Changes in Properties of Succinate Dehydrogenase during Senescence of Pea Cotyledons

Comparisons were made between succinate dehydrogenases (EC 1.3.99.1 [EC] )from 1-day-old and 5-day-old pea cotyledons. The enzyme wasloosely bound to the mitochondrial inner membrane in 5-day-oldcotyledons, but tightly in 1-day-old cotyledons. In addition,the enzyme partially purified from 5-day-old cotyledons wasmuch more labile than that from 1-day-old cotyledons. Succinaterapidly inactivated partially purified succinate dehydrogenasefrom 1-day-old cotyledons, but not from 5-day-old cotyledons.Dithiothreitol caused a change in the charge of the enzyme proteinfrom either 1- or 5-day-old cotyledons, only when succinatewas present. The enzyme from 5-day-old cotyledons differed fromthe succinate-induced labile form of the enzyme from 1-day-oldcotyledons in electrophoretic properties on a polyacrylamidegel. There was also a difference in the pattern of polyacrylamidegel electrophoresis between succinate dehydrogenases partiallypurified from 1- and 5-day-old cotyleodns. The partially purifiedenzyme from either 1- or 5-day-old cotyledons in the presenceof succinate had a molecular weight of 92,000. The molecularweight of the large subunit was suggested to be 65,000. Thepartially purified enzyme prepared from 1-day-old cotyledonsin the absence of succinate was in a form with a molecular weightof 113,000. (Received August 29, 1980; Accepted December 3, 1980)     

Studies on the Development of Mitochondria of Mung Bean Cotyledons During Imbibition

The development of mitochondria in the course, of imbibition ofmung bean cotyledons was studied. Mitochondria were prepared by differential centrifugation, and purified by discontinuous sucrose, density gradient centrifugation. Theelectron micrographs revealed that the mitochondria isolated from dry seeds cotyledonslook like small vesicles, when cotyledons were imbibed for two hours, the mitochondrial cristae were not observed, but after four hours, a few cristae appeared on the innermembrane. Till 12 hours, the inner membrane systems wore well-developed. Al this time.all the space in mitochondria are filled with cristae. With the structural integrity of the mitochondria, the functions of oxidation and phosphorylation were graduallyshown, For instance, ADP/O ratio and RCR were not able to be measured in the imbibition of 2 and 4 hours, but at 6th hours, ADP/O Was increased by 0.6, RCR nearly 2.0, After 24 hours imbibition, ADP/O and RCR were increased to 1.5 and 3.5 respectively. The activity of cytochrome oxidase reached the highest after imbibition for 3 hours (2.54 OD/mg protein/min). If cotyledons were imbibed continuously, the activityof this enzyme in mitochondria remained constant. The activity of ATPase located onthe inside of the mitochondrial inner membrane was gradually enhanced from the beginnning of imbibition. These results suggest that the assembly of cytochrome oxidaseand ATPase on mitochondrial membrane may not be synchronous.     

Partial purification and properties of rat liver mitochondrial polynucleotide phosphorylase

1. Polynucleotide phosphorylase was partially purified from the inner membrane of rat liver mitochondria. 2. The partially purified particulate enzyme catalyses phosphorolysis of poly(A), poly(C), poly(U) and RNA to nucleoside diphosphates. 3. It is devoid of nucleoside diphosphate-polymerization activity. 4. Variable amounts of ADP/P(i)-exchange activity are associated with the polynucleotide phosphorylase and are probably due to a different enzyme. 5. ADP is the preferred substrate for exchange, and little or no reaction occurs with other nucleoside diphosphates, but ATP/P(i)-exchange takes place at one-third the rate observed with ADP. 6. The partially purified enzyme is free from the phosphatases found in the crude mitochondrial inner membrane, but is associated with an endonuclease activity and some adenylate kinase activity; no cytidylate kinase activity analogous to the latter was detectable.     

