Entecavir competitively inhibits deoxyguanosine and deoxyadenosine phosphorylation in isolated mitochondria and the perfused rat heart

Deoxyguanosine kinase (dGK) is reported responsible for the phosphorylation of deoxyadenosine (dA) and deoxyguanosine (dG) in the mitochondrial purine salvage pathway. Antiviral nucleoside analogs known as nucleoside reverse transcriptase inhibitors (NRTIs) must be phosphorylated by host enzymes for the analog to become active. We address the possibility that NRTI purine analogs may be competitive inhibitors of dGK. From a group of such analogs, we demonstrate that entecavir (ETV) competitively inhibited the phosphorylation of dG and dA in rat mitochondria. Mitochondria from the brain, heart, kidney, and liver showed a marked preference for phosphorylation of dG over dA (10-30-fold) and ETV over dA (2.5-4-fold). We found that ETV inhibited the phosphorylation of dG with an IC50 of 15.3 ± 2.2 μM and that ETV and dG were both potent inhibitors of dA phosphorylation with IC50s of 0.034 ± 0.007 and 0.028 ± 0.006 μM, respectively. In addition, the phosphorylation of dG and ETV followed Michaelis-Menten kinetics and each competitively inhibited the phosphorylation of the other. We observed that the kinetics of dA phosphorylation were strikingly different from those of dG phosphorylation, with an exponentially lower affinity for dGK and no effect of dA on dG or ETV phosphorylation. Finally, in an isolated heart perfusion model, we demonstrated that dG, dA, and ETV were phosphorylated and dG phosphorylation was inhibited by ETV. Taken together, these data demonstrate that dGK is inhibited by ETV and that the primary role of dGK is in the phosphorylation of dG rather than dA.

Effects of Zidovudine Treatment on Heart mRNA Expression and Mitochondrial DNA Copy Number Associated with Alterations in Deoxynucleoside Triphosphate Composition in a Neonatal Rat Model

The prevention of mother-to-child transmission (MTCT) of HIV is a crucial component in HIV therapy. Nucleoside reverse transcriptase inhibitors (NRTIs), primarily 3'-azido-3'-thymidine (AZT [zidovudine]), have been used to treat both mothers and neonates. While AZT is being replaced with less toxic drugs in treating mothers in MTCT prevention, it is still commonly used to treat neonates. Problems related to mitochondrial toxicity and potential mutagenesis associated with AZT treatment have been reported in treated cohorts. Yet little is known concerning the metabolism and potential toxicity of AZT on embryonic and neonatal tissues, especially considering that the enzymes of nucleoside metabolism change dramatically as many tissues convert from hyperplastic to hypertrophic growth during this period. AZT is known to inhibit thymidine phosphorylation and potentially alter deoxynucleoside triphosphate (dNTP) pools in adults. This study examines the effects of AZT on dNTP pools, mRNA expression of deoxynucleoside/deoxynucleotide metabolic enzymes, and mitochondrial DNA levels in a neonatal rat model. Results show that AZT treatment dramatically altered dNTP pools in the first 7 days of life after birth, which normalized to age-matched controls in the second and third weeks. Additionally, AZT treatment dramatically increased the mRNA levels of many enzymes involved in deoxynucleotide synthesis and mitochondrial biogenesis during the first week of life, which normalized to age-matched controls by the third week. These results were correlated with depletion of mitochondrial DNA noted in the second week. Taken together, results demonstrated that AZT treatment has a powerful effect on the deoxynucleotide synthesis pathways that may be associated with toxicity and mutagenesis.

Nonclinical and pharmacokinetic assessments to evaluate the potential of tedizolid and linezolid to affect mitochondrial function

Prolonged treatment with the oxazolidinone linezolid is associated with myelosuppression, lactic acidosis, and neuropathies, toxicities likely caused by impairment of mitochondrial protein synthesis (MPS). To evaluate the potential of the novel oxazolidinone tedizolid to cause similar side effects, nonclinical and pharmacokinetic assessments were conducted. In isolated rat heart mitochondria, tedizolid inhibited MPS more potently than did linezolid (average [± standard error of the mean] 50% inhibitory concentration [IC50] for MPS of 0.31 ± 0.02 μM versus 6.4 ± 1.2 μM). However, a rigorous 9-month rat study comparing placebo and high-dose tedizolid (resulting in steady-state area under the plasma concentration-time curve values about 8-fold greater than those with the standard therapeutic dose in humans) showed no evidence of neuropathy. Additional studies explored why prolonged, high-dose tedizolid did not cause these mitochondriopathic side effects despite potent MPS inhibition by tedizolid. Murine macrophage (J774) cell fractionation studies found no evidence of a stable association of tedizolid with eukaryotic mitochondria. Monte Carlo simulations based on population pharmacokinetic models showed that over the course of a dosing interval using standard therapeutic doses, free plasma concentrations fell below the respective MPS IC50 in 84% of tedizolid-treated patients (for a median duration of 7.94 h) and 38% of linezolid-treated patients (for a median duration of 0 h). Therapeutic doses of tedizolid, but not linezolid, may therefore allow for mitochondrial recovery during antibacterial therapy. The overall results suggest that tedizolid has less potential to cause myelosuppression and neuropathy than that of linezolid during prolonged treatment courses. This, however, remains a hypothesis that must be confirmed in clinical studies.

