Linearity and Functioning Forms in the Carboxylase Reaction of Spinach Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase

The reaction of spinach RuBisCO activated with CO₂ and Mg²⁺proceeded in two phases, an initial burst for a few minutesand the subsequent linear phase, in the presence of saturatingconcentrations of CO₂, ribulose 1,5-bisphosphate (RuBP), andMg²⁺. The percentage of the activity in the linear phase tothat in the initial burst was 55% with RuBisCO prepared withpolyethylene glycol, and very close to the value with the enzymereleased immediately from isolated chloro-plasts. RuBisCO preparedwith ammonium sulfate had a much larger decrease of the activityin the linear phase. The Euglena enzyme had a linear courseof reaction with time for up to 20 minutes. The K_m for CO₂ of spinach RuBisCO activated beforehand was 20µM in the initial burst, and 28 µM in the linearphase. In the carboxylase reaction initiated with inactive enzyme,the activity was initially negligible, but in 5 minutes increasedto the level observed in the linear phase of the activated enzyme.The K_m for CO₂ in the linear phase of the pre-inactivated enzymewas 70 µM. The concentration of RuBP was the immediate cause of the two-phasiccourse of the carboxylase reaction of spinach RuBisCO. The curvatureof the time course was not observed below 35 µM RuBP.The enzyme required over 88 µM RuBP for the conventionaltwo-phasic course. Further increase of the concentration ofRuBP increased the extent of the curvature, but did not startthe curvature sooner after the start of the reaction. Even ifspinach RuBisCO was in the linear phase, dilution of RuBP orits consumption by the enzymatic reaction to less than 30 µMcaused the enzyme to show the resumed biphasic reaction courseafter addition of a high concentration of RuBP. ¹This paper is the twenty-fourth in a series on PhotosyntheticCarbon Metabolism in Euglena gracilis. (Received September 19, 1988; Accepted November 25, 1988)     

Activation of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase by Inorganic Phosphate under Nocturnal Conditions

In vivo activation states of ribulose 1,5-bisphosphate carboxylase/oxygenase(RuBisCO; EC 4.1.1.39 [EC] ) in the dark and light phases were measuredin intact leaves of Phaseolus and radish. The activation statewas high in the dark and comparable to the activation stateunder illumination at saturating light intensity. Then, we examined,using RuBisCO purified from spinach leaves, a mechanism forthe activation of RuBisCO in the dark when the stroma is neutralizedand lossess Mg²⁺ partly. Activation was not obserevd when theenzyme was incubated at air-level CO₂ and 10 mM Mg²⁺ at pH rangingfrom 6.2 to 7.5. However, the activation was highly promotedin this pH range when the activation mixture contained 10 mMinorganic phosphate. The activation state was 50 to 60% betweenpH 7.0 and 7.8 and maximum over pH 8.2 in the presence of 10mM inorganic phosphate. Studies of the initial rate of activationshow that the promotion of activation was through stabilizationof the active form of the enzyme by inorganic phosphate, notby altering the pKa of the activator -amino group of Lys-201.The physiological significance of the activation of RuBisCOby inorganic phosphate in the dark is discussed. ³ Present address: Department of Biochemistry, University ofNebraska, Lincoln, NE 68588-0664, U.S.A.     

The Effects of Benzyladenine on the Accumulation of Messenger RNAs That Encode the Large and Small Subunits of Ribulose-l,5-bisphosphate Carboxylase/oxygenase and Light-Harvesting Chlorophyll a/b Protein in Excised Cucumber Cotyledons

Benzyladenine promoted rapid accumulation of mRNAs that encodedthe small subunit of ribulose-l,5-bisphosphate carboxylase/oxygenase(RuBisCO) and light-harvesting chlorophyll a/b protein in etiolatedcucumber cotyledons, but only after prior incubation of thecotyledons in water. However, benzyladenine hardly affectedthe level of mRNA for the large subunit of RuBisCO. ¹ Present address: Shonan Junior College, Yokosuka, Kanagawa238, Japan (Received September 3, 1990; Accepted March 4, 1991)     

Regulation of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase Activation by Inorganic Phosphate through Stimulating the Binding of the Activator CO2 to the Activation Sites

