Contents of Cytochromes, Quinones and Reaction Centers of Photosystems I and II in a Cyanobacterium Synechococcus sp.

The contents of photosystem I and photosystem II reaction centers,cytochrome c-553, cytochrome c-550, cytochrome f, cytochromeb-559, cytochrome b-563, plastoquinone and vitamin K₁ in thecyanobacterium Synechococcus sp. were determined. About threephotosystem I reaction centers were present for each photosystemII reaction center. The amounts of cytochromes functioning betweenthe two photosystems were approximately half those of the photosystemI reaction center. Plastocyanin was not detected, while plastoquinoneand vitamin K₁ were present in excess of other electron carriersand reaction centers. The results indicate the importance ofplastoquinone and cytochrome c-553 for cooperation of the tworeaction centers through electron transport. ¹Present address: Toray Basic Research Laboratory, 1111 Tebiro,Kamakura, Kanagawa 248, Japan. (Received June 17, 1982; Accepted January 17, 1983)     

Cytochrome b560 in chromatophores from Chromatium vinosum

A membrane-bound cytochrome of the B-type in Chromatium chromatophores,cytochrome b₅₆₀, was reduced both by flash light activationand continuous illumination in the presence of antimycin atcontrolled ambient redox potentials. The light-minus-dark differencespectra had peaks at 560 and 430 nm, and troughs at 445 and415 nm. The reduction was observed in the ambient redox potentialfrom 400 to about 200 mV. However, below 200 mV, a re-reductionof photooxidized C-type cytochrome superimposed the reductionof cytochrome b₅₆₀ In the absence of antimycin, the reductionwas not observed, suggesting that the reoxidation of cytochromeb₅₆₀ was faster than the reduction. Dark titrations at various pH values showed that E_m7 of thecytochrome b₅₆₀ was about 40 mV and the E_m value was pH-dependent(–60 mV/pH) from pH 6 to 9. Cytochrome b₅₆₀ had a pK ataround pH 9. The content and some properties of cytochrome b₅₆₀ were similarin chromatophores from either photoautotrophically or photoheterotrophicallygrown cells. The possibility of involvement of cytochrome b₅₆₀ in the photosyntheticelectron transfer is discussed. (Received April 19, 1980; )     

Studies on cytochromes in photosynthetic electron transport system I. photoreduction and photo-oxidation of cytochrome b559 by photosystem II in spinach chloroplasts

Light-induced redox-reactions of cytochrome b₅₅₉ in spinachchloroplasts were investigated. Illumination of chloroplastsinduced photoreduction of cytochrorne b₅₅₉ Red light (650 nm)was more effective than far-red light (725 nm), indicating thatthe photoreduction is a photosystem II-mediated reaction. Onaddition of DCMU, the photoreduction was eliminated and a photooxidationof cytochrome b₅₅₉ was observed. The rate of this photooxidationwas faster with photosystem II light than with photo-systemI light. On addition of Mn⁺⁺ the photooxidation was partly suppressed;far-red light became as effective as red light in inducing photooxidationof cytochrome b₅₉₉, in the presence of DCMU and Mn⁺⁺. Ascorbate completely suppressed photooxidation of cytochromeb⁵⁵⁹ In the presence of ascorbate, however, photooxidation wasobserved in the presence of inhibitors or after inhibitory treatmentsof chloroplasts which affected the oxidizing side of systemII. These inhibitors and inhibitory treatments, but not DCMU,decreased the redoxpotential of cytochrome b₅₅₉. Reactivationof Hill reaction in Tris-washed chloroplasts by indophenol-ascorbatetreatment was not accompanied by an abolishment of photooxidationof cytochrome b₅₅₉. A possible mechanism is proposed to account for these reactionsof cytochrome b₅₅₉ in the photosynthetic electron transportin chloroplasts. (Received April 4, 1972; )     