Heart mitochondrial TTP synthesis and the compartmentalization of TMP

The primary pathway of TTP synthesis in the heart requires thymidine salvage by mitochondrial thymidine kinase 2 (TK2). However, the compartmentalization of this pathway and the transport of thymidine nucleotides are not well understood. We investigated the metabolism of [(3)H]thymidine or [(3)H]TMP as precursors of [(3)H]TTP in isolated intact or broken mitochondria from the rat heart. The results demonstrated that [(3)H]thymidine was readily metabolized by the mitochondrial salvage enzymes to TTP in intact mitochondria. The equivalent addition of [(3)H]TMP produced far less [(3)H]TTP than the amount observed with [(3)H]thymidine as the precursor. Using zidovudine to inhibit TK2, the synthesis of [(3)H]TTP from [(3)H]TMP was effectively blocked, demonstrating that synthesis of [(3)H]TTP from [(3)H]TMP arose solely from the dephosphorysynthase pathway that includes deoxyuridine triphosphatelation of [(3)H]TMP to [(3)H]thymidine. To determine the role of the membrane in TMP metabolism, mitochondrial membranes were disrupted by freezing and thawing. In broken mitochondria, [(3)H]thymidine was readily converted to [(3)H]TMP, but further phosphorylation was prevented even though the energy charge was well maintained by addition of oligomycin A, phosphocreatine, and creatine phosphokinase. The failure to synthesize TTP in broken mitochondria was not related to a loss of membrane potential or inhibition of the electron transport chain, as confirmed by addition of carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone and potassium cyanide, respectively, in intact mitochondria. In summary, these data, taken together, suggest that the thymidine salvage pathway is compartmentalized so that TMP kinase prefers TMP synthesized by TK2 over medium TMP and that this is disrupted in broken mitochondria.

Metabolism of deoxypyrimidines and deoxypyrimidine antiviral analogs in isolated brain mitochondria

The goal of this project was to characterize deoxypyrimidine salvage pathways used to maintain deoxynucleoside triphosphate pools in isolated brain mitochondria and to determine the extent that antiviral pyrimidine analogs utilize or affect these pathways. Mitochondria from rat brains were incubated in media with labeled and unlabeled deoxynucleosides and deoxynucleoside analogs. Products were analyzed by HPLC coupled to an inline UV monitor and liquid scintillation counter. Isolated mitochondria transported thymidine and deoxycytidine into the matrix, and readily phosphorylated both of these to mono-, di-, and tri-phosphate nucleotides. Rates of phosphorylation were much higher than rates observed in mitochondria from heart and liver. Deoxyuridine was phosphorylated much more slowly than thymidine and only to dUMP. 3'-azido-3'-deoxythymidine, zidovudine (AZT), an antiviral thymidine analog, was phosphorylated to AZT-MP as readily as thymidine was phosphorylated to TMP, but little if any AZT-DP or AZT-TP was observed. AZT at 5.5 ± 1.7 μM was shown to inhibit thymidine phosphorylation by 50%, but was not observed to inhibit deoxycytidine phosphorylation except at levels > 100 μM. Stavudine and lamivudine were inert when incubated with isolated brain mitochondria. The kinetics of phosphorylation of thymidine, dC, and AZT were significantly different in brain mitochondria compared to mitochondria from liver and heart.

Pyrimidine deoxynucleoside and nucleoside reverse transcriptase inhibitor metabolism in the perfused heart and isolated mitochondria

BACKGROUND

The metabolism of pyrimidine deoxynucleosides and nucleoside reverse transcriptase inhibitors has been studied in growing cells. However, many of these drugs are associated with mitochondrial toxicities observed in non-replicating tissues, such as in the heart, where their metabolism has not been investigated.

METHODS

The aims of this study were twofold. The first was to investigate the metabolism of the thymidine analogues 3'-azido-3'deoxythymidine (AZT) and 2',3'-didehydrodideoxy-thymidine (d4T), and the deoxycytidine (dCyd) analogues 2'-deoxy-3'-thiacytidine (3TC) and 2',3'-dideoxycytidine (ddC) with regard to phosphorylation and breakdown. The second was to investigate their potential effects, singly or in combination with AZT, on metabolism of the naturally occurring deoxynucleosides in the perfused rat heart and in isolated heart mitochondria.