The mechanism of the regulation of the activation of ribulose1,5-bisphosphate carboxylase/ oxygenase (RuBisCO) by inorganicphosphate (P_i) in the presence of limiting concentrations ofCO₂ was explored. The activation state of RuBisCO increasedsigmoidally following a biphasic kinetics against the concentrationof P_i in the activation mixture with an intermediary plateauat 2 to 3 mM P_i when the enzyme was activated for 30 min. Theintermediary plateau could not be seen when the preincubationtime was 10 min and the activation was completed at 10 mM P_i.RuBisCO from Euglena also showed a quite similar activationkinetics. The activation was not due to the contaminating CO₂included in the stock P_i solution or in the activation buffercontaining the enzyme. The experiments with 2-carboxyarabinitol1,5-bisphosphate showed that the P_i stimulated activation wasdue to the promotion of binding of the activator CO₂ to theactivation sites. It was also found that P_i increased the affinityof RuBisCO for the activator CO₂ 5.4-fold accompanied by a decreaseof the half-saturating concentration of CO₂ to 1.6 µMat 20 mM MgCl₂. Physiological significance of the effects ofP_i on the activation of RuBisCO is discussed. ²Present address: Laboratory of Plant Molecular Physiology,Research Institute of Innovative Technology for the Earth (RITE),9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto, Japan.     

Ribulose bisphosphate-induced, slow conformational changes of spinach ribulose bisphosphate carboxylase cause the two types of inflections in the course of its carboxylase reaction.

The cause of the inflection in the course of the carboxylase reaction and the changes in the functioning form of spinach ribulose bisphosphate carboxylase (RuBisCO) during the reaction were elucidated by relating the activity to the protein conformation of RuBisCO using a fluorescence probe, 2-p-toluidinylnaphthalene sulfonate. The activity of RuBisCO in the linear phase was 50 to 60% of that in the initial burst at 0.5 to 1.0 mM ribulose bisphosphate (RuBP) and 65 to 80% at 2 to 5 mM RuBP. The amount and the progress of the decrease in the activity during the reaction had a close relationship to a change in the protein conformation of RuBisCO. The enzyme, the substrate binding sites of which were masked beforehand with carboxyarabinitol bisphosphate, still showed a change of its protein conformation upon addition of RuBP, suggesting that RuBisCO has two (substrate and regulatory) RuBP-binding sites per RuBisCO promoter. RuBisCO required over 2 mM RuBP for binding on the regulatory sites. Both sites also bound 6-phosphogluconate. When both sites were masked with 6-phosphogluconate beforehand, the course of the subsequent carboxylase reaction was linear with time. From these results, I propose that the inflection in the course of the reaction of spinach RuBisCO is a hysteretic response of the enzyme to RuBP bound to both substrate and regulatory sites.     

Structural and regulatory studies on the ribulose bisphosphate carboxylase-oxygenase of Anabaena cylindrica

Ribulose 1,5-bisphosphate (RuBP) carboxylase-oxygenase fromthe cyanobacterium Anabaena cylindrica has been purified tohomogeneity by the criterion of polyacrylamide gel electrophoresisand shown to consist of two types of subunits of molecular weights51K (large) and 12K (small). The enzyme is of the higher planttype and probably consists of 8 large plus small subunits. Isoelectricfocusing of the S-carboxymethylated protein in 8 M urea revealeda profile of consisting of 3 major polypeptides plus 1 minorpolypeptide. Some characteristics of the carboxylase and oxygenasereactions were studied using simultaneous measurements of bothactivities. Pyridoxal 5'-phosphate inhibited both activitiesequally. Neither the carboxylase nor oxygenase reaction wasaffected by glutamate (5 mM), although 6-phosphogluconate andfructose 1,6-bisphosphate inhibited both reactions. RuBP oxygenasewas more sensitive to 6-phosphogluconate (0.5 and 1.0 mM) thanRuBP carboxylase. Marked changes in the oxygenase to carboxylaseactivity ratio of the purified enzyme were effected by homologousantiserum (which preferentially inhibited carboxylation). ¹Present address: Institute of Applied Microbiology, Universityof Tokyo, Bunkyo-ku, Tokyo 113, Japan. (Received May 22, 1980; )     

Differential Modulation of Carboxylase and Oxygenase Activities of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase Released from Freshly Ruptured Spinach Chloroplasts

