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Regulation of Acetyl-Coenzyme A Carboxylase and Acetyl-Coenzyme A Synthetase in Spinach Chloroplasts Andreas Sauer and K laus-P eter H eise Lehrstuhl für Biochemie der Pflanze der Universität G öttingen. Untere Karspüle 2, D-3400 Göttingen. Bundesrepublik Deutschland Z. Naturforsch. 39c, 2 6 8 -2 7 5 (1984); received N ovem ber 24, 1983 Acetyl-CoA Carboxylase, A cetyl-CoA Synthetase, Light D ependence o f Fatty Acid Synthesis in Chloroplasts In analogy to chloroplast fatty acid synthesis from acetate the key enzym es o f acetate fixation, acetyl-CoA synthetase and acetyl-CoA carboxylase, in rapidly Triton X-100 lysed spinach chloroplasts show an activation by light and deactivation in the dark. The stim ulation o f acetyl-CoA carboxylase by dithiothreitol in darkened chloroplasts points to an involvem ent o f reducing equivalents in the light activation o f this enzyme. But more than by alterations o f the activation state per se, these enzymes appear to be effected by changes in their catalytic activity due to differences in the proton-, M g2+- and adenine nucleotide levels o f the chloroplast stroma. Thus the pH dependence o f both enzymes, as im m ediately extracted from Triton X-100 lysed chloroplasts, resembles that recently found for lipid incorporation o f acetate into intact spinach chloroplasts in the light with an identical pH optim um o f about pH 8.5 for the acetyl-CoA carboxylase. Moreover, in the same extracts both enzym e activities show the already postulated requirement for MgATP and free Mg and are com petitively inhibited by free ATP and A D P with respect to MgATP. But on account o f the fact, that the extractable acetyl-CoA synthetase as opposed to the carboxylase activities exceed by far the lipid incorporation rates o f acetate by illuminated chloroplasts before disruption, acetyl-C oA synthetase will be excluded as rate limiting step in fatty acid synthesis from acetate. From key enzym es o f acetate fixation only the carboxylase appears to be involved therefore in the light regulation o f acetate incorporation into long-chain fatty acids. Introduction Materials and Methods Chloroplast fatty acid synthesis from acetate appears to be regulated by light [1 -6 ]. L ike the Calvin cycle enzymes this effect o f light seem s to be m ediated by light dependent variab le p ara m ete rs in the chloroplast strom a, e.g. the concentration o f H +, M g2+ and ATP [6 ] which are know n essential factors for acetate activation [3,7], F u rth e rm o re lightenhancem ent of fatty acid synthesis from acetate may be due to an enzym e interconversion m e d iated by changes in the redox poising [6 , 8 ]. A cetyl-C oA carboxylase has been exam ined as possible site for such a light regulation o f fatty acid synthesis [8 ]. In this study we have exam ined: Spinach (Spinacia oleracea, U.S. H ybrid 4 2 4 , Ferry-M orse Co., M ountain View, CA, USA) was grown in hydroponic culture [ 1 1 ] and intact ch lo ro plasts were prepared as in [12]. T he chloroplasts ( 1 0 0 (ig Chi • m l-1) w hich show ed an C ^-evolution rate in the range o f 8 0 - 1 4 0 jim ol • m g - 1 Chi • h - 1 were suspended in a reaction m ixture co n tain in g 0 . 3 3 m sorbitol, 5 0 m M H epes/K O H pH 7.6, 1 m M M gC b, 1 m M M n C h , 2 m M ED TA , 0 . 5 m M K 2 H P 0 4, 10 m M N a H C 0 3 and 4 0 |ig • m f 1 catalase from bovine liver (Boehringer, M annheim ), unless otherw ise stated. . the acvtivation state o f the key enzym es o f acetate fixation in chloroplasts, e.g. acetyl-C oA synthetase and acetyl-C oA carboxylase in im m ediately lysed chloroplasts [9] durin g a lightdark transient, and 2 . the kinetic dependence o f these enzym e activities rapidly extracted from illu m in ated chloroplasts [ 1 0 ] on various strom al solutes and m etabolites. Assay o f acetyl-CoA synthetase 1 Reprint requests to Dr. K.