RESULTS

The analogue d4T was not metabolized in perfused heart or in isolated mitochondria, and had no effect on either thymidine or dCyd metabolism. The dCyd analogues were both phosphorylated in perfused heart to the triphosphate, but only at the limit of detection and they were not phosphorylated in isolated mitochondria. Neither ddC nor 3TC had any effect on thymidine or dCyd metabolism in either perfused heart or in isolated mitochondria. AZT has been previously shown to inhibit thymidine phosphorylation. When d4T, 3TC or ddC were given with AZT, only ddC caused a significant further decrease in thymidine phosphorylation.

CONCLUSIONS

These results indicate that with the exception of the competition between AZT and thymidine, there was little competition for phosphorylation among and between these other nucleoside reverse transcriptase inhibitors and the naturally occurring deoxynucleosides in cardiac tissue and isolated heart mitochondria.

Effect of AZT on thymidine phosphorylation in cultured H9c2, U-937, and Raji cell lines

3'-azido-3'-deoxythymidine (AZT) has been shown to be a potent inhibitor of thymidine kinase 2 in work from this laboratory. Inhibition results in decreased salvage of thymidine to TTP, which may lead to depletion of the TTP pool and result in the mitochondrial dysfunction and mt-DNA depletion observed with AZT toxicity. The effect of AZT on thymidine phosphorylation in growing cells expressing thymidine kinase 1 has not been shown. Three cell lines were used in these experiments: H9c2, derived from rat cardiomyoblasts; U-937, derived from human monocytes; and Raji, derived from human lymphoblasts. AZT inhibited growth in a concentration-dependent manner in U-937 cells, but not the other cell lines. The phosphorylation of [3H]-thymidine or [3H]-AZT was determined during log growth. All cell lines salvaged and phosphorylated thymidine to TTP, with TTP the major product. The U-937 cells had a much more active salvage pathway than the other cells. All cell lines phosphorylated AZT to the triphosphate, but the major product was AZTMP. The AZT inhibition of growth of the U-937 cells did not correlate with levels of the phosphorylated AZT. In contrast, pro-drug AZT was shown to inhibit thymidine phosphorylation in all lines with 50% inhibition concentrations (IC50) ranging from 4.4 to 21.9muM. Since the U-937 cells expressed higher activity of the salvage pathway than the other cell lines, the U-937 cells may rely more heavily on the salvage pathway for TTP synthesis, accounting for AZT inhibition of growth.

Zidovudine inhibits thymidine phosphorylation in the isolated perfused rat heart

Zidovudine (AZT; 3'-azido-3'-deoxythymidine), a thymidine analog, has been a staple of highly active antiretroviral therapy. It is phosphorylated in the host to the triphosphate and functions by inhibiting the viral reverse transcriptase. However, long-term use of AZT is linked to various tissue toxicities, including cardiomyopathy. These toxicities are associated with mitochondrial DNA depletion, which is hypothesized to be caused by AZT triphosphate inhibition of mitochondrial DNA polymerase gamma. In previous work with isolated heart mitochondria, we demonstrated that AZT phosphorylation beyond the monophosphate was not detected and that AZT itself was a potent inhibitor of thymidine phosphorylation. This suggests an alternative hypothesis in which depletion of the TTP pool may limit mitochondrial DNA replication. The present work extends these studies to the whole cell by investigating the metabolism of thymidine and AZT in the intact isolated perfused rat heart. [3H]thymidine is converted to [3H]TTP in a time- and concentration-dependent manner. The level of [3H]TMP is low, suggesting that the reaction catalyzed by thymidine kinase is the rate-limiting step in phosphorylation. [3H]AZT is converted in a time- and concentration-dependent manner to AZT monophosphate, the only phosphorylated product detected after 3 h of perfusion. Both compounds display negative cooperativity, similar to the observations with cloned and purified mitochondrial thymidine kinase 2. The presence of AZT in the perfusate inhibits the phosphorylation of [3H]thymidine with a 50% inhibitory concentration of 24+/-4 microM. These data support the hypothesis that AZT-induced mitochondrial cardiotoxicity may be caused by a limiting pool of TTP that lowers mitochondrial DNA replication.

Inhibition of mammalian mitochondrial protein synthesis by oxazolidinones

The effects of a variety of oxazolidinones, with different antibacterial potencies, including linezolid, on mitochondrial protein synthesis were determined in intact mitochondria isolated from rat heart and liver and rabbit heart and bone marrow. The results demonstrate that a general feature of the oxazolidinone class of antibiotics is the inhibition of mammalian mitochondrial protein synthesis. Inhibition was similar in mitochondria from all tissues studied. Further, oxazolidinones that were very potent as antibiotics were uniformly potent in inhibiting mitochondrial protein synthesis. These results were compared to the inhibitory profiles of other antibiotics that function by inhibiting bacterial protein synthesis. Of these, chloramphenicol and tetracycline were significant inhibitors of mammalian mitochondrial protein synthesis while the macrolides, lincosamides, and aminoglycosides were not. Development of future antibiotics from the oxazolidinone class will have to evaluate potential mitochondrial toxicity.