The carboxylase activity of ribulose 1,5-bisphosphate (RuBP)carboxylase/oxygenase released from freshly ruptured spinachchloroplasts was stimulated preferentially by Mg²⁺ while oxygenaseactivity was higher with Mn²⁺. Only Mg²⁺ could reactivate eitheractivity of desalted enzyme. The results suggest that carboxylaseand oxygenase activities of RuBP craboxylase/oxygenase can bemodulated selectively by Mg²⁺ or Mn²⁺. ¹ Present address: Department of Botany, Sri Venkateswara University,Tirupati 517 502, India. (Received March 5, 1981; Accepted June 26, 1981)     

Induction of Cytosolic Acetyl-Coenzyme A Carboxylase in Pea Leaves by Ultraviolet-B Irradiation

Levels of subunits of two acetyl-coenzyme A carboxylases werehigh in small leaves of Pisum sativum, decreased with growth,and remained constant in fully expanded leaves. Irradiationof fully expanded leaves induced the cytosolic isozyme only.This result suggests a key role for the cytosolic enzyme inprotection against UV-B. ¹Present address: Laboratory of Molecular Genetics, BiotechnologyInstitute, Akita Prefectural College of Agriculture, 2-2 Minami,Ohgata, Akita, 010-04 Japan ²Present address: Laboratory of Plant Molecular Biology, Schoolof Agricultural Sciences Nagoya University, Nagoya, 464-01 Japan     

A Rapid and Sensitive Method for Determination of Relative Specificity of RuBisCO from Various Species by Anion-Exchange Chromatography

A rapid and sensitive method to determine the relative specificity() of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO)using anion-exchange chromatography is described. We employedthe open gas system for the reaction of RuBisCO to get the mostreliable CO₂ and O₂ concentrations in the reaction mixture.3-Phosphoglycerate and 2-phosphoglycolate which were formedin the RuBisCO reaction were completely separated and directlymeasured with anion-exchange chromatography without using radioisotopes.The determination of the value was accomplished in 3 h. The values of RuBisCO enzymes from higher land plants were between90 and 96, and those from bacteria including cyanobacteriumwere close to 45. These values were in agreement with previouslyreported values. The enzyme of the red macroalga Porphyra yezoensisexhibited a value of over 140, as expected from the reportedvalue of the enzyme from the red microalga Porphyridium cruenteum.RuBisCO from the green macroalga Ulva pertusa had a value closeto 70 and similar to that of the enzyme from the green microalgaChlamydomonas reinhardtii. ⁴On leave from Research and Development Center, Unitika Ltd.,23 Kozakura, Uji, Kyoto, 611 Japan. ⁵Present address: Nara Institute of Science and Technology,8916-5 Takayama-Cho, Ikoma, Nara, 630-01 Japan     

Photosynthetic and Photorespiratory Enzymes in Widely Divergent Plant Species with Special Reference to the Moss Ceratodon purpureus: Properties of Ribulose Bisphosphate Carboxylase/ Oxygenase, Phosphoenolpyruvate Carboxylase and Glycolate Oxidase

Rintamäki, E. and Aro, E.-M. 1985. Photosynthetic and photorespiratoryenzymes in widely divergent plant species with special referenceto the moss Ceratodon purpureus: Properties of ribulose bisphosphatecarboxylase/oxygenase, phosphoenolpyruvate carboxylase and glycolateoxidase.—J. exp. Bot. 36: 1677–1684. Km(CO₂) values and maximal velocities of ribulose bisphosphatecarboxylase/oxygenase (E.C. 4.1.1.39 [EC] ) were determined for sixplant species growing in the wild, consisting of a moss, a fernand four angiosperms. The maximum velocities of the RuBP carboxylasesvaried from 0.13 to 0.;62 µmol CO₂ fixed min^–1 mg^–1soluble protein and the Km(CO₂) values from 15 to 22 mmol m^–3CO₂. The highest Km(CO₂) values found were for the moss, Ceratodonpurpureus, and the grass, Deschampsia flexuosa. These plantsalso had the highest ratios of the activities of RuBP carboxylaseto RuBP oxygenase. Glycolate oxidase (E.C. 1.1.3.1 [EC] ) activitieswere slightly lower in D.flexuosa, but not in C. purpureus,than for typical C₃ species. Phosphoenolpyruvate carboxylase(E.C. 4.1.1.31 [EC] ) was not involved in the photosynthetic carboxylationby these two plants. However, another grass, Phragmites australis,was intermediate in PEP carboxylase activity between C₃ andC₄ plants The properties of RuBP carboxylase/oxygenase are discussedin relation to the activities of PEP carboxylase and glycolateoxidase and to the internal CO₂ concentration. Key words: RuBP carboxylase, oxygenase, Km(CO₂), moss     

