limits the respiration rate or not. This conclusion was
reached by Laties (1953b) on the basis of experiments involving an inhibition by malonate of the oxidation of malate or citrate. When cauliflower-bud mitochondria, in the absence of exogenous hexokinase, were
given high concentrations of malate or citrate (0.03-0.05 M), together
with catalytic quantities of adenylate, there was no inhibition of the
respiratory activity by malonate, which is known to inhibit succinic dehydrogenase competitively. When substrate concentrations were lowered
(0.01 M), however, an inhibition by malonate was demonstrable. It was
established that the failure of malonate to inhibit at high substrate
levels was attributable not to the extensive formation of succinate from
the initial substrate, as might be expected, but rather to the absence
of succinate formation, and hence to the lack of participation of succinic
dehydrogenase in the over-all respiration. The observations were thereupon explained in terms of a competition among the oxidative components of the cycle for a common phosphorylative intermediate. That
component of the cycle for which a high concentration of substrate was
supplied was thought to preempt the common intermediate. As a conse-