3'-Azido-3'-deoxythymidine (AZT) is a competitive inhibitor of thymidine phosphorylation in isolated rat heart and liver mitochondria

Long-term use of 3'-azido-3'-deoxythymidine (AZT) is associated with various tissue toxicities, including hepatotoxicity and cardiomyopathy, and with mitochondrial DNA depletion. AZT-5'-triphosphate (AZTTP) is a known inhibitor of the mitochondrial DNA polymerase gamma and has been targeted as the source of the mitochondrial DNA depletion. However, in previous work from this laboratory with isolated rat heart and liver mitochondria, AZT itself was shown to be a more potent inhibitor of thymidine phosphorylation (IC50 of 7.0+/-1.0 microM AZT in heart mitochondria and of 14.4+/-2.6 microM AZT in liver mitochondria) than AZTTP is of polymerase gamma (IC50 of >100 microM AZTTP), suggesting that depletion of mitochondrial stores of TTP may limit replication and could be the cause of the mitochondrial DNA depletion observed in tissues affected by AZT toxicity. The purpose of this work is to characterize the nature of AZT inhibition of thymidine phosphorylation in isolated rat heart and rat liver mitochondria. In both of these tissues, AZT was found to be a competitive inhibitor of the phosphorylation of thymidine to TMP, catalyzed by thymidine kinase 2. The inhibition constant (Ki) for heart mitochondria is 10.6+/-4.5 microM AZT, and for liver mitochondria Ki is 14.0+/-2.5 microM AZT. Since AZT is functioning as a competitive inhibitor, increasing thymidine concentrations may be one mechanism to overcome the inhibition and decrease AZT-related toxicity in these tissues.

3'-Azido-3'-deoxythymidine (AZT) inhibits thymidine phosphorylation in isolated rat liver mitochondria: a possible mechanism of AZT hepatotoxicity

3'-azido-3'-deoxythymidine (AZT) is a staple of highly active antiretroviral therapy (HAART). Prior to HAART, long-term use of high-dosage AZT caused myopathy, cardiomyopathy, and hepatotoxicity, associated with mitochondrial DNA depletion. As a component of HARRT, AZT causes cytopenias and lipodystrophy. AZT-5'-triphosphate (AZTTP) is a known inhibitor of the mitochondrial polymerase gamma and has been targeted as the source of the mitochondrial DNA depletion. However, in previous work from this laboratory with isolated rat heart mitochondria, AZT phosphorylation beyond AZT-5'-monophosphate (AZTMP) was not detected. Rather, AZT was shown to be a more potent inhibitor of thymidine phosphorylation (50% inhibitory concentration (IC50) of 7.0+/-1.0 microM) than AZTTP is of polymerase gamma (IC50 of >100 microM), suggesting that depletion of mitochondrial stores of TTP may limit replication. This work has investigated whether an identical mechanism might account for the hepatotoxicity seen with long-term use of AZT. Isolated rat liver mitochondria were incubated with labeled thymidine or AZT, and the rate and extent of phosphorylation were determined by HPLC analysis of acid-soluble extracts of the incubated mitochondria. The results showed that in the phosphorylation of thymidine to TMP, liver mitochondria exhibit a higher Vmax and Km than heart mitochondria, but otherwise heart and liver mitochondria display similar kinetics. AZT is phosphorylated to AZTMP, but no further phosphorylated forms were detected. In addition, AZT inhibited the production of TTP, with an IC50 of 14.4+/-2.6 microM AZT. This is higher, but comparable to, the results seen in isolated rat heart mitochondria.

Phosphorylation of thymidine and AZT in heart mitochondria: elucidation of a novel mechanism of AZT cardiotoxicity

Antiretroviral nucleoside analogs used in highly active antiretroviral therapy (HAART) are associated with cardiovascular and other tissue toxicity associated with mitochondrial DNA depletion, suggesting a block in mitochondrial (mt)-DNA replication. Because the triphosphate forms of these analogs variably inhibit mt-DNA polymerase, this enzyme has been promoted as the major target of toxicity associated with HAART. We have used isolated mitochondria from rat heart to study the mitochondrial transport and phosphorylation of thymidine and AZT (azidothymidine, or zidovudine), a component used in HAART. We demonstrate that isolated mitochondria readily transport thymidine and phosphorylate it to thymidine 5'-triphosphate (TTP) within the matrix. Under identical conditions, AZT is phosphorylated only to AZT-5'-monophosphate (AZT-MP). The kinetics of thymidine and AZT suggest negative cooperativity of substrate interaction with the enzyme, consistent with work by others on mitochondrial thymidine kinase 2. Results show that TMP and AZT-MP are not transported across the inner membrane, suggesting that AZT-MP may accumulate with time in the matrix. Given the lack of AZT-5'-triphosphate (AZT-TP), it seems unlikely that the toxicity of AZT in the heart is mediated by AZT-TP inhibition of DNA polymerase gamma. Rather, our work shows that AZT is a potent inhibitor of thymidine phosphorylation in heart mitochondria, having an inhibitory concentration (IC)(50) of 7.0 +/- 0.9 microM. Thus, the toxicity of AZT in some tissues may be mediated by disrupting the substrate supply of TTP for mt-DNA replication.