Effect of salicylic acid on the metabolic activity of plant mitochondria

The effects of salicylic acid (SA) on mitochondrial respiration and generation of membrane potential across the inner membrane of mitochondria isolated from stored taproots of sugar beet (Beta vulgaris L.) and etiolated seedling cotyledons of yellow lupine (Lupinus luteus L.) were studied. When malate was oxidized in the presence of glutamate, low SA concentrations (lower than 1.0 mM) exerted predominantly uncoupling action on the respiration of taproot mitochondria: they activated the rate of oxygen uptake in State 4 (in the absence of ADP) and did not affect oxidation in State 3 (in the presence of ADP). In contrast, in lupine cotyledon mitochondria these SA concentrations inhibited oxygen uptake in the presence of ADP and much weaker activated substrate oxidation in State 4. Thus, SA (0.5 mM) reduced the respiratory control ratio according to Chance (RCR) by 25% in the taproots and 35% in cotyledons. When the concentration of phytohormone was increased (above 1.0 mM), malate oxidation in State 3 was inhibited and in State 4 — activated independently of the plant material used. In this case, the values of RCR and ADP/O were reduced by 50–60%. The effect of high SA concentrations (2 mM and higher) on malate oxidation depended on the duration of incubation and had a biphasic pattern: the initial activation of oxygen uptake was later replaced by its inhibition. The parallel studying the SA effect on the generation of membrane potential (ΔΨ) at malate oxidation in the mitochondria of beet taproots and lupine cotyledons showed that ΔΨ dissipation was observed because of SA uncoupling and inhibiting action on respiration. The degree of ΔΨ dissipation depended on the phytohormone concentration and duration on mitochondria treatment, especially at its high concentrations. In general, a correlation was found between the effects of SA on mitochondrial respiration and ΔΨ values in the coupling membranes. Furthermore, these results show that the responses of mitochondria to SA were determined not only by its concentration but also by treatment duration and evidently by the sensitivity to the phytohormone of mitochondria isolated from different plant tissues.     

Effects of extramitochondrial ADP on permeability transition of mouse liver mitochondria

Carboxyatractylate (CAT) and atractylate inhibit the mitochondrial adenine nucleotide translocator (ANT) and stimulate the opening of permeability transition pore (PTP). Following pretreatment of mouse liver mitochondria with 5 microM CAT and 75 microM Ca2+, the activity of PTP increased, but addition of 2 mM ADP inhibited the swelling of mitochondria. Extramitochondrial Ca2+ concentration measured with Calcium-Green 5N evidenced that 2 mM ADP did not remarkably decrease the free Ca2+ but the release of Ca2+ from loaded mitochondria was stopped effectively after addition of 2 mM ADP. CAT caused a remarkable decrease of the maximum amount of calcium ions, which can be accumulated by mitochondria. Addition of 2 mM ADP after 5 microM CAT did not change the respiration, but increased the mitochondrial capacity for Ca2+ at more than five times. Bongkrekic acid (BA) had a biphasic effect on PT. In the first minutes 5 microM BA increased the stability of mitochondrial membrane followed by a pronounced opening of PTP too. BA abolished the action about of 1 mM ADP, but was not able to induce swelling of mitochondria in the presence of 2 mM ADP. We conclude that the outer side of inner mitochondrial membrane has a low affinity sensor for ADP, modifying the activity of PTP. The pathophysiological importance of this process could be an endogenous prevention of PT at conditions of energetic depression.     

Presence in Dry Pea Cotyledons of Soluble Succinate Dehydrogenase That Is Assembled into the Mitochondrial Inner Membrane during Seed Imbibition

SUCCINATE DEHYDROGENASE (SUCCINATE: phenazine methosulfate oxidoreductase, EC 1.3.99.1) activity in crude mitochondrial fraction from pea (var. Alaska) cotyledons increased during seed imbibition to reach a maximum after about 12 hours. The increase was not inhibited by either cycloheximide or d(-)threo-chloramphenicol. The postmicrosomal fraction from dry cotyledons, but not that from fully imbibed ones, contained a soluble form of succinate dehydrogenase. The soluble enzyme was partially purified by ammonium sulfate fractionation and diethylaminoethyl-cellulose and Sepharose 6B column chromatography. The enzyme showed no succinate-coenzyme Q oxidoreductase activity and had a molecular mass of about 100,000 daltons. The soluble enzyme seemed to differ only slightly from succinate dehydrogenase solubilized from the mitochondrial inner membrane from fully imbibed cotyledons by a detergent. It is proposed that the soluble succinate dehydrogenase is associated with an inert mitochondrial inner membrane in dry cotyledons to form an active one during seed imbibition.     