-P. Heise. 0341-0382/84/0300-0268 $ 0 1 .3 0 /0 The radioactivity assay of acetyl-C oA synthetase in chloroplasts by [ l - 14C ]acetate in co rp o ratio n [13] was carried out with 100 m M T ricin e /K O H (pH 8.0), 2 m M M gC l2, 1 m M ATP. 0.5 m M CoA , 0.5 m M [ l - , 4 C]acetate (specific activity: 57,2 m C i • m m o r 1) and 0.2% (w /v) T riton X-100 (final volume: 0.3 ml), if not stated otherw ise. T he reaction was stopped w ith 0.3 ml o f an activated charcoal slurry: glacial acetic acid ( 1 : 1 ) m ix tu re and labelled acetyl-CoA, adsorbed to charcoal, was Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung-Keine Bearbeitung 3.0 Deutschland Lizenz. This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution-NoDerivs 3.0 Germany License. Zum 01.01.2015 ist eine Anpassung der Lizenzbedingungen (Entfall der Creative Commons Lizenzbedingung „Keine Bearbeitung“) beabsichtigt, um eine Nachnutzung auch im Rahmen zukünftiger wissenschaftlicher Nutzungsformen zu ermöglichen. On 01.01.2015 it is planned to change the License Conditions (the removal of the Creative Commons License condition “no derivative works”). This is to allow reuse in the area of future scientific usage. A .Sauer and K.-P. H eise • R egulation o f A cetyl-CoA C arboxylase an d A cetyl-C oA S ynthetase m easured according to K uhn et al. [13]. C ontrol sam ples contained neither CoA nor ATP, both of w hich were essential for high rates o f [ l - 14C ]acetate adsorption to the charcoal. Assay o f acetvl-CoA carboxylase Acetyl CoA carboxylase activity in chloroplasts was measured as an acetyl-C oA -dependent fixation of H I4C 0 3 into acid-soluble, h eat-stable products [14]. The standard reaction m ixture contained 100 m M T ricine/K O H (pH 8,5), 0.5 m M Acetyl-CoA , 1 m M ATP, 2 m M M gC l2, 30 m M KC1, 7 m M N a H C 0 3, 3 m M N a H l4 C 0 3 (specific activity: 1 m Ci • m m o l-1) and 0.2% (w /v) T rito n X-100 in a final volum e o f 0.3 ml. The reaction was term in ated by adding 0.3 ml o f 2 n HC1. T he sam ples w ere dried at 70 °C in a scintillation vial and 0.2 ml 2 n HC1 and 7 ml scintillator ( 6 g butyl-PBD , 600 ml toluene, 400 ml methylglycol) were then added and the acid-stable radioactivity was counted by liquid scintillation. Control sam ples w ith out acetyl-C oA were included in each series o f the assays. Measurement o f enzyme interconversion F or the study o f the kinetics and the state of activation and inactivation o f the enzymes ch lo ro plasts in the stan d ard m edium w ere preillu m in ated or kept dark in the absence and presence o f d iffe r ent D TT concentrations at 25 °C for th e tim e intervals indicated (Figs. 1 and 2) and 50 (il aliquots were w ithdraw n and im m ediately lysed in 250 |il o f a reaction m ixture containing 0.2% w /v T riton X-100 [9, 15] and the essential unlabelled or ra d io active substrates o f the enzym atic reaction to be assayed. The final chlorophyll conc. was 20 |ig / sample. This procedure allow s an instantaneous start of enzym e activity m easurem ents. T he enzym ic reaction, as initiated by the disru p tio n o f the chloroplasts, was term inated 30 s later by ad d itio n o f hydrochloric acid, in o rd er to keep the tim e of the assay short in relation to the kinetics o f activa tion and inactivation [15] and to m easure in the linear region o f the reaction (up to 90 s). Preparation o f chloroplast crude extracts In order to keep the investigated enzym es u nder approxim ate physiological conditions, m o d u latio n o f their catalytic activity by ions (H +, M g2+) and adenine nucleotides (ATP, A D P) has been m ea 269 sured in substrate free chloroplast crude extracts, prepared by rapid elution o f T rito n X-100 (final conc. 0 . 