Optimization of in vitro protein synthesis by isolated mouse adrenal mitochondria

Isolation of rat adrenocortical mitochondria

This report describes a relatively simple and reliable method for isolating adrenocortical mitochondria from rats in good, reasonably pure yield. These organelles, which heretofore have been unobtainable in isolated form from small laboratory animals, are now readily accessible. A high degree of mitochondrial purity is shown by the electron micrographs, as well as the structural integrity of each mitochondrion. That these organelles have retained their functional integrity is shown by their high respiratory control ratios. In general, the biochemical performance of these adrenal cortical mitochondria closely mirrors that of typical hepatic or cardiac mitochondria.

Acute depression of mitochondrial protein synthesis during anoxia: contributions of oxygen sensing, matrix acidification, and redox state

Mitochondrial protein synthesis is acutely depressed during anoxia-induced quiescence in embryos of Artemia franciscana. Oxygen deprivation is accompanied in vivo by a dramatic drop in extramitochondrial pH, and both of these alterations strongly inhibit protein synthesis in isolated mitochondria. Here we show that the oxygen dependence is not explained simply by blockage of the electron transport chain or by the increased redox state. Whereas oxygen deprivation substantially depressed protein synthesis within 5 min and resulted in a 77% reduction after 1 h, aerobic incubations with saturating concentrations of cyanide or antimycin A had little effect during the first 20 min and only a modest effect after 1 h (36 and 20% reductions, respectively). Yet the mitochondrial NAD(P)H pools were fully reduced after 2-3 min with all three treatments. This cyanide- and antimycin-insensitive but hypoxia-sensitive pattern of protein synthesis depression suggests the presence of a molecular oxygen sensor within the mitochondrion. Second, we show for the first time that acidification of extramitochondrial pH exerts inhibition on protein synthesis specifically through changes in matrix pH. Matrix pH was 8.2 during protein synthesis assays performed at the extramitochondrial pH optimum of 7.5. When this proton gradient was abolished with nigericin, the extramitochondrial pH optimum for protein synthesis displayed an alkaline shift of approximately 0.7 pH unit. These data suggest the presence of proton-sensitive translational components within the mitochondrion.

Translation rates of isolated liver mitochondria under conditions of hepatic mitochondrial proliferation

The hepatic mitochondrial content is increased in rats by treatment with the hypolipidaemic drug clofibrate and by administration of the cobalamin analogue hydroxycobalamin[c-lactam] (HCCL), an inhibitor of hepatic L-methylmalonyl-CoA mutase activity. As a first step in defining the mechanisms regulating liver mitochondrial contents in these models, the current studies were designed to test the hypothesis that hepatic mitochondrial proliferation is associated with enhanced translation rates of mitochondrial DNA gene products. Incorporation of [35S]methionine and [3H]leucine into protein was quantified in mitochondria isolated from control, clofibrate- and HCCL-treated rats. Use of multiple amino acid substrate concentrations permitted the maximal rate of translation (Vmax.) to be determined independent of endogenous amino acid concentrations. The Vmax. for methionine incorporation was not different in the models evaluated (0.062, 0.057 and 0.061 pmol/min per mg of mitochondrial protein in control, clofibrate- and HCCL-treated rats respectively). Similar results were obtained for leucine incorporation when absolute fractional radiolabel incorporation rates were analysed and when conventional Lineweaver-Burk analysis was employed. These results demonstrate no change in the intrinsic capacity of mitochondrial translation in these two models of hepatic mitochondrial proliferation.

The effects of cryopreservation on protein synthesis and membrane transport in isolated rat liver mitochondria

Protein synthesizing activity and membrane transport were examined in fresh and cryopreserved isolated rat liver mitochondria. In the presence of 0.6, 1.2, and 1.8 M final concentrations of dimethyl sulfoxide (Me2SO), both metabolic parameters were considerably inhibited in the fresh samples and even more inhibited in the cryopreserved specimens. However, simple exposure to this penetrating cryoprotectant, followed by its subsequent removal by washing, did not seem to affect significantly the examined functions. When different freeze-thaw regimes were investigated, it was observed that optimal recovery of protein synthesis and membrane transport functions were obtained when fast freezing took place in the absence of Me2SO.

Effects of chronic ethanol consumption on the synthesis of polypeptides encoded by the hepatic mitochondrial genome

Liver mitochondria from rats fed ethanol chronically demonstrate an impaired ability to incorporate [35S]methionine into polypeptide products in vitro. This ethanol-induced effect on mitochondrial translation in vitro could not be attributed to significant differences in the methionine precursor pool sizes of ethanol and control mitochondria or to the acute effects of residual ethanol. The observed reduction of radiolabeled methionine incorporation into mitochondrial gene products of ethanol mitochondria in vitro reflects a decrease in the synthesis of all the mitochondrial gene products. However, the percentage of total radiolabel incorporated into each gene product is unaffected by ethanol, suggesting an ethanol-induced coordinate depression of mitochondrial protein synthesis. Moreover, SDS-PAGE and densitometry of submitochondrial particles from ethanol-fed and control rats demonstrated that the steady-state concentration of each of the mitochondrial gene products is decreased in ethanol-fed rats. This reduction of the steady-state concentration of the mitochondrial gene products may be related to the observed depressions of oxidative phosphorylation activities associated with hepatic mitochondria from ethanol-fed rats.

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)

Mitochondrial DNA, RNA and protein synthesis in normal, hypothyroid and mildly hyperthyroid rat liver during cold exposure

We have examined in isolated liver mitochondria the effect of cold exposure on DNA, RNA and protein synthesis in normal, hypothyroid and mildly hyperthyroid rats. In normal rats DNA polymerase activity increased from the first day of cold exposure remaining high up to the fifteenth day. RNA polymerase and protein synthesis were stimulated from the fifth day of cold exposure, maintaining a high level up to the fifteenth day. These activities were related to serum triiodothyronine (T3) levels. Indeed propylthiouracil (PTU) administration to cold-exposed rats drastically depressed the above activities, whereas T3 administration to PTU-treated cold-exposed rats restored them to about the values prevalent in normal cold-exposed rats. The translation products analyzed by gel electrophoresis showed that different effects may be exerted by T3 depending on whether its circulating levels are physiologically or pharmacologically modified. These findings suggest that T3 may be involved in the regulation of the acclimation process by acting, presumably with a permissive role, on those activities which determine a modification of the mitochondrial morphometric features and an increase in mitochondria number and turnover.

Mitochondrial DNA expression in Drosophila melanogaster: neosynthesized polypeptides in isolated mitochondria

The expression of mitochondrial genome of D. melanogaster in isolated mitochondria was followed by incorporation of 35S methionine in neosynthesized polypeptides. A high level of protein synthesis was obtained after optimization of all the incubation parameters. Two kinds of energy-generating systems were used: an endogenous system where an oxidizable substrate were added for ATP synthesis; an exogenous system with an energy-rich compound for ATP regeneration, the latter proved to be the most effective. The effect of the oxidative phosphorylation uncoupler (Clccp), and an ATPase inhibitor (oligomycine) allow us to postulate the role of the electrochemical potential in the expression of the mitochondrial genome. Electrophoresis and autoradiography of neosynthesized mitochondrial proteins exhibits 18 to 24 protein bands, ranging from 6.5 to 65 Kd; incubation of KC 0% drosophila cells with 35S methionine and cycloheximide gave similar results. Both our results and those published elsewhere suggest that the expression of mitochondrial genome in higher organisms could be more complex than simple translation of the 13 genes presents on these genomes.