Artificial Quinones Replace the Function of Quinone Electron Acceptor (QA) in the Isolated D1-D2-Cytochrome b559 Photosystem II Reaction Center Complex

Various benzo- and naphthoquinone derivatives were introducedinto the purified photosystem II Dl-D2-cytochrome b₅₅₉ reactioncenter complex, which lacks the intrinsic plasto-quinone electronacceptors. Effects of these quinones on the electron transferreactions in nanoseconds to milliseconds time range were studiedat room and cryogenic temperatures. 1) The addition of quinonesto the purified photosystem II reaction center complex suppressedthe nanosecond charge recombination between oxidized reactioncenter chlorophyll a (P680⁺) and reduced pheophytin a (Ph^–),and stabilized P680⁺ up to millisecond time range at 280 K andat 77 K. 2) In the reaction center complex supplemented withdibromothymoquinone (DBMIB), P68O was almost fully oxidizedand cytochrome b₅₅₉ was partially reduced by flash excitation.A semi-quinone-like signal with a peak around 320 nm was alsoinduced but the shift of pheophytin absorption band (C55O) wasnot observed. 3) Halogenated quinones, especially DBMIB, werebetter electron acceptors than unsubstituted or methylated quinones.4) The affinities of quinones to the reaction center complexwere weakly dependent on their molecular structure. (Received July 9, 1991; Accepted August 15, 1991)     

Comparison between Electron Transfers through Plastocyanin in Spinach Chloroplasts and Cytocbrome C2 in Rhodopseudomonas sphaeroides

Photosynthetic electron transfers through the water-solubleperipheral membrane proteins of plastocyanin and cytochromec₂, were studied in spinach chloroplasts and the photosyntheticbacterium Rhodopseudomonas sphaeroides. In spinach chloroplasts,the rate of flash-induced oxidation of cytochrome f was highlydependent on the salt concentration in the suspending medium.The maximum rate with a half time of 200 µs was observedin the presence of 50 mas KCl or 5 mM MgCl₂. The salt effectwas similar to that on the reaction rate between P700 in thylakoidfragments and externally added plastocyanin. On the other hand,in intact cells of R. sphaeroides, in which cytochrome c₂ islocated in the periplasmic space exposed to the outer ionicenvironment, the rate of cytochrome c₁ oxidation via cytochromec₂ was almost independent of salt concentration. This independencewas a contrast to the strong dependence on salt concentrationof reactions between isolated reaction centers and cytochromec₂ These results suggest that plastocyanin reacts collisionallywith the photosystem I reaction center and cytochrome b₆f complexin a manner that is controlled by the surface electrostaticpotential. Cytochrome c₂, on the other hand, reacts with thebacterial reaction center and cytochrome bc₁ complex probablyby forming a complex prior to activation of the reaction center. ¹ Present address: Department of Biology, Faculty of Science,Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo158, Japan.     

Isolation and Purification of a Ubiquinone-Cytochrome b-c1 Complex from a Photosynthetic Bacterium, Rhodopseudomonas sphaeroides

A ubiquinone-cytochrome b-c₁ complex was removed from chromatophoremembranes of a Rhodopseudomonas sphaeroides green mutant bydeoxycholate-cholate treatment of the chromatophores. The complexwas purified by ammonium sulfate fractionation and gel filtration. The molecular weight of the purified complex was 240,000 (240kD) and it was composed of seven subunits with molecular weightsof 47 kD, 42 kD, 38 kD, 32 kD, 30 kD, 24 kD and 16 kD. The complexcontained 1.54 and 3.42 nmol of cytochrome c₁ and two differentcytochrome b species per mg protein, respectively. It also contained7.07 nmol of ubiquinone, 6.37 nmol of non-heme iron and about3 nmol of carotenoids per mg protein. No flavins were detected.Heme staining indicated that the 32 kD-and 24 kD-subunits werecytochromes. The midpoint potential of cytochrome c₁ was 245 mV, and thevalues for the cytochromes b were 60 mV and –75 mV atpH 7.2. The peak of the -band of the reduced-minus-oxidizeddifference spectrum of cytochrome c₁ was located at 552.5 nm,arid peaks of the b-type cytochromes with higher and lower midpointpotentials were located at 562 nm and 563 nm. The chemical and the subunit compositions of the purified complexreported here were similar to those obtained for the inner membranesof mitochondria of various organisms. (Received April 5, 1982; Accepted June 14, 1982)     