2 % w /v) lysed p reillu m in ated ch lo ro p lasts over a short Sephadex G 25 colum n acco rd in g to Laing and R oughan [10] and ad ju sted to 0.1 - 0 .2 m g Chi m r 1. In order to d iscrim inate betw een the effect o f free and adenine nucleotide b o u n d M g2+ on the activities o f acetyl-C oA synthetase and carboxylase in above chloroplast extracts we calculated th e free nucleotide concentration in the in cu b atio n by using the stability constants o f K räm er [16]. Extraction and detection o f labelled fatty acids was as described earlier [17], Results and Discussion Recent m easurem ents o f the endogenous acyl-acyl carrier protein pool sizes d u rin g stead y -state fatty acid synthesis by isolated spinach ch lo ro p lasts [18] dem onstrated the well know n light d ep e n d en ce o f fatty acid synthesis from acetate [ 1 -3 ] at th e level of its thioester interm ediates. T hese results im ply that the level o f acyl-carrier p ro tein th io esters reaches a “steady sta te” w ithin 1 m in o f illu m in a tion and declines im m ed iately after d ark e n in g o f the chloroplasts. Because the latter co m p o u n d s are direct substrates for specific acyl-thioesterases and transferases, the decline o f the acy l-carrier p ro tein pool in the dark leads to an im m ed ia te sto p in the release of free fatty acids and reduced g ly cero lip id synthesis [5]. Enzyme interconversion In order to localize the light d ep e n d en t steps in the reaction sequence from acetate to lo n g -ch ain fatty acids we started to investigate to w h at extent the activities o f th e key enzym es in acetate fixation, e.g. acetyl-CoA synthetase and acetyl-C oA car boxylase, are controlled by light. Fig. 1 shows a tim e course for th e light activ atio n and dark inactivation o f acetyl-C oA syn th etase and acetyl-CoA carboxylase o f chloroplasts in situ. A fter the onset o f illu m in atio n the activity o f th e acetylCoA synthetase increases ab o u t 2-fold an d th a t of acetyl-CoA carboxylase increases a b o u t 3-fold. D arkening leads to a decrease o f b o th enzym e activities. C om pared to the rap id red u ctio n o f acylacyl carrier protein synthesizing capacity o f chloro- 270 A .Sauer and K.-P. Heise • R egulation o f A cetyl-CoA C arboxylase and A cetyl-C oA S ynthetase Light t E - Dark I 2.0 03 Acetyl - CoA carboxylase 0 .2 o 01 Acetyl - CoA synthetase 10 Time ( min) Fig. 1. Changes in the level o f m alonyl-CoA ( • ) and acetyl-CoA ( a ) formed from [ l - 14C]acetate in spinach chloroplasts during a light-dark transition. The tim e o f light activation and dark inactivation o f intact chloroplasts before their disruption in the Triton X-100 containing reaction mixture is plotted on the abscissa. In order to keep the time o f the assay short in relation to the kinetics o f activation and inactivation and to measure in the linear region o f the reaction, the test was terminated after 30 s by addition of acid. plasts after turning o ff the light [18] the d eactiv atio n of acetyl-CoA synthetase and acetyl-C oA carb o x ylase appears relatively low (Fig. 1). L atter c o m p a r ison suggests, alterations o f the activation state o f these enzymes, as caused for exam ple by reducing equivalents, to play a su b o rd in ate role in th e lightenhancem ent o f p lastidary fatty acid synthesis from acetate. However, recent reinvestigations on the effect o f the sulfhydryl reagent d ith io th reito l (D T T ) on dark activation o f acetate in c o rp o ratio n into long-chain fatty acids o f isolated chloroplasts [6 ], which appeared contradictory in the literatu re [4, 19], led to the conclusion, th a t beside changes in stromal solutes (H +, M g2+ and A TP) reducing equivalents may be in som ehow involved in the light control o f fatty acid synthesis. L ast notio n directed our attention to the localization o f the DTT-sensible step w ithin the reaction sequence o f the plastidary acetate fixation, particu larly because o f the recent finding, light to be essential only for acetyl-CoA carboxylase activity