Properties of Mitochondria Prepared by Two Different Isolation Procedures From the Cotyledons of Germinating Bean Seedlings

Mitochondria from 1-day-old bean cotyledons were isolated bya ‘slow’ isolation procedure involving a wash andby a ‘rapid’ procedure. The mitochondria isolatedby the ‘rapid’ procedure were more tightly coupledthan those isolated by the ‘slow’ procedure. Anexogenous supply of cytochrome c or NAD was shown to improvethe activity of mitochondria isolated by the ‘slow’procedure, but not those isolated by the ‘rapid’procedure. The phosphorylative abilities of the latter weremuch greater than the former and were retained for longer periods.It was found that there was a leakage of NAD out of the mitochondria.     

Changes of Adenine Nucleotide in the Course of Mitochondrial Development of Mung Bean Cotyledon and Their Effect on Cellular Energy Status

The changes of adenine nucleotide and adenylate energy charge (AEC) during the development of mitochondria in imbibed mung bean cotyledons and the relationship between these changes and cellular energy status are studied. After cotyledons were imbibed in water for two hours, mitochondrial cristae were not observed, but for 12 hours, they appeared obviously on the inner membrane. With the structural integrity of the mitochondria, the functional mitochondria were graduately shown. For instance, the activity of H+-ATPase of cotyledons imbibed for 24 hours was about twice higher than that of 2 hours. The ATP content and the AEC value in the cotyledons imbibed for 24 hours increased sharply and the AMP decreased, but these were not observed in the mitochondria of the cotyledons imbibed either for 24 hours or 2 hours. When the cotyledons were imbibed in 1 × 10-4 mol/l or 5 × 10-4 mol/l DNP solution for 24 hours, the ATP and the AEC in the Cells exhibited a rapid decrease, but in the mitochondria they remained canstant. In the same DNP solution with cotyledons for 24 hours, the activity of mitochondrial adenylate kinase (AK) not only was not decreased but also increased by about 50% over the control. This result shows that the energy equilibration in the mitochondria seems likely to be regulated by adenylate kinase locating between inner and out membranes of the mitochondria.     

萌发绿豆子叶衰老期间的呼吸作用与线粒体功能的改变

暗中培养的绿豆幼苗子叶在萌发后3—4天时,外观出现衰老征状,6天后子叶凋落。随子叶日龄的增加,子叶的呼吸强度一直下降,呼吸商始终小于1。当外加L—苹果酸、a—酮戊二酸、琥珀酸和NADH为底物测定离体线粒体氧化活性时,衰老子叶的线粒体对上述四种底物的氧化活性有不同程度的增加;抗氰呼吸也有所升高。子叶衰老时,线粒体的ADP／O和呼吸控制(RC值均降低);线粒体ATPase水解ATP的活性升高。衰老绿豆子叶线粒体氧化磷酸化偶联效率的降低和ATPase水解活性的增强是与线粒体结构改变相联系的一种功能变化,它导致能量亏缺,并进一步加速了衰老的恶化进程。     

The development of oxidative enzymes in rat liver mitochondria.

During early postnatal development there was an increase in the specific activity of a number of oxidative enzymes localized on the outer and inner mitochondrial membrane. The succinic oxidase complex of the inner mitochondrial membrane, whose activity in 1-day-old rats was 50% of the value in adult animals, attained the maximum on about the 10th day after birth. Activity of the choline and the proline oxidase complex, both of which are also localized in the inner mitochondrial membrane, was minimal in 1-day-old rats and went on rising after the 10th day. Rotenone-insensitive NADH-cytochrome c reductase activity, which is localized on the outer mitochondrial membrane, remained stable up to the 10th day, and rose between the 10th and the 90th day. Developmental changes in monoaminooxidase activity, which is likewise localized on the outer mitochondrial membrane, followed a similar course to the choline and proline oxidase complexes. The amount of cytochromes a+alpha3 and cytochrome b in isolated mitochondria did not alter during development. The protein spectrum of the mitochondrial particles, determined by polyacrylamide gel electrophoresis in sodium dodecyl sulphate, likewise displayed no marked changes during postnatal development. The above findings show that the metabolic functions of the mitochondria mature during development and that changes in the different enzymes have their own characteristic time course.     

