76
Ν. G. PON
dark reactions of photosynthesis (497). The rate of C0 2 fixation, however, is low and amounts to a maximum of 1-2.5% that in the intact leaf
(498), which may be due to a loss of photosynthetic pyridine nucleotide
reductase (499). Recently, Heber and Tyszkiewicz obtained an extrapolated rate equal to 15% the maximum rate of the intact leaf by conducting
the experiment with a concentrated suspension of aqueous prepared
chloroplasts supplemented with proteins from nonaqueous prepared
chloroplasts (500).
Trebst et al. first showed that the light and dark reactions of photosynthesis can be physically separated by centrifuging osmotically swollen
spinach chloroplasts (501). The resulting pale-colored supernatant solution is able to carry out the C0 2 fixation whereas the green precipitate
can carry out the light reactions, generating ATP and reducing power
along with the production of oxygen. Later, Park and Pon confirmed
these findings and showed further that even very small green fragments,
obtained by sonically rupturing chloroplasts, are able to carry out the
light reaction at full capacity (502).
When isolated chloroplasts from various species are exposed to C
14 0 2 ,
the fixation patterns for the different species are nearly identical (503).
Most of the label is located in the sugar monophosphates, and lesser
amounts are found in the diphosphates, dihydroxyacetone, and phosphoglycerate. The species studied were spinach, sugar beet, pokeweed, sunflower, and New Zealand spinach (Tetragonia expansa). The rates of
C0 2 fixation, under illumination, ranged from 1.4 to 3.5 /xmoles carbon
fixed per hour per milligram chlorophyll. Maximum rates for the intact
spinach leaves are about 200 //moles per hour per milligram chlorophyll.
Evidence for the presence of carboxydismutase in spinach chloroplasts was obtained by Fuller and Gibbs (425) and by Lyttleton and
Ts'o (504). The former workers found that the chloroplasts contain the
bulk of the carboxylase activity when compared with the whole cell. By
using nonaqueous techniques for the isolation of the chloroplasts, Smillie
and Fuller showed an excellent correlation between the carboxydismutase activity and the chlorophyll content of different fractions (94),
indicating that the enzyme is associated with the chloroplasts. Finally,
Heber et al. obtained quantitative data that clearly show the complete
localization of carboxydismutase in the chloroplasts of tobacco, spinach,
and New Zealand spinach (95).
The occurrence of carboxydismutase in higher plants is widespread.
Some examples of higher plants containing this carboxylase activity are
itemized in Table XVII. In these higher plant tissues, the enzymatic
activity was assayed directly in the presence of bicarbonate and ribulose
diphosphate, but in some cases ribose-5-phosphate plus ATP were sub-
Ν. G. PON
dark reactions of photosynthesis (497). The rate of C0 2 fixation, however, is low and amounts to a maximum of 1-2.5% that in the intact leaf
(498), which may be due to a loss of photosynthetic pyridine nucleotide
reductase (499). Recently, Heber and Tyszkiewicz obtained an extrapolated rate equal to 15% the maximum rate of the intact leaf by conducting
the experiment with a concentrated suspension of aqueous prepared
chloroplasts supplemented with proteins from nonaqueous prepared
chloroplasts (500).
Trebst et al. first showed that the light and dark reactions of photosynthesis can be physically separated by centrifuging osmotically swollen
spinach chloroplasts (501). The resulting pale-colored supernatant solution is able to carry out the C0 2 fixation whereas the green precipitate
can carry out the light reactions, generating ATP and reducing power
along with the production of oxygen. Later, Park and Pon confirmed
these findings and showed further that even very small green fragments,
obtained by sonically rupturing chloroplasts, are able to carry out the
light reaction at full capacity (502).
When isolated chloroplasts from various species are exposed to C
14 0 2 ,
the fixation patterns for the different species are nearly identical (503).
Most of the label is located in the sugar monophosphates, and lesser
amounts are found in the diphosphates, dihydroxyacetone, and phosphoglycerate. The species studied were spinach, sugar beet, pokeweed, sunflower, and New Zealand spinach (Tetragonia expansa). The rates of
C0 2 fixation, under illumination, ranged from 1.4 to 3.5 /xmoles carbon
fixed per hour per milligram chlorophyll. Maximum rates for the intact
spinach leaves are about 200 //moles per hour per milligram chlorophyll.
Evidence for the presence of carboxydismutase in spinach chloroplasts was obtained by Fuller and Gibbs (425) and by Lyttleton and
Ts'o (504). The former workers found that the chloroplasts contain the
bulk of the carboxylase activity when compared with the whole cell. By
using nonaqueous techniques for the isolation of the chloroplasts, Smillie
and Fuller showed an excellent correlation between the carboxydismutase activity and the chlorophyll content of different fractions (94),
indicating that the enzyme is associated with the chloroplasts. Finally,
Heber et al. obtained quantitative data that clearly show the complete
localization of carboxydismutase in the chloroplasts of tobacco, spinach,
and New Zealand spinach (95).
The occurrence of carboxydismutase in higher plants is widespread.
Some examples of higher plants containing this carboxylase activity are
itemized in Table XVII. In these higher plant tissues, the enzymatic
activity was assayed directly in the presence of bicarbonate and ribulose
diphosphate, but in some cases ribose-5-phosphate plus ATP were sub-