Guanine nucleotide transport by atractyloside-sensitive and -insensitive carriers in isolated heart mitochondria

In previous work (McKee EE, Bentley AT, Smith RM Jr, and Ciaccio CE, Biochem Biophys Res Commun 257: 466-472, 1999), the transport of guanine nucleotides into the matrix of intact isolated heart mitochondria was demonstrated. In this study, the time course and mechanisms of guanine nucleotide transport are characterized. Two distinct mechanisms of transport were found to be capable of moving guanine nucleotides across the inner membrane. The first carrier was saturable, displayed temperature dependence, preferred GDP to GTP, and did not transport GMP or IMP. When incubated in the absence of exogenous ATP, this carrier had a V(max) of 946 +/- 53 pmol. mg(-1). min(-1) with a K(m) of 2.9 +/- 0.3 mM for GDP. However, transport of GTP and GDP on this carrier was completely inhibited by physiological concentrations of ATP, suggesting that this carrier was not involved with guanine nucleotide transport in vivo. Because transport on this carrier was also inhibited by atractyloside, this carrier was consistent with the well-characterized ATP/ADP translocase. The second mechanism of guanine nucleotide uptake was insensitive to atractyloside, displayed temperature dependence, and was capable of transporting GMP, GDP, and GTP at approximately equal rates but did not transport IMP, guanine, or guanosine. GTP transport via this mechanism was slow, with a V(max) of 48.7 +/- 1.4 pmol. mg(-1). min(-1) and a K(m) = 4.4 +/- 0.4 mM. However, because the requirement for guanine nucleotide transport is low in nondividing tissues such as the heart, this transport process is nevertheless sufficient to account for the matrix uptake of guanine nucleotides and may represent the physiological mechanism of transport.

Origin of guanine nucleotides in isolated heart mitochondria

Presence of guanine nucleotide within the matrix of mitochondria is uncontested; the mechanism by which GTP takes up residence in the matrix is unknown. In this report, we demonstrate for the first time that direct transport of guanine nucleotide across the inner membrane of heart mitochondria is possible. Transport of guanine nucleotides from the medium to the matrix was suggested by inhibition of translation in isolated rat heart mitochondria when GTP-gamma-S was added to the medium. This result suggested that GTP was one source of matrix GTP. Other sources were investigated by measuring matrix uptake and conversion to GTP of several purines, purine nucleosides, and purine nucleotides. Results demonstrated that [14C]-guanine and [3H]-guanosine were not taken up by isolated mitochondria and were not converted to any other compound. While [14C]-ATP and [3H]-AMP were taken up readily into the matrix, radioactivity was never associated with a guanine compound. [3H]-IMP was not taken up into the matrix and was never converted to another compound. Our data showed that label added as [3H]-GTP, [3H]-GDP, or [3H]-GMP was readily taken up and concentrated in the matrix of isolated mitochondria.

Mitochondrial gene expression in Saccharomyces cerevisiae. IV. Effects of yeast cytosol on mitochondrial protein synthesis, degradation, and respiration

It has been known for some time that the addition of a crude yeast cytosolic fraction to isolated mitochondria stimulates the rate of amino acid incorporation into protein in the isolated organelles. However, the mechanism and importance of this phenomenon relative to mitochondrial function has not been established. While it has been assumed that this effect is at the level of translation, the recognition that newly synthesized mitochondrial translation products are rapidly degraded in isolated yeast mitochondria raises the possibility that cytosol affects amino acid incorporation by inhibiting proteolysis. Using pulse-chase experiments we demonstrate that the rate constants of degradation of the nascent products are not affected by yeast cytosol. Further, not only is proteolysis not inhibited by cytosol, but the loss of label caused by proteolysis is actually increased. This increase is directly related to an increase in the size of the nascent product pool which increases simply as a consequence of increasing the rate of translation. By utilizing an approach in which the loss of label due to proteolysis is minimized, the true stimulatory activity of the cytosolic fraction on synthesis was determined (2.1-fold vs. 1.3-fold by the previous method). Pulse-chase experiments in the presence of pactamycin, an initiation inhibitor, demonstrate that yeast cytosol causes an initial increase in the rate of translational initiation without increasing the rate of elongation. However, at later intervals the yeast cytosol acts primarily to maintain the rate of elongation which falls steadily in the controls. Finally, the presence of yeast cytosol dramatically increases the length of incubation time in which the mitochondrial preparation consumes oxygen and maintains coupled respiration, parameters that fall rapidly in the controls. Thus, a yeast cytosolic fraction may function to promote the stability of the mitochondrial preparation, which in turn may account for the increase in rates of translation, particularly with regard to maintaining rates of elongation.