in fatty acid synthesis o f spinach chloroplasts [8 ], Therefore, in analogy to the enzym e in terco n version m easurem ents (see m ethods), intact darkened chloroplast suspensions w ere in cu b ated for 10 min with different D T T concentrations in the dark (Fig. 2), before they were lysed in th e T ri ton X-100 containing reaction m ixture. In spite o f a five-fold dilution o f D T T in the reaction m ix tu re this method was not fit for a q u an tita tiv e d etectio n o f the acetyl-CoA synthetase activity by ad so rp tio n o f l4C-labelled acetyl-CoA (from [ 14 C ]acetate) to charcoal [13], because D T T undergoes a nonenzymic acetylation with acyl-CoA th io esters [19]. On the other hand carboxylase activities, d e te r mined by ,4 C 0 2-fixation to acetyl-C oA , could be determ ined quantitatively if one allow ed for adequate acetyl-CoA concentrations (0.5 mM) in the reaction mixture, to ensure th a t the fo rm a tio n o f radioactive m alonyl-CoA was not lim ited by D T T binding [19] o f the substrate acetyl-CoA . F u rth e r m ore through the acid stop in this test only th e reaction products were m easured w hile u n reacted C 0 2 was released. The observed stim u latio n o f the acetyl-CoA carboxylase by D T T, show n in Fig. 2, appears to reflect therefore an in volvem ent o f reducing equivalents in the light activ atio n o f this enzyme. 0 20 O 0 15 Acetyl- CoA carboxylase 0 10 0 I 5 10 DTT(mM) Fig. 2. Effect o f dithiothreitol (D D T ) on the activity o f acetyl-CoA carboxylase measured as m alonyl-CoA form a tion in spinach chloroplasts in the dark. Intact chloroplast suspensions were preincubated in the dark for 10 m in with the DDT concentrations indicated and then disrupted in Triton X-100 containing reaction-mixture (further procedure as in Fig. 1). 271 A .Sauer and K.-P. Heise • R egulation o f A eetyl-C oA C arboxylase and A cetyl-C oA S ynthetase p H Dependence As shown in Fig. 3, in chloroplast extracts the optim al activity o f acetyl-C oA synthetase is o b served at about pH 8.0 and that o f acetyl-C oA carboxylase at about pH 8.5. T he latter pH -optim um appears identical w ith the strom a pH d eter m ined for optim al acetate incorporation into the lipid fraction o f intact spinach chloroplasts [6 ]. D uring a light-dark transition a decrease o f the pH from 8.4 to 7.4 was m easured in intact chloroplasts incubated at pH 7.9 [6 , 12]. But low ering o f the stroma pH by 1 unit led only to a partial inhibition of the activities o f acetyl-C oA synthetase and acetylCoA carboxylase (Fig. 3). T he latter observation suggests that light-induced pH changes in the strom a may control the activities o f both enzym es, a suggestion that is consistent w ith recent findings on the pH dependence o f total fatty acid synthesis from acetate in intact chloroplasts [6 ]. Effect o f Mg The absolute req u irem en t for M g2+ in the initial steps o f fatty acid synthesis from acetate in ch lo ro plasts [14, 20, 21] has been confirm ed in dark activation experim ents o f fatty acid synthesis in intact chloroplasts [6 ]. F u rth e rm o re a dual re q u ire m ent o f M g2+ for the form atio n o f M gA TP and for enzyme activation has been established [14, 2 1 ]. Figures 4 and 5 show the activity o f th e two key enzymes o f acetate fixation in ch lo ro p last extracts on the concentration o f free and A T P -b o u n d M gi+. The prim ary stim ulation o f b oth enzym e activities in incubations o f a constant A T P content (1 m M ) , caused by increasing M g2+ up to eq u im o la r concen trations (1 m M ) , w ould be d u e to an accu m u latio n o f MgATP [22] at the expense o f free A T P (Fig. 4). The relatively small secondary stim u latio n o f acetylCoA synthetase and carboxylase, found at h ig h er M g2+ concentrations ( ^ 1 m M ) , w hen th e A T P con centration for com plexation w ith Mg is lim ited , can be ascribed