Polypeptide Composition of Envelopes of Spinach Chloroplasts: Two Major Proteins Occupy 90% of Outer Envelope Membranes

Outer and inner envelope membranes of spinach chloroplasts wereisolated using floatation centrifugation followed by sedimentationsucrose density gradient centrifugation after disruption ofintact chloroplasts by freezing and thawing. Two major fractionswith buoyant densities of 1.11 and 1.08 g cm^–3 and a minorfraction with a density of 1.15 g cm^–3 were obtained.They were identified as innei and outer envelope and thylakoidfractions, respectively, by analyzing their polypeptide compositionby high-resolution SDS-PAGE and the N-terminal sequences oftheir protein components. Due to the refinement of the isolation procedure, most of theribulose-l,5-bisphosphate carboxylase/oxygenasc (RuBisCO), whichhad always been observed as a contaminant, was eliminated fromthe outer envelope fraction. Application of high-resolutionSDS-PAGE revealed that this fraction was rich in the low-molecular-massouter envelope protein, E6.7 [Salomon et at. (1990) Proc. Natl.Acad. Sci. USA 87: 5778] and a protein with a molecular massof 15 kDa which is homologous to the 16 kDa outer envelope proteinof pea [Pohlmeyer et al. (1997) Proc. Natl. Acad. Sci. USA 94:9504]. The two proteins account for 90% of the total proteinspresent in outer envelope membranes. Proteins which are suggestedto function in translocation of nuclear-encoded polypeptideswere not identified in the envelopes from spinach in the presentstudy. Differences in the protein composition of outer envelopemembranes arc discussed based on the developemental stages ofchloroplasts. ¹Present address: Biological Function Section, Kansai AdvancedResearch Center, Communications Research Laboratory, Ministryof Posts and Telecommunications, Kobe, Hyogo, 651-24 Japan.     

Carboxysomal Diffusion Resistance to Ribulose 1,5-Bisphosphate and 3-Phosphoglycerate in the Cyanobacterium Synechococcus PCC7942

Two preparations of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase(RuBisCO; EC 4.1.2.39 [EC] ) were obtained from the cyanobacteriumSynechococcus PCC7942. In one preparation, the majority of RuBisCOwas insoluble to be localized in carboxysomes ("CarboxysomalRuBisCOrdquo;), whereas in the other, the enzyme was solubilized("Solubi-lized RuBisCO"). The kinetic properties of both RuBisCOpreparations were determined to elucidate the changes of theactivity based on packing the enzyme in carboxysomes. The activityof Carboxysomal RuBisCO decreased with increasing reaction time.The apparent affinity of the Carboxysomal RuBisCO for RuBP waslower than that of the Solubilized RuBisCO. The 3-phosphoglycerateproduced in carboxysomes was detected with a lag of three minutes.These results suggest that RuBisCO activity changes dependenton packing in carboxysomes and have a diffusion resistance toRuBP and PGA (Received October 31, 1996; Accepted April 11, 1997)     

Changes in Levels of Phosphoenolpyruvate Carboxylase with Induction of Crassulacean Acid Metabolism in Mesembryanthemum crystallinum L.