Myosin heavy chain turnover during cardiac mass changes by glucocorticoids

One aim of this investigation was to determine whether the cardiac enlargement observed with glucocorticoid treatment is temporary or remains a permanent adaptation if steroid treatment is prolonged. A second aim was to study whether myosin heavy chain (MHC) synthesis rates are coordinated with the cardiac mass responses. Female rats received either a vehicle (1% aqueous carboxymethyl cellulose in saline) or hydrocortisone 21-acetate for 1, 3, 7, 11, and 15 days. Peak cardiac enlargement (10-15%) was observed after 7 days of hormone treatment in two separate series of experiments. The enlargement was maintained through 11 days of steroid injections but by 15 days had declined toward control levels. MHC synthesis measurements were performed by constant infusion of [3H]leucine. Leucine specific activities were similar among precursor pools (intracellular, extracellular, and leucyl-tRNA) and did not vary with steroid treatments. Fractional synthesis rates of ventricular MHC (%/day) did not change during the period of increase in ventricular mass but were reduced to 56-59% of controls (-11/19.5) at 7 and 11 days of treatment, when ventricular mass increases were highest. MHC breakdown (%/day) was reduced to approximately 60% (-11.5/18.7) of controls at 7 and 11 days. Changes in total protein synthesis, which was measured in isolated perfused hearts, were similar to the MHC responses and indicated that the alterations in MHC synthesis are synchronized with the hormonal effects on total protein metabolism. These results demonstrate that peak cardiac enlargement is not maintained with long-term glucocorticoid treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin stimulates mitochondrial protein synthesis and respiration in isolated perfused rat heart

The rates of synthesis of mitochondrial proteins by both the cytoplasmic and mitochondrial protein synthetic systems, as well as parameters of respiration, were measured and compared in mitochondria isolated from fresh, control perfused, and insulin-perfused rat hearts. The respiratory control ratio (RCR) in mitochondria from fresh hearts was 8.1 +/- 0.4 and decreased to 6.0 +/- 0.2 (P less than 0.001 vs. fresh) in mitochondria from control perfused hearts and to 6.7 +/- 0.2 (P less than 0.005 vs. fresh and P less than 0.02 vs. control perfused) for mitochondria from hearts perfused in the presence of insulin. A positive correlation between the RCR and the rate of mitochondrial translation was demonstrated in mitochondria from fresh hearts. In mitochondria isolated from control perfused hearts, the rate of protein synthesis decreased to 84 +/- 3% of the fresh rate after 30 min of perfusion and fell further to 64 +/- 3% after 3 h of perfusion. The inclusion of insulin in the perfusion buffer stimulated mitochondrial protein synthesis 1.2-fold by 1 h (P less than 0.005) and 1.34-fold by 3 h of perfusion (P less than 0.001). The addition of insulin to 1-h control perfused hearts shifted the rate of mitochondrial protein synthesis from the control level to the insulin-perfused level within 30 min of additional perfusion, whereas 1 h was required to shift the RCR values of these mitochondria from control levels to insulin-perfused levels. Thus, whereas RCR was a useful predictor of mitochondrial translation rates, it did not account for the effects of insulin on mitochondrial translation.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling of mitochondrial metabolism and protein synthesis in heart mitochondria

Although much is now known with regard to the processes of mammalian mitochondrial gene expression, relatively little is known concerning the quantitative regulation of this pathway in response to hormones or other physiological stimuli. In this paper the potential coupling of mitochondrial metabolism to mitochondrial protein synthesis was investigated and the concentration of nucleotides and substrates for optimal translation in isolated rat heart mitochondria was determined. It was demonstrated that optimal isolated heart mitochondrial protein synthesis required the presence of an oxidizable substrate. Of the substrates tested, glutamate (20 mM) supported translation best followed by malate, succinate, and alpha-ketoglutarate, whereas pyruvate supported synthesis poorly. Unlike other recent mammalian mitochondrial systems, the presence of an oxidizable substrate was required for translation even in the presence of medium ATP and an exogenous energy-generating system. Mitochondrial translation also required the presence of adenine nucleotide that could be added as ADP or ATP; however, ATP added above 0.5 mM became progressively inhibitory. As a result, synthesis was supported significantly better by ATP synthesized by the system from added ADP, than by ATP added directly to the system. However, if the phosphorylation of ADP was prevented by limiting the phosphate concentration, ADP itself strongly inhibited mitochondrial protein synthesis. This inhibition appeared to be closely related to the energy charge of the system rather than to absolute levels of ADP, indicating for the first time that mitochondrial translation, like its cytoplasmic counterpart is regulated by energy charge. Last, this system did not require the inhibition of guanine nucleotide or exogenous energy-generating systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and incubation conditions to study heart mitochondrial protein synthesis