to free M g2+ (Fig. 5). T o find an ex planation for the activating influence o f free M g2+, the dependence o f the K m and Fmax o f b o th enzym es on the concentration o f free M g2+ (Fig. 6 ) has been investigated under a constant M gA TP level (1 m M ) , calculated according to K räm er [16]. W hile the o f the carboxylase for M gA TP (A'm(MgATP) = 0.24 m M ) is significantly reduced by free Mg2+, its Fmax (Kmax (M gA TP) = 0.36 nm ol • mg “ ‘ C h l - h -1 ) for the sam e su b strate rem ains unchanged. In contrast, free M g2+ increases the Fmax ( Vm (M gA TP) = 1.85 |im ol • mg Chi • h -1) Acetyl - CoA carboxylase MgATP E Z 0.14 0 ° io >E 010 1 0 O.O6 !§ Ö - 0 02 ifreeATP 01 02 03 0.4 05 ATP, MgATP (mM) Assay pH Fig. 3. Effect o f pH on the activities o f acetyl-CoA carbox ylase measured as malonyl-CoA formation and acetyl-CoA synthetase in spinach chloroplast extracts. Fig. 4. Effect o f free ATP and MgATP on the activities o f acetyl-CoA synthetase and acetyl-CoA carboxylase in spinach chloroplast extracts. The free nucleotide concen tration during incubation was calculated by using the stability constants o f Krämer [16]. A .Sauer and K.-P. Heise ■ R egulation o f A cetyl-CoA C arboxylase and A eetyl-C oA S ynthetase 272 Km (mM) A cetyl - C o A synthetase free Mg gZ Eo o, cr < 0 EcO_ 1 o ^ 0.5 E 0 Acetyl - CoA carboxylase Vmax ( Hm°l m<3 o < a° 1Chi h 1) free Mg ° 0.05 CT> bp 0 01 0 1 2 3 4 5 Mg ATP, free Mg (mM) free Mg ( mM) Fig. 5. Effect o f Mg2+ on the activities o f acetyl-CoA synthetase and acetyl-CoA carboxylase in extracts o f spinach chloroplasts under a constant MgATP level (1 mM ), calculated according to Krämer [16]. Fig. 6. Effect o f Mg on (Fig. A) and Kmax (Fig. B) o f acetyl-CoA synthetase and acetyl-CoA carboxylase in spinach chloroplast extracts. The concentration o f M gATP, calculated according to Krämer [16], was a constant 1 mM. o f the synthetase but does not change the K m for M gATP (A^m(M gA TP) = 0.14 m M ; Fig. 6 ). T he decrease of the M ichaelis-M enten constant o f the carboxylase for M gA TP by free M g2+ m ay explain the difference in observed here w ith th a t o f the enzyme (A^mATP = 0.039 m M ) recently found in crude carboxylase p rep aratio n s from spinach [23]. An evaluation o f the d ata by H ill plots (not show n) clearly indicated th a t there is no cooperativ ity between the catalytic binding sites o f b oth enzym es for the substrates exam ined. T he results o f this kinetic study support the u n iq u e relatio n sh ip b e tween MgATP as substrate and free M g2+ as an activator of acetyl CoA carboxylase activity as well as the notion, th at both ions p ro b ab ly ad d to this enzyme in an eq u ilib riu m o rd ered m a n n er [14]. F urtherm ore the kinetic constants d eterm in e d for acetyl CoA carboxylase in the ap p lied cru d e extracts (Fig. 6 , T able I) resem ble th a t recently found for partially purified enzym e p rep a ra tio n s Table I. Kinetic constants o f acetyl-CoA synthetase and acetyl-C oA carboxylase from spinach chloroplast extracts (mean values). K m [mM] ^max [nmol • m g 1 Chi • h ' 1] MgATP Free Mg A cetyl-C oA M gATP acetyl-CoA synthetase 0.14 0.14 - 1.85 acetyl-CoA carboxylase 0.24 0.08 0.06 0.36 Enzymes Substrate Free Mg 0.36 K x[mM ] Acetyl-CoA Free ATP ADP - 1.50 0.71 0.10 0.44 0.10 A .Sauer and K..