Studies on energy status and mitochondrial respiration during growth and senescence of mung bean cotyledons

Several characteristics of mitochondrial respiration and energy status have been studied during growth and senescence of mung bean ( Phaseolus radiatus L.) cotyledons. The results showed that mitochondrial oxygen consumption, respiratory control, ADP:O ratios, and energy charge changed in the cotyledons during germination and growth of the seedlings. The respiration rate of intact cotyledons approximately reflected the trend of the oxidative activities of the isolated mitochondria. An increase was observed in both whole cotyledon respiration and mitochondrial oxygen uptake at the onset of senescence of mung bean cotyledons (day 3 after germination), which thereafter declined gradually. The capacity and activity of the alternative pathway increased markedly in mitochondria isolated from senescent cotyledons. After the onset of senescence, the mung bean cotyledon mitochondria exhibited a decrease both in the respiratory control ratios and ADP:O ratios, and the cotyledons exhibited a gradual decline in energy charge. All these results showed an irreversible deterioration of energy conservation in mung bean cotyledons. The role(s) of the alternative pathway in senescent mung bean cotyledons is discussed.     

Senescence induced structural reorganization of thylakoid membranes in Cucumis sativus cotyledons; LHC II involvement in reorganization of thylakoid membranes

We report the formation and appearance of loosely stacked extended grana like structures along with plastoglobuli in the chloroplasts isolated from 27-day old senescing cucumber cotyledons. The origin and the nature of these extended grana structures have not been elucidated earlier. We isolated Photosystem I complexes from 6-day-old control and 27-day-old senescing cotyledons. The chlorophyll a/b ratio of the isolated Photosystem I complex obtained from 6-day cotyledons was 5–5.5 as against a ratio of 2.9 was found in Photosystem I complexes obtained from 27-day-old senescing cotyledons. We also found that the presence of LHC II in the Photosystem I complexes isolated from 27-day cotyledonary chloroplasts. The presence of LHC II in Photosystem I complexes in senescing and not in control samples, clearly suggest the detachment and diffusion of LHC II complexes from stacked grana region to Photosystem I enriched stroma lamellar region thereby, forming loose disorganized extended grana structures seen in the transmission electron microscope. Furthermore, we show that under in vitro condition the senescing cotyledon chloroplasts exhibited lower extent of light induced phosphorylation of LHC II than the control samples suggesting a possible irreversible phosphorylation and diffusion of LHC II in vivo during the progress of senescence in Cucumis cotyledons. From these findings, we suggest that the senescence induced phosphorylation of LHC II and its migration towards Photosystem I may be a programmed one some how causing the destruction of the thylakoid membrane. The released membrane components may be stored in the plastoglobuli prior to their mobilization to the younger plant parts. This revised version was published online in June 2006 with corrections to the Cover Date.     

ADENINE NUCLEOTIDE-INDUCED CONTRACTION OF THE INNER MITOCHONDRIAL MEMBRANE : I. General Characterization

The inner membranes of isolated bovine heart mitochondria undergo pronounced contraction upon being exposed to exogenous adenosine diphosphate (ADP), adenosine triphosphate (ATP), and certain other high-energy phosphate compounds. Contraction results in decrease of inner membrane expanse which in turn results in decrease of intracristal space and increase of mitochondrial optical density (OD). The magnitude of the OD change appears to be proportional to the degree of contraction Half-maximal contraction can be achieved with ADP or ATP at concentrations as low as about 0 3 µM. Atractyloside at concentrations as low as about 1.2 nmol/mg mitochondrial protein completely inhibits the contraction. It is concluded from these and other observations that inner membrane contraction occurs as a result of adenine nucleotide binding to the carrier involved in the exchange of adenine nucleotides across the inner mitochondrial membrane.     

Structural Development during Germination of Different Populations of Mitochondria from Pea Cotyledons

The crude mitochondrial fraction from pea cotyledons can, from days 1 to 7 of germination, be separated into three fractions by sucrose density gradient centrifugation. When seeds were grown in water (control) or cycloheximide (120 micrograms per milliliter of medium) for 4 days, the originally different populations of mitochondria acquired a uniform density and separated together in band 1 (density, 1.205 grams per milliliter). The oxidative and phosphorylative activities of mitochondria obtained from 4-day-old control and 4-day-old cycloheximide-treated pea seeds were the same. However, mitochondria from pea seeds that were grown in d-threo-chloramphenicol (1.5 milligrams per milliliter of medium) or erythromycin (0.5 milligram per milliliter of medium) for 4 days separate into three bands (fully developed mitochondria in the top band [band 1] and partially developed mitochondria in the lower two bands [bands 2 and 3]). Separation patterns and oxidative and phosphorylative activities were the same for mitochondria separated from 4-day-old cotyledons treated with d-threo-chloramphenicol or erythromycin and from 1-day-old cotyledons grown in water. This indicated that these inhibitors prevented the partially developed mitochondria originally in bands 2 and 3 from developing further. In contrast, cycloheximide did not seem to interfere with the mitochondrial structural development. These results along with those obtained from the experiments on the effects of d-threo-chloramphenicol, erthromycin, and cycloheximide on ¹⁴C-leucine incorporation into mitochondrial membrane proteins suggest that the increase in mitochondrial activity during germination may be a result of structural development (membrane synthesis) in pre-existing mitochondria.     