Properties of the Cytochrome c Oxidase Activity of Cytochrome b561 from Photoanaerobically Grown Rhodopseudomonas sphaeroides

Cytochrome b₅₆₁ from Rhodopseudomonas sphaeroides had cytochromec (c2) oxidase activity and a pH optimum at 6.0 for this activity.The activity was affected by the ionic strength of the reactionmixture. The apparent K_m and maximal velocity (V_max) valuesin the absence of addea salts were 14 µM and 120 nmoloxidized per min per mg protein for horse heart cytochrome c.Reduced horse heart cytochrome c was reoxidized in first-orderkinetics by this cytochrome b₅₆₁. The specific activity was0.7 s^–1 per mg protein at 20°C at the concentrationof 30 µMM cytochrome c. Activity was inhibited by KCN and NaN₃, but not by antimycin.The addition of a low concentration of KCN to the cytochromeb₅₆₁ produced a change in the absorption spectrum, evidencethat KCN interacts with the heme moiety of cytochrome b₅₆₁.Results of this and preceeding studies show that the cytochromeoxidase (cytochrome "o") described earlier (Sasaki et al. 1970)is cytochrome b₅₆₁. (Received May 16, 1983; Accepted September 8, 1983)     

Specific Inhibition of Electron Transfer for Nitrate Reduction by a Low Concentration of Cyanide in Rhodobacter sphaeroides f. s. denitrificans

The effects of cyanide on the electron flow in NO₃^– andNO₂^– reductions and photosynthetic electron transfer wereinvestigated with intact cells of a photodenitrifier, Rhodobactersphaeroides f. s. denitrificans. In the presence of 30 µMKCN, electron transfer for NO₃^– reduction was inhibitedby about 70% and the concomitant H translocation was completelyinhibited. However, neither NO₂^– reduction nor photosyntheticcyclic electron transfer was affected at 30 µM. Theseresults suggested that the electron transfer pathway to NO₃^–has, in addition to a b-type cytochrome and the nitratereductase,a component sensitive to a low concenration of cyanide whichis not involvedin the cytochrome bc₁ complex. (Received April 13, 1987; Accepted July 23, 1987)     

Effect of 1,1,1-Trichloro-2,2-bis(p-chlorophenyl)ethane on Cytochromes of the Photosynthetic Electron Transport System

It is shown that in susceptible barley DDT has a marked effecton cytochrome f responses, and on measurable levels of cytochromesb₅₅₉LP, b₅₅₉HP, and b₆. These effects, not shown by treatedresistant barley, are discussed in the light of known sitesof inhibition by DDT of photosynthetic electron transport.     

Cytochrome b oxidation and reduction reactions in the ubiquinone-cytochrome b/c2 oxidoreductase from Rhodopseudomonas Sphaeroides