Expanded leaves of Mesembryanthemum crystallinum L. performingC₃ photosynthesis were induced to perform pronounced Crassulaceanacid metabolism (CAM) by exposing the plant roots to higherNaCl concentration. Levels of phosphoenolpyruvate (PEP) carboxylaseactivity increased 10-fold during the 7-day induction period.Densitometric analysis of Coomassie-stained sodium dodecyl sulfate(SDS) polyacrylamide gradient slab gels of leaf extracts, preparedduring the course of CAM induction, revealed that at least fivebands of polypeptides increased in content (kilodalton valuesof 98, 91, 45, 41, 38). Higher levels of three additional polypeptides(kilodalton values of 102, 76, 33) became apparent after tissuehad been grown for 2 weeks at 400 mM NaCl. Of these polypeptides,that having a mass of 98 kilodaltons was identified as the subunitof PEP carboxylase by comparison with the corresponding bandfrom partially purified PEP carboxylase from the same tissue.Only a faint 98 kilodalton band was evident on SDS gels fortissue operating in the C₃ mode; staining intensity at thislocation increased with increasing NaCl-salinity in the rootingmedium until CAM was fully induced. These data provide evidencefor net synthesis of PEP carboxylase and several other proteinsduring the induction of CAM in M. crystallinum. ¹ Present address: USDA, P. O. Box 867 Airport Rd., Beckley,WV. 25801, U.S.A. ² Present address: Department of Botany, Washington State University,Pullman, Washington 99164, U.S.A. ³ Present address: Botanisches Institut der Universit?t, MittlererDallenbergweg 64, 8700 W?rzburg, W.-Germany. (Received October 27, 1981; Accepted March 15, 1982)     

SOME PROPERTIES OF THE CYTOCHROME C REDUCING SUBSTANCE, A FACTOR FOR LIGHT-INDUCED REDOX REACTION OF CYTOCHROME C IN PHOTOSYNTHETIC LAMELLAE

Cytochrome c reducing substance (CRS), a redox substance discoveredin photoreactive lamellar fragments, was purified by Sephadexcolumn chromatography. Chromatographic behaviours of CRS ofAnabaena and spinach were essentially the same. Purified CRSof Anabaena showed an absorption spectrum having one absorptionmaximum around 260 mµ. The absorption peak disappearedon addition of excess amount of borohydride. Similar absorptionchange on borohydride addition was observed with spinach CRSpreparation. Purified preparations of Anabaena and spinach CRS supportedphotophosphorylation in spinach broken chloroplasts. The phosphorylationwas found to couple the electron flow from water to molecularoxygen. ¹This work was supported by grant GM-11300 from the NationalInstitute of Health, U. S. A. ²Present address: Institute of Applied Microbiology, The Universityof Tokyo, Tokyo, Japan.     

Characterization of a Maize Ca2+-Dependent Protein Kinase Phosphorylating Phosphoenolpyruvate Carboxylase

Phosphoenolpyruvate carboxylase (PEPC) [EC 4.1.1.31 [EC] ] of plantsundergoes regulatory phosphorylation in response to light ornutritional conditions. However, the nature of protein kinase(s)for this phosphorylation has not yet been fully elucidated.We separated a Ca²⁺-requiring protein kinase from Ca²⁺-independentone, both of which can phosphorylate maize leaf PEPC and characterizedthe former kinase after partial purification. Several linesof evidence indicated that the kinase is one of the characteristicCa²⁺-dependent but calmodulin-independent protein kinase (CDPK).Although the M_r, of native CDPK was estimated to be about 100kDa by gel permeation chromatography, in situ phosphorylationassay of CDPK in a SDS-polyacrylamide gel revealed that thesubunit has an M_r of about 50 kDa suggesting dimer formationor association with other protein(s). Several kinetic parameterswere also obtained using PEPC as a substrate. Although the CDPKshowed an ability of regulatory phosphorylation (Ser-15 in maizePEPC), no significant desensitization to feedback inhibitor,malate, could be observed presumably due to low extent of phosphorylation.The kinase was not specific to PEPC but phosphorylated a varietyof synthetic peptides. The possible physiological role of thiskinase was discussed. ¹Present address: NEOS Central Research Laboratory, 1-1 Ohike-machi,Kosei-cho, Shiga, 520-3213 Japan. ²Present address: Chugai Pharmaceutical Co., Ltd., 1-135 Komakado,Gotemba, 412-0038 Japan. ⁴N.O. and N.Y. contributed equally to this work.     