Although much is now known with regard to the processes of mammalian mitochondrial gene expression, relatively little is known concerning the quantitative regulation of this pathway in response to hormones or other physiological stimuli. This has been caused, in large part, by the lack of adequate assay systems in which such processes can be meaningfully measured. The purpose of this and the companion paper [E. E. McKee, B. L. Grier, G. S. Thompson, A. C. F. Leung, and J. D. McCourt. Am. J. Physiol. 258 [Endocrinol. Metab. 21):E503-E510, 1990] is to describe a system in which the quantitative regulation of mitochondrial protein synthesis in rat heart can be investigated. In this report the conditions for mitochondrial isolation and labeling are described, and the importance of isolating intact, tightly coupled mitochondria in obtaining high and reliable rates of protein synthesis is demonstrated. The highest levels of protein synthesis are obtained in mitochondria isolated from hearts perfused and homogenized in the presence of subtilisin, conditions in which the fastest rates of state 3 respiration and the highest respiratory control ratios are also observed. Analysis of the free amino acid pools indicates that isolated heart mitochondria have a negligible level of endogenous methionine as well as other amino acids. As a result, the concentration and specific radioactivity of the [35S]methionine pool serving protein synthesis could be easily determined. Optimal translation occurred at 30 degrees C at a pH of 7.0-7.2 and required the addition of methionine (20 microM), the other 19 amino acids (0.1 mM each), K+ (60-90 mM), Cl- (30-90 mM), Mg2+ (0.5-5 mM), and bovine serum albumin (1 mg/ml). As shown in the companion paper, adenine nucleotide (0.5-4.0 mM) and oxidizable substrate (10-20 mM glutamate) are also required for isolated heart mitochondrial protein synthesis. Analysis of labeled mitochondrial translation products demonstrated that bona fide mitochondrial peptides were synthesized.

Mitochondrial protein synthesis during thyroxine-induced cardiac hypertrophy

The goal of this paper was to determine the effects of 3,5,3'-triiodothyronine (T3)-thyroxine-induced cardiac hypertrophy on the rates of synthesis of mitochondrial proteins by both the cytoplasmic and mitochondrial protein synthesis systems and to compare the results with total protein synthesis and cardiac enlargement. Daily injections of T3-thyroxine in the rat resulted in a 25% increase in the growth of the ventricle compared with controls. The cytoplasmic synthesis of both mitochondrial and total proteins as measured in the isolated perfused heart was stimulated by T3-thyroxine injection to a peak of 155 and 146%, respectively, of vehicle-injected controls after 3 days of hormone treatment. This peak was followed by a gradual decline in stimulation in total protein synthesis to 132% of control by 9 days of injection, whereas the decline in stimulation of cytoplasmic synthesis of mitochondrial proteins was significantly steeper, falling to 119% of vehicle control. The rate of protein synthesis within the mitochondrial compartment was also measured during the time course of T3-thyroxine-induced hypertrophy. These rates were measured in an isolated intact heart mitochondrial protein synthesis system described and characterized in the companion papers [E. E. McKee, B. L. Grier, G. S. Thompson, and J. D. McCourt. Am. J. Physiol. 258 (Endocrinol. Metab. 21): E492-E502, 1990; and E. E. McKee, B. L. Grier, G. S. Thompson, A. C. F. Leung, and J. D. McCourt. Am. J. Physiol. 258 (Endocrinol. Metab. 21): E503-E510, 1990]. Rates of mitochondrial protein synthesis were dramatically stimulated by T3-thyroxine injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoid receptor activation in isolated perfused rat hearts

The formation of unactivated and activated glucocorticoid receptor complexes was studied in intact, isolated, perfused rat hearts in the presence of [3H]triamcinolone acetonide. Receptor activation, as quantified by the DNA-cellulose-binding assay, began to increase within 30 s of perfusion and reached a final steady-state level (t 1/2 = 4.6 min) with 46% of the steroid-receptor complexes bound to DNA-cellulose. With the use of a linear potassium phosphate (KP) gradient (5-400 mM), unactivated receptors eluted from DEAE-cellulose anion exchange columns at approximately 250 mM KP. Two activated receptor forms appeared, which eluted either in the wash fraction (binder IB) or between 50 and 100 mM KP (binder II) and occurred with half times of 1.3 and 2.7 min, respectively. Postperfusion cytosol preparation did not markedly influence the results as receptor binding was reduced by 10% or less when a 100-fold excess of unlabeled triamcinolone acetonide was included in the homogenizing buffer. We conclude from these results that glucocorticoids are able to exert a direct effect on the heart through binding to their own receptor in the absence of endogenous hormones. The time dependency of receptor activation supports a physiological role for this process. However, activation rates, determined from conformational changes associated with altered DEAE-cellulose elution profiles and appearance of activated receptor forms, occur earlier and may not be coordinated with the rate of activation as quantified by DNA-cellulose binding.