Changes in the response of mitochondrial calcium transport to exogenous phosphate during development in flight muscle of the sheep blowfly Lucilla cuprina.

1. Ca2+ transport by mitochondria isolated from flight muscle of the sheep blowfly Lucilla cuprina does not occur in the absence of added P1. Maximum rates of transport are attained when about 2.5 mM-phosphate is present. 2. As mitochondria develop, high but not low phosphate concentrations begin to inhibit Ca2+ transport markedly; those isolated from 2-day-old flies for example, are inhibited by about 75% by 20 mM-phosphate. Maximum rates of transport, i.e. those measured in the presence of 2.5 mM-phosphate, begin to decline only when the fly is about 3 days old. 3. Mitochondrial phosphate transport activity does not change during development of the blowfly, but the endogenous concentration of the anion does. At emergence it is about 6nmol/mg of protein, increases to about 17 nmol/mg of protein at 2-3h and then rapidly declines to reach less than 5 nmol/mg of protein after 2 days of adult life. 4. Studies on the effect of phosphate on oxidation of alpha-glycerophosphate in the absence and presence of ADP reveal a lack of inhibition by high phosphate concentrations indicating that the anion does not influence Ca2+ transport by preventing the generation of the proton electrochemical gradient across the inner membrane. 5. It is concluded that the molecular assembly in the inner membrane of Lucilla mitochondria responsible for transporting Ca2+ is fully developed at emergence and remains so for at least 2-3 days of adult life. The possibility exists that Ca2+-transport activity in these mitochondria is controlled at least in part by P1.     

Effect of the pea embryo on formation and degeneration of the mitochondrial membrane in cotyledons during germination

The effect of removal of the embryo on the properties of mitochondriain pea cotyledons was investigated. During imbibition of theseeds, mitochondrial activity was enhanced in the cotyledons.In later stages of germination, respiratory activity of themitochondria decreased gradually, and no response of the mitochondriato exogenous ADP was observed. Moreover, considerable activityof cytochrome oxidase wasrecovered in the post-mitochondrialfraction. Mitochondrial fractions isolated from senescent cotyledonscontained only fragmented particles of mitochondria. On theother hand, in cotyledons excised from the seeds and cultivatedunder wet condition, the initial development of mitochondriademonstrated in the attached cotyledons was suppressed. However,respiratory activity of the mitochondria increased in the laterstages of cultivation. The mitochondria remained unfragmentedand responded to exogenous ADP during all stages of cultivation.Also, a change in the density of mitochondria which occurredin the germinating attached cotyledons was delayed in the cultivatedexcised cotyledons. (Received February 27, 1973; )     

Bax and heart mitochondria: uncoupling and inhibition of respiration without permeability transition

The effects of Bax (full-length, FL, and C-terminal truncated, DeltaC) on respiration rate, membrane potential, MgATPase activity and kinetics of regulation of respiration were studied in isolated rat heart mitochondria and permeabilized cardiomyocytes. The results showed that while both Bax-FL and Bax-DeltaC permeabilized the outer mitochondrial membrane, released cytochrome c and reduced the respiration rate, the latter could be fully restored by exogenous cytochrome c only in the case of Bax-DeltaC, but not in presence of Bax-FL. In addition, Bax-FL but not Bax-DeltaC increased the MgATPase activity, and their effects on the mitochondrial membrane potential were quantitatively different. None of these effects was sensitive to cyclosporin A (CsA).It is concluded that Bax-FL affects both the outer and the inner mitochondrial membranes by: (1) opening large pores in the outer membrane; (2) inhibiting some segments of the respiratory chain in the inner membrane; and (3) uncoupling the inner mitochondrial membrane by increasing proton leak without opening the permeability transition pore (PTP).     