1. The kinetics of cytochrome b reduction and oxidation in the ubiquinone-cytochrome b/c₂ oxidoreductase of chromatophores from Rhodopseudomonas sphaeroides Ga have been measured both in the presence and absence of anti-mycin, after subtraction of contributions due to absorption changes from cytochrome c₂, the oxidized bacteriochlorophyll dimer of the reaction center, and a red shift of the antenna bacteriochlorophyll.2. A small red shift of the antenna bacteriochlorophyll band centered at 589 nm has been identified and found to be kinetically similar to the carotenoid bandshift.3. Antimycin inhibits the oxidation of ferrocytochrome b under all conditions; it also stimulates the amount of single flash activated cytochrome b reduction 3- to 4-fold under certain if not all conditions.4. A maximum of approximately 0.6 cytochrome b-560 (E_m(7) = 50 mV, n = 1, previously cytochrome b₅₀) hemes per reaction center are reduced following activating flashes. This ratio suggests that there is one cytochrome b-560 heme functional per ubiquinone-cytochrome b/c₂ oxidoreductase.5. Under the experimental conditions used here, only cytochrome b-560 is observed functional in cyclic electron transfer.6. We describe the existence of three distinct states of reduction of the ubiquinone-cytochrome b/c₂ oxidoreductase which can be established before activation, and result in markedly different reaction sequences involving cytochrome b after the flash activation. Poising such that the special ubiquinone (Q_z) is reduced and cytochrome b-560 is oxidized yields the conditions for optimal flash activated electron transfer rates through the ubiquinone-cytochrome b/c₂ oxidoreductase. However when the ambient redox state is lowered to reduce cytochrome b-560 or raised to oxidize Q_z, single turnover flash induced electron transfer through the ubiquinone-cytochrome b/c₂ oxidoreductase appears impeded; the points of the impediment are tentatively identified with the electron transfer step from the reduced secondary quinone (Q_II) of the reaction center to ferricytochrome b-560 and from the ferrocytochrome b-560 to oxidized Q_z, respectively.     

Chromatic Regulation of Cytochrome Composition and Electron Transfer from Cytochrome b6-f Complex to Photosystem I in Anacystis nidulans (Tx 20)

Cytochrome composition of the cyanobacterial photosyntheticsystem was studied with Anacystis nidulans (Tx 20) in relationto the chromatic regulation of photosystem composition. Comparisonof cytochrome compositions in cells with a high PS I/II ratio(3.0, grown under weak orange light) and with a low ratio (1.6,grown under weak red light) indicated that cytochrome compositionwas also changed in the chromatic regulation of photosystemcomposition. Two types of cytochrome change were observed: 1)contents of cytochromes C₅₅₃ and c₅₄₈ were changed in parallelwith the changes in PS I content, and 2) cytochrome b₅₅₃ andcytochrome b₆-f complex were held at a constant molar ratioto PS II. The molar ratio, PS II : cytochrome b₅₅₉ : cytochromeb₆-f complex : cytochrome c₅₅₃ : PS I : cytochrome C₅₄₈, inthe red-grown cells was 1 : 2.5 : 1.3 : 0.17 : 1.6 : 0.67, andthe ratio in the orange-grown cells, 1:2.4:0.9:0.32:3.0:1.2.In both types of cells, almost all cytochrome f in the cytochromeb₆-f complex was rapidly oxidized after multiple flash activation,indicating that all cytochrome b₆-f complexes in cells of bothtypes are functionally connected to PS I, even when the molarratio to PS I is largely changed. The content of cytochromeC₅₅₃ was at most 0.14 of PS I, suggesting that the cytochrometurns over several times per one turnover of PS I. ¹Present address: Department of Biology, Faculty of Science,Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo158, Japan. (Received January 20, 1986; Accepted March 17, 1986)     

Cyt b-559 (Fd) Participating in Cyclic Electron Transport in Spinach Chloroplasts: Evidence for Kinetic Connection with the Cyt b6/f Complex

A small fraction of low potential Cyt b-559, amounting to only13% of total Cyt b-559 in spinach chloroplasts, is analyzedwith the help of a highly selective, computer-controlled spectrophotometer,which simultaneously applies 16 pulse modulated narrow bandmeasuring beams with wavelengths in the cytochrome -band (500–600nm) for recordings of time resolved difference spectra. ThisCyt b-559 fraction remains oxidized upon dark incubation withascorbate and is reduced upon illumination. It can be reducedby cyclic PSI in an antimycin A-sensitive reaction or in thecourse of antimycin A-insensitive linear electron transportvia the Cyt b₆/f complex. Reduction by NADPH in the dark requiresferredoxin. Simultaneous recordings of Cyt b-563 and Cyt f revealclose kinetic connection between this Cyt b-559 fraction andthe low potential chain of the Cyt b₆/f complex. These resultsconfirm and extend previous observations of Miyake et al. 1995(Plant Cell Physiol. 36: 743) in maize mesophyll thylakoids,which led to the hypothesis that Cyt b-559 (Fd) occupies theposition of the postulated ferredoxin-plastoquinone reductase(FQR) in cyclic electron transport. (Received March 9, 1999; Accepted May 21, 1999)     