Glyoxylate Inhibition of Ribulosebisphosphate Carboxylase/Oxygenase Activation State in vivo

Photosynthetic CO₂ exchange in photorespiration mutants of Arabidopsisthaliana showed a time-dependent inhibition at 350 µl/literCO₂ in 50% O₂ but not in 2% O₂. In a glycolate-P phos-phatasedeficient mutant, inhibition of photosynthesis was due to adepletion of ribulosebisphosphate. In the remaining mutants,which have defects in photorespiratory enzymes which metabolizeamino acids, reduced photosynthesis was accompanied by a declinein the activation level of ribulosebisphosphate carboxylase/oxygenase(Chastain and Ogren 1985), a decline in ribulosebisphosphateconcentration, and an accumulation of glyoxylate. Addition ofglyoxylate at submillimolar concentrations to intact spinach(Spinacea oleracea L.) chloroplasts inhibited light activationof ribulosebisphosphate carboxylase/oxygenase (rubisco) andCO₂ fixation. Similar concentrations of glyoxylate had no effecton A. thaliana rubisco activity in vitro. These results suggestthat glyoxylate accumulation indirectly inhibited rubisco activationstate in vivo. The inhibition of photosynthesis in mutants whichaccumulate glyoxylate may be attributed to a decline in ribulosebisphosphateconcentration, a reduction in rubisco activation state, or acombination of both phenomena. ³Present address: CSIRO, Division of Plant Industry, GPO Box1600, Canberra, ACT 2601, Australia. (Received May 12, 1989; Accepted July 8, 1989)     

Aldolase--An Important Enzyme in Controlling the Ribulose 1,5-bisphosphate Regeneration Rate in Photosynthesis

This study was done to explore an enzymatic mechanism for thephotosynthetic carbon reduction cycle whereby the rate of synthesisof ribulose 1,5-bisphosphate (RuBP) could be changed while thelevels of intermediates other than 3-phosphoglycerate and RuBPwere kept constant. Chloroplast aldolase was purified to homogeneityfrom spinach leaves. When the enzyme was assayed in the directionof fructose 1,6-bisphosphate synthesis in the presence of theconcentrations of the substrates reported in vivo, the activitywas severely inhibited by physiological concentrations of RuBP.The aldolase reaction proceeded with a sequential mechanism.The K_m for dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphatewere 0.45 mM and 40 µM, respectively. The activity wascompetitively inhibited by RuBP with respect to dihydroxyacetonephosphate. The K^I was 0.78 mM. The maximum activity of aldolasein spinach leaves was calculated as 1,360µmol (mg Chl)^–1h^–1 An equation to express the reaction for the synthesisof fructose 1,6-bisphosphate by aldolase was constructed topredict the metabolic rate of this reaction in vivo. The calculationclearly showed that aldolase is an important enzyme in controllingthe rate of RuBP regeneration. (Received March 25, 1991; Accepted August 12, 1991)     

The Deduced Amino Acid Sequence of Phytochrome from Adiantum Includes Consensus Motifs Present in Phytochrome B from Seed Plants

A cDNA for the phytochrome of the fern Adiantum capillus-venerisL. was cloned and sequenced. The deduced phytochrome is 50{smalltilde}55% identical to phytochromes of seed plants, and 68%identical to Selaginella phytochrome. Regions resemble thosein previously characterized phytochromes from ferns, lower plantsand seed plants. ³Present address: Yamanouchi Pharmaceutical Co., Ltd., 21 Miyukigaoka,Tsukuba-shi, Ibaraki, 305 Japan ⁴Present address: Plant Growth Regulation Laboratory, The Instituteof Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi,Saitama, 351-01 Japan ⁵Present address: Advanced Research Laboratory, Hitachi, Ltd.,Hatoyama, Saitama, 350-03 Japan     

Some Properties of Ribulose Bisphosphate Carboxylase Extracted from Tomato Leaves

Ribulose bisphosphate carboxylase (EC 4.1.1.39 [EC] ) activity wasvery low in tomato leaf extracts unless prepared in the presenceof Mg²⁺, and polyclar AT. With young leaves, but not with fully-expanded leaves, the RuBP carboxylase activityextracted was increased by prolonged illumination of the leaves(2 h). The main effect of the light treatment was to increasethe specific activity of the enzyme but there was also a smallincrease in RuBP carboxylase protein. Tomato leaf RuBP carboxylasein extracts had specific activities in the range 0.2–0–6µmol CO₂ min^–1 mg^–-1 total protein extracted,or 0.5–1.2 µmol CO₂ min^–1 mg^–1 RuBPcarboxylase, and an apparent K_m (CO₂) at 20 ?C of 9.3 ? 1.2µM (using a of 6.407). Key words: Tomato leaf, RuBP carboxylase, Properties