-P. H eise • R egulation o f A cetyl-C oA C arboxylase and A cetyl-C oA S ynthetase from spinach chloroplasts [21]. T he data supp o rt earlier findings [20], th a t free and A T P-bound M g2+ are required for o ptim al activity o f acetyl CoA synthetase even though the kinetic m echanism for the activation o f this enzym e by free M g2+ appears to differ from th at o f the carboxylase (Fig. 6 ). F ree Mg2+ seems to increase the affinity o f the catalytic centre for M gA TP in carboxylase. In the synthetase it accelerates the enzym e catalyzed reaction possibly by a conform ational change o f eith er the enzym e or the enzyme substrate com plex. The observed dependence o f the kinetic constants o f acetyl CoA synthetase and carboxylase on free M g2+ m ay be o f regulatory significance for com pensating changes in the level o f chelating adenine nucleotides, w hich has been found to influence the activities o f both enzymes [24], L atter assum ption is sup p o rted by the observation, that an activation o f fatty acid syn thesis in darkened chloroplasts by supplying co factors (ATP, N A D P H ) via a dihydroxyacetone phosphate shuttle [ 1 2 ] was considerably stim ulated by increasing the strom al Mg concentration beyond the supposed dark value ( l - 3 m M ; [25]), although the calculated increase o f the sim ultaneously av ail able ATP level (from 0 .2 -0 .4 mM) was com parably low. Thus, the observed activation by free M g2+ may be ascribed to a com pensation o f the inhib ito ry effect o f increasing A D P concentrations in the d ark [23, 25], Inhibitors It has been show n th a t free A TP caused a considerable decrease o f acetyl CoA carboxylase activity [14, 21]. T his effect has been recently attributed to co m tam in atio n s o f the carboxylase by A TPase and adenylate kinase generating significant am ounts o f A D P and AM P, w hich are inh ib ito rs o f the carboxylase [23]. T his in terp retatio n cannot be applied in the present investigation because the disruption o f chloroplasts in the presence o f T riton X-100 and subsequent gel filtration [10] elim inate cofactors and substrates for A TPase and adenylate kinase. F urther, du rin g the short assay tim es o f only 5 - 1 0 min, A TP hydrolysis in the extracts will play only a m inor role, as suggested by sim ultaneous m easurem ents o f the phosphorylation p otential (data not shown). T herefore, the reduction o f acetylCoA carboxylase and synthetase activities at A TP concentrations exceeding the adjusted M g2+ content in the reaction m ixture (Fig. 7), w hich is super- 273 o E fi <0 t£ o _ 1 o ^ E A T P (m M ) Fig. 7. Effect o f ATP at a constant M g2+ (1 m M ) in the assay medium on the activities o f acetyl-CoA synthetase and acetyl-CoA carboxylase in spinach chloroplast extracts. im posed on the stim ulating effect o f M gA T P on both enzyme activities (Figs. 4 and 5), seem s to be related m ore to a com petitive in h ib itio n by free ATP than to an in h ib itio n by A T P hydrolysis [23], The Kj values for free A TP o b ta in ed for acetyl CoA carboxylase and acetyl CoA synthetase w ere 0.44 and 1.5 m M respectively (T able I). But th e relatively high strom al M g2+ concentrations present in intact chloroplasts (0 .4 -1 |im ol • m g - 1 C hi) [26], suggest that an inhibition by free A TP is unlikely to be o f physiological significance. O n th e o th er h an d , the level o f A D P is know n to increase in d ark e n ed chloroplasts [25]. T h erefore th e above m en tio n ed evidence for the inhibitory effect o f A D P [23] has been extended to b oth key enzym es o f acetate fix a tion in the chloroplast extracts. T he o f both enzymes for M gA TP increases w ith A D P co n cen tra tion (Table II). T his indicates th a t A D P is not only a com petitive in h ib ito r for acetyl CoA carboxylase ( ^ = 0.1 m M ) but also for acetyl CoA synthetase (A^ = 0.71 m M ) w ith respect to M gA TP (T ab le I). In Table II. Effect o f A D P on the K m o f acetyl-C oA syn thetase and acetyl-CoA carboxylase in spinach chloro plast extracts for MgATP. [mM] ADP Concentration Acetyl-CoA carboxylase Acetyl-CoAsynthetase 0.14 0.18 0.24 0.32 0.02 0.12 0.26 0.52 [mM] 0 0.2 0.5 1.0 274 c o_ O SI E _ A .Sauer and K.