Mitochondrial gene expression in saccharomyces cerevisiae. I. Optimal conditions for protein synthesis in isolated mitochondria

An in vitro mitochondrial protein-synthesizing system, which makes use of intact yeast mitochondria, has been developed in order to study mitochondrial gene expression and its control by nuclear-coded proteins. Studies with this system have revealed that: isolated mitochondria synthesize polypeptide gene products which can be radiolabeled to high specific radioactivities when incubated in a "protein-synthesizing medium" that has been optimized with respect to each of its components; two energy-generating systems, endogenous oxidative phosphorylation and an exogenous ATP-regenerating system, support the highest level of protein synthesis; and the omission of an oxidizable substrate results in the synthesis of two new polypeptides (19.5 and 18 kDa) and a decrease in the amounts of cytochrome c oxidase subunits I and II which are synthesized. They have also revealed that added adenine and guanine nucleotides increase the overall level of protein synthesis and that the added guanine nucleotides facilitate polypeptide chain elongation. Although isolated mitochondria which have been optimized for protein synthesis synthesize normal gene products (McKee, E. E., McEwen, J. E., and Poyton, R. O., (1984) J. Biol. Chem. 259, 9332-9338) they still respond to an added dialyzed S-100 fraction from yeast cells by increasing their level of protein synthesis. This stimulation is observed in the presence of optimal concentrations of GTP, making it unlikely that guanyl nucleotides or enzymes which synthesize them are the sole stimulatory factors present in cellular cytosolic fractions, as suggested by Ohashi and Schatz (Ohashi, A., and Schatz, G. (1980) J. Biol. Chem. 255, 7740-7745).

Mitochondrial gene expression in saccharomyces cerevisiae. II. Fidelity of translation in isolated mitochondria from wild type and respiratory-deficient mutant cells

The fidelity of mitochondrial translation has been examined in isolated yeast mitochondria incubated in an optimized protein-synthesizing medium (McKee, E. E., and Poyton, R. O., (1984) J. Biol. Chem. 259, 9320-9331). These studies have revealed: that isolated mitochondria synthesize bona fide mitochondrial gene products which are identical in kind and relative amounts to those synthesized in vivo; that mitochondria isolated from both mitochondrial mit- mutants and nuclear Pet mutants, which retain the capacity for mitochondrial protein synthesis, produce a mutant pattern of mitochondrial gene products which is similar to that produced in vivo; and that isolated mitochondria synthesize up to 7% of their protein mass in vitro at a rate of about one polypeptide bond/polypeptide chain/s. These studies also reveal that isolated wild type yeast mitochondria are competent in all steps in mitochondrial translation, including initiation. Using pactamycin as a specific inhibitor of translational initiation we have demonstrated that polypeptide chain initiation continues throughout a 60-min incubation period. By using this in vitro system to calculate the stoichiometry of synthesis of the major proteins coded by yeast mitochondrial DNA we have found that the var1 polypeptide is synthesized at a level which is significantly lower than all other mitochondrial gene products and that cytochrome c oxidase subunits I, II, and III and ATPase subunit 9 are synthesized in nearly equimolar amounts. These results suggest that the synthesis of these four gene products is controlled coordinately.

Transfer RNA and aminoacyl transfer RNA in developing rats

The total transfer RNA (tRNA) level in the liver, kidney, skeletal muscle and heart muscle of developing rats was determined by purification using (3H)tRNA as an internal standard. Liver and kidney contained almost twice as much tRNA per gram tissue as heart and skeletal muscle. There were no apparent differences between the sexes. The aminoacylation capacities of six tRNA species (alanyl, aspartyl, leucyl, methionyl, phenylalanyl, and tryptophanyl) from rat liver were not different during 3 developmental stages (suckling, weaning and young adult), and there were also no differences noted between males females. The in vivo percent aminoacylation of 4 tRNAs (aspartyl, leucyl, methionyl, and phenylalanyl) was lower during the newborn and suckling periods than in weaning and young adult rat livers. The tRNA of young adults was almost completely aminoacylated in vivo with the exception of alanyl-tRNA.

Increase in rat liver ribonuclease inhibitor levels during the neonatal period

Alkaline ribonuclease (RNase) activity in nuclear, mitochondrial, and cytoplasmic fractions of rat liver is higher in newborn animals than in young adults. The level of cytoplasmic RNase inhibitor is low in newborn rat liver and increases more than two fold by 10 days of age.