COMPONENTS OF THE ELECTRON-TRANSFERRING SYSTEM IN ACETOBACTER SUBOXYDANS AND RECONSTRUCTION OF THE LACTATE OXIDATION SYSTEM

1. Cytochromes a₁⁵⁹⁰, b⁵⁶⁰, c₁⁵⁵⁴ and c₁⁵⁵² were isolated andpurifiedfrom a strain of Acetobacter suboxydans. The proceduresusedwere described in detail.
2. The main cytochrome band at550-560 mµ in intact cellssplitted at liquid air temperatureinto two bands, 551 mµ(strong) and 559 mµ (weak).
3. Optical and physiological properties of the four cytochromeswere investigated. Lactic dehydrogenase activity was found tobe associated with cytochrome c₁⁵⁵⁴. The two c₁-type cytochromes,especially cytochrome c₁⁵⁵⁴, persisted in their reduced formafter the purification through many steps.
4. By some combinationsof isolated components reconstruction ofthe oxygen uptake systemcould be realized.
5. The oxygen-consuming activity of purifiedoxidase preparationswas accelerated by a-tocopherol but notby Emasoll 4130 andTween 80.
6. Some discussions were made onthe nature of terminal oxidase,the role of cytochrome c₁⁵⁵²in the electron-transport system,and persistence of reducedstate of c₁-type cytochromes.
7. A possible scheme of the electron-transferringsystem of Acetobactersuboxydans was presented.

(Received May 16, 1960; )     

Electron Transport Coupled to Nitrate Reduction in a Photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

The electron transport system involved in nitrate reductionand its relationship to photosynthetic cyclic electron transportin a photodenitrifier, Rhodopseudomonas sphaeroides forma sp.denitrificans, were studied. Nitrate oxidized only b-type cytochromein the presence of cyanide, which inhibits nitrite reductase.Heptylhydroxyquinoline-N-oxide (HOQNO) inhibited the oxidationof b-type cytochrome by nitrate, but not the oxidation of b-and c-type cytochrome by nitrite. The inhibition by HOQNO wasovercome by phenazine methosulfate (PMS). Absorption changesof b-type cytochrome induced by illumination were in just theopposite directions for oxygen- and nitrate-oxidized cells;the cytochrome was reduced in oxygen-oxidized cells and oxidizedin nitrate-oxidized cells. Antimycin enhanced the reductionand inhibited the oxidation, but had no inhibitory effect onthe oxidation of b-type cytochrome by nitrate. Dithionite-reducedminus ferricyanide-oxidized difference spectra of cells at 77?Kshowed two b-type cytochrome components with a bands at 556.5and 562 nm. The proportion of the b-562 component decreasedin cells grown under denitrifying conditions. It was concludedthat a b-type cytochrome is involved in the nitrate reduction.The b-type cytochrome was presumed to be an alternative to thecytochrome b in the photosynthetic cyclic electron transport. ¹ Present address: Japanese Red Cross Tokyo-to Komagome BloodCenter, Komagome 2-2-2, Toshima-ku, Tokyo 170, Japan. (Received August 13, 1981; Accepted December 5, 1981)     

The Isolation and Characterization of a Cytochrome b6f Complex from the Cyanobacterium Spirulina sp.