Electron transfer properties of NADH:ubiquinone reductase in the ND1/3460 and the ND4/11778 mutations of the Leber hereditary optic neuroretinopathy (LHON)

We report the electron transfer properties of the NADH:ubiquinone oxidoreductase complex of the respiratory chain (Complex I) in mitochondria of cells derived from LHON patients with two different mutations in mitochondrial DNA (mtDNA). The mutations occur in the mtDNA genes coding for the ND1 and ND4 subunits of Complex I. The ND1/3460 mutation exhibits 80% reduction in rotenone-sensitive and ubiquinone-dependent electron transfer activity, whereas the proximal NADH dehydrogenase activity of the Complex is unaffected. This is in accordance with the proposal that the ND1 subunit interacts with rotenone and ubiquinone. In contrast, the ND4/11778 mutation had no effect on electron transfer activity of the Complex in inner mitochondrial membrane preparations; also Km for NADH and NADH dehydrogenase activity were unaffected. However, in isolated mitochondria with the ND4 mutation, the rate of oxidation of NAD-linked substrates, but not of succinate, was significantly decreased. This suggests that the ND4 subunit might be involved in specific aggregation of NADH-dependent dehydrogenases and Complex I, which may result in fast ('solid state') electron transfer from the former to the latter.     

Creatine kinase of rat heart mitochondria. The demonstration of functional coupling to oxidative phosphorylation in an inner membrane-matrix preparation

To define more clearly the interactions between mitochondrial creatine kinase and the adenine nucleotide translocase, the outer membrane of rat heart mitochondria was removed by digitonin, producing an inner membrane-matrix (mitoplast) preparation. This mitoplast fracton was well-coupled and contained a high specific activity of mitochondrial creatine kinase. Outer membrane permeabilization was documented by the loss of adenylate kinase, a soluble intermembrane enzyme, and by direct antibody inhibition of mitochondrial creatine kinase activity. With this preparation, we documented four important aspects of functional coupling. Kinetic studies showed that oxidative phosphorylation decreased the value of the ternary enzyme-substrate complex dissociation constant for MgATP from 140 to 16 microM. Two approaches were used to document the adenine nucleotide translocase specificity for ADP generated by mitochondrial creatine kinase. Exogenous pyruvate kinase (20 IU/ml) could not readily phosphorylate ADP produced by creatine kinase, since added pyruvate kinase did not markedly inhibit creatine + ATP-stimulated respiration. Additionally, when ADP was produced by mitochondrial creatine kinase, the inhibition of the translocase required 2 nmol of atractyloside/mg of mitoplast protein, while only 1 nmol/mg was necessary when exogenous ADP was added. Finally, the mass action ratio of the mitochondrial creatine kinase reaction exceeded the apparent equilibrium constant when ATP was supplied to the creatine kinase reaction by oxidative phosphorylation. Overall, these results are consistent with much data from intact rat heart mitochondria, and suggest that the outer membrane plays a minor role in the compartmentation of adenine nucleotides. Furthermore, since the removal of the outer membrane does not alter the unique coupling between oxidative phosphorylation and mitochondrial creatine kinase, we suggest that this cooperation is the result of protein-protein proximity at the inner membrane surface.     

Regulation of biosynthesis of the rat liver inner mitochondrial membrane by thyroid hormone

Regulation of mitochondrial protein synthesis by thyroid hormone has been studied in isolated rat hepatocytes and liver mitochondria. Small doses (5 micrograms/100 g body wt) of triiodothyronine (T3) injected into hypothyroid rats increased both state 3 and 4 respiration by approximately 100%, while the ADP:O ratio remained constant. This suggests that T3 increases the numbers of functional respiratory chain units. T3 also induces mitochondrial protein synthesis by 50-100%. Analysis of the mitochondrial translation products show that all of the products were induced. No differential translation of the peptides involved in the respiratory chain was found. Regulation of the cytoplasmically made inner membrane peptides was also investigated in isolated hepatocytes. The majority of these peptides were not influenced by T3, in contrast to the finding with mitochondrial translation products. Those found to be regulated by T3 belong to two subsets, which were either induced or repressed by hormone. Thus, T3 stimulated a general increase in the synthesis of mitochondrially translated inner membrane peptides, but regulates selectively those inner membrane peptides translated on cytoplasmic ribosomes. The findings suggest that hormone regulation of the respiratory chain is exerted through a few selective proteins, perhaps those which require subunits made from both nuclear and mitochondrial genes.