-P. Heise • R egulation o f A cetyl-C oA C arboxylase and A cetyl-C oA S ynthetase 030 0.24 £ 6 < 7 0.18 O oi ^ E >* *5 0.12 g o i. 2 0 06 Fig. 8. Effect o f the A T P /A D P quotient in the assay medium on the activities o f acetyl-CoA carboxylase and acetyl-CoA synthetase in spinach chloroplast extracts. The samples were incubated for 10 min in the standard medium with 2 m M M gCl2 and 1 m M ATP at different ADP-concentrations. The ÄTP used for the A T P /A D P quotient is total ATP. order to determ ine the regulatory significance o f th e stromal adenine nucleotide levels on the activities o f both enzymes during a light-dark tran sitio n , the dependence o f th eir activities on the A T P /A D P quotient in incubations o f chloroplast extracts has been com pared w ith corresponding values d e te r mined in the strom a o f illum inated (A T P /A D P = 2 - 3 ) and darkened (A T P /A D P = 0 .4 -0 .6 ) iso lated chloroplast [25, 27], T he results (Fig. 8 ) suggest that the calculated decrease o f the strom al A T P/A D P quotient during a light-dark transient, at least in isolated chloroplasts, led to a strong red u c tion o f the activities o f acetyl CoA synthetase and carboxylase and could th erefore exert a regulato ry function in fatty acid synthesis [6 , 23]. T he relatively higher stromal A T P /A D P q u o tie n t ( ^ 1) found in chloroplasts from w heat le af protoplasts [27] in the dark, which appears to be m ain tain ed by A TP im port from the cytosol, m ay fu rth er explain, why leaf discs [5] and w heat le a f protoplasts (u n p u b lished results) as opposed to isolated chloroplasts [5, 18] were still capable o f fatty acid synthesis (about 2 0 % o f the corresponding light rates) in the dark. [1] B. P. Smirnov, Biokhimia 2 5 ,4 1 9 -4 2 2 (1960). [2] J. B. Mudd and T. T. McManus, J. Biol. Chem. 237, 2056-2063 (1962). [3] P. K. Stumpf and A. T. James, Biochim. Biophys. Acta 70, 2 0 -3 2 (1963). The above evidence suggests th a t a co o p e ra tio n o f enzyme interconversions and light d ep e n d e n t changes in the H +- and M g2+-co n cen tratio n and A TP/A D P quotient affect the catalytic activ ities o f acetyl-CoA synthetase and carboxylase in the chloroplast strom a. Evidently these play an im portant role in the light activation o f acetate in corporation into long-chain fatty acids [ 1 -6 ] . T he rates of lipid incorporation o f acetate by illu m i nated chloroplasts before d isru p tio n w ere 100-360 nmol • m g “ 1 Chi • h _1. In ch lo ro p last ex tracts under optim ized conditions, m alo n y l-C o A was synthesized w ith sim ilar rates (up to 360 nm ol • mg - 1 Chi • h_l), w hile acetyl-C oA fo rm a tio n show ed significantly higher values (3 (.tmol • m g - 1 C hi • h -1). Even allowing for the effect o f stro m al p a ra m eters (e.g. H +, M g2+ and A T P /A D P q u o tie n ts) on enzyme activity in illum inated ch lo ro p lasts th e h igh activity o f acetyl-CoA synthetase m easu red suggests that it is unlikely a rate lim itin g step in th e lig h t regulation o f fatty acid synthesis from acetate. T his conclusion is consistent w ith the findings th a t no substantial difference in acetyl-C oA fo rm a tio n was observed between illum inated and d ark e n ed c h lo ro plasts [28], although fatty acid synthesis in th e d ark was nearly totally in h ib ited [ 1 -6 ]. In co n trast, the activity o f acetyl-CoA carboxylase in ch lo ro p last extracts, calculated w ith respect at th e know n levels o f above strom al solutes (H +, M g2+ and A T P /A D P quotient) in illum inated chloroplasts [27], was sim ilar to the rate o f lipid in co rp o ratio n o f acetate in intact chloroplasts in the light. T his suggests th at, as proposed [8 , 23], the carboxylase is the key enzym e o f acetate fixation that w ould a p p e a r to be involved in the light control o f acetate in c o rp o ratio n into long-chain fatty acids in chloroplasts. Acknowledgements This work was supported by the D eu tsch e F o r schungsgemeinschaft. The au th o rs are g ratefu l to Prof. H. W. H eldt and Dr. M. 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