A cytochrome b₆f complex was isolated and purified from Spirulinasp. The complex was solubilized with n-heptyl ß-D-thioglucosideand chromatographed on a DEAE-Toyopearl 650M column. The purifiedcomplex contained a small amount of chlorophyll and carotenoid.At least four polypeptides were present in the complex: cytochromef (29 kDa), cytochrome b₆(23 kDa), iron-sulfur protein (ISP,23 kDa), and a 17 kDa polypeptide. Each polypeptide was separatedfrom the complex treated with 2-mercaptoethanol or urea. Theabsorption spectra of cytochrome b₆ and cytochrome f were similarto those of Anabaena and spinach as expected. The complex wasactive in supporting ubiquinol-cytochrome c oxidoreductase activity.Fifty percent inhibition of the activity was accomplished by1 µM dibromothymoquinone (DBMIB). The K_m values for ubiquinol-2and cytochrome c (horse heart) were 5.7 µM and 7.4 µM,respectively. (Received August 15, 1988; Accepted November 14, 1988)     

Exposure of the Cyanobacterium Synechocystis PCC6803 to Salt Stress Induces Concerted Changes in Respiration and Photosynthesis

In order to survive and to grow in the presence of a high salinity(550 mM NaCl) Synechocystis PCC6803 increases its energeticcapacity. The salt-induced increase of electron transport ratesinvolves both cytochrome c oxidase and photosystem I. In contrast,electron transport rates measured through complexes I plus IIIof the respiratory chain, or through the photosystem II pluscytochrome b₆f complexes of the photosynthetic chain, do notshow appreciable changes. The time at which changes in electrontransport rates occur in the photosystem I and cytochrome coxidase complexes after the onset of salt stress indicates similaritiesin the adaptation of dark respiration and (cyclic) photosyntheticelectron flow. Given an increase of whole cell respiration andof PSI cyclic electron flow larger than the neosynthesis ofcytochrome aa3 and PSI reaction centers would predict, it appearsthat both adaptations require more than just synthesis of thesetwo complexes. (Received April 12, 1993; Accepted August 10, 1993)     

Purification and characterization of CMP-N-acetylneuraminic acid hydroxylase from pig submandibular glands

N-Glycoloylneuraminic acid (Neu5Gc) is synthesized as its CMP-giycosideby the action of CMPN-acetylneuramlnic acid (CMP-Neu5Ac) hydroxylase.This enzyme is a soluble cytochrome bs-dependent monooxygenaseand has been purified to apparent homogeneity from pig submandibularglands by precipitation with N-cetyN,N,N-trimethylam-moniumbromide and fractionation on Q-Sepharose, Cibacron Blue 3GA-Agarose,Reactive Brown 10-Agarose, Hexyl-Agarose and Superose S.12.This procedure resulted in an 8960-fold purification of thehydroxylase with a recovery of 0.8%. The molecular mass of thisprotein was shown to be 65 kDa on SDS-PAGE and 60 kDa as determinedby gel filtration on Superose S.12, which suggests that theenzyme is a monomer. The purified CMP-Neu5Ac hydroxylase isactivated by FeSO₄ and inhibited by iron-binding reagents suchas o-phenanthroline, KCN, Tiron and ferro-zine. An apparentKm of 11 µM was determined for the substrate CMP-Neu5Acusing purified hydroxylase in the presence of Triton X-100-solubilizedmicrosomes. In a reconstituted system consisting of purifiedhydroxylase, cytochrome b₅, cytochrome b₅ reductase and catalase,an apparent K_m of 3 µM was measured. The apparent K_mforcytochrome b₅ in this system was 0.24 µM. Immunizationof a rabbit with enriched and purified hydroxylase led to anantiserum that inhibited CMP-Neu5Ac hydroxylase activity andreacted with the purified 65 kDa protein on a Western blot afterSDS-PAGE. Antibodies specific for this 65 kDa protein were isolatedand showed a strong reaction with the purified CMP-Neu5Ac hydroxylasefrom mouse liver after immunoblotting. Initial experiments withthis monospecific antibody suggest that the activity of thehydroxylase in a particular tissue correlates with the amountof immuno-reactive protein. cytochrome b₅ N-glcoloylneuraminic acid hydroxylase pig submandibular gland mucin sialic acid     

Demonstration of Two Sites of Inhibition of Electron Transport by Protein Phosphorylation in Wheat Thylakoids

The effect of protein phosphorylation on electron transportactivities of thylakoids isolated from wheat leaves was investigated.Protein phosphorylation resulted in a reduction in the apparentquantum yield of whole chain and photosystem II (PSII) electrontransport but had no effect on photosystem I (PSI) activity.The affinity of the D1 reaction centre polypeptide of PSII tobind atrazine was diminished upon phosphorylation, however,this did not reduce the light-saturated rate of PSII electrontransport. Phosphorylation also produced an inhibition of thelight-saturated rate of electron transport from water or durohydroquinoneto methyl viologen with no similar effect being observed onthe light-saturated rate of either PSII or PSI alone. This suggeststhat phosphorylation produces an inhibition of electron transportat a site, possibly the cytochrome b₆/f complex, between PSIIand PSI. This inhibition of whole-chain electron transport wasalso observed for thylakoids isolated from leaves grown underintermittent light which were deficient in polypeptides belongingto the light-harvesting chlorophyll-protein complex associatedwith photosystem II (LHCII). Consequently, this phenomenon isnot associated with phosphorylation of LCHII polypeptides. Apossible role for cytochrome b₆/f complexes in the phosphorylation-inducedinhibition of whole chain electron transport is discussed. Key words: Electron transport, light harvesting, photosystem 2, protein phosphorylation, thylakoid membranes, wheat (Triticum aestivum)     

Oxygenated cytochrome o (Vitreoscilla) formed by treating oxidized cytochrome with superoxide anion

Oxygenated cytochrome o can be formed experimentally in twoways, i) by reaction of reduced cytochrome o with molecularoxygen, or ii) by reaction of oxidized cytochrome o with superoxideanion generated by the action of the xanthine oxidase system.It is thermodynamically feasible for oxidized cytochrome o plusO₂–, and reduced cytochrome o plus O₂ to appear as intermediatesin reactions i) and ii), respectively. Superoxide dismutase completely inhibits the xanthine oxidase-catalyzedconversion of oxidized cytochrome o into the oxygenated formbut it has relatively little effect on the oxygenated cytochromeo formation in the reaction system consisting of NADH, NADH-cytochromeo reductase, and cytochrome o. Thus, if superoxide anion doesplay a significant role in the latter system it must be efficientlycoupled to react with cytochrome o and inaccessible to superoxidedismutase. Direct electron transfer from the reductase to thecytochrome without the involvement of superoxide anion is analternative mechanism. (Received December 16, 1976; )     

Cytochromes of Mitochondria from Rice Seedlings Germinated under Water and Their Changes during Air Adaptation

Rice seeds were germinated for up to 5 days under water (submerged)and some for another day in air (air-adapted). Control seedswere germinated for 6 days throughout in air. Low-temperaturedifference spectra of shoot mitochondria were compared amongthese three types of seedlings. All cytochromes found in theaerobic seedlings were present in the submerged seedlings. However,there were some differences in the cytochromes b₅₅₃ and c ofthese two types of seedlings. The cytochrome aa₃ peak heightand cytochrome oxidase activity per mitochondrial protein increased1.6- and 2.8-fold, respectively, during air adaptation. Slightlyhigher concentrations of the b-type cytochromes than found inair-adapted mitochondria were already present in submerged mitochondria.The computed difference between the dithionite-reduced differencespectra of mitochondria from submerged seedlings before andafter air adaptation, showed that cytochromes aa₃ and c hadincreased more than cytochrome b₅₅₇ during air adaptation. (Received November 16, 1987; Accepted March 16, 1988)