The inner membrane of the rat liver mitochondrion as the site of incorporation of radioactively labeled precursors into nucleic acid and protein in vitro

The inhibition of mitochondrial DNA replication in vitro by the metabolites of benzene, hydroquinone and p-benzoquinone

Rat liver mitochondria incubated with the metabolites of benzene, p-benzoquinone or 1,2,4-benzenetriol, showed a dose-dependent inhibition of [3H]dTTP incorporation into mtDNA with median inhibitory concentrations of 1 mM for each compound. Benzene and the metabolites phenol, catechol and hydroquinone did not inhibit at concentrations up to 10 mM. Similarly, incubation of p-benzoquinone or hydroquinone with rabbit bone marrow mitochondria showed a dose-dependent inhibition of mtDNA synthesis with 50% inhibition at 1 mM and 10 mM, respectively. That these metabolites inhibit mitochondrial replication was evidenced by the fact that [3H]dTTP incorporation into characteristic 38S, 27S and 7S mitochondrial replication intermediates was decreased by the quinones, as analyzed on 5-20% neutral sucrose velocity gradients. p-Benzoquinone, hydroquinone and 1,2,4-benzenetriol inhibited the activity of partially purified rat liver mtDNA polymerase gamma using either activated calf thymus DNA or poly(rA) X p(dT)12-18 as primer/template, with 50% inhibitory concentrations of 25 microM, 25 microM and 180 microM, respectively. Preincubation of the metabolites with polymerase gamma or primer/template, followed by removal of the unreacted metabolite by gel filtration, indicated that inhibition resulted from interaction of the metabolites with the enzyme, rather than with the template. Binding appeared to involve a sulfhydryl residue on the enzyme since the binding of [14C]hydroquinone was prevented by N-ethylmaleimide. The ability of hydroquinone or p-benzoquinone to inhibit binding of [14C]hydroquinone to the enzyme suggests that the compounds bind to a common site or are converted to a common intermediate. Inhibition of, or changes in, replication in mitochondria of bone marrow cells by hydroquinone and p-benzoquinone may explain the changes in the mitochondrial genome observed in marrow stem cells in acute myelogenous leukemia and may suggest a mechanism for benzene leukemogenesis.

Mesosomes: membranous bacterial organelles

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Synthesis of ribonucleic acid by isolated rat liver mitochondria

Rat liver mitochondria isolated in sucrose-N-tris(hydroxymethyl)methyl-2-aminoethane-sulphonic acid (TES) incorporated [(3)H]UTP into RNA for 1h. Incorporation was inhibited 50% by 1mug of actinomycin D/ml, 1mug of acriflavine/ml and 0.5mug of ethidium bromide/ml but was insensitive to rifampicin, rifamycin SV, streptovarcin and deoxyribonuclease. After the first 10min of incubation, the synthesis was insensitive to ribonuclease. RNA synthesis by mitochondria isolated in sucrose-EDTA was insensitive to actinomycin D and sensitive to ribonuclease during the first 10min of the incubation but thereafter the sensitivities were the same as for mitochondria isolated in sucrose-TES. In a hypo-osmotic medium the relative extent of incorporation of the four ribonucleoside triphosphates into RNA was CTP>UTP=ATP>>GTP. In an iso-osmotic medium the incorporation of CTP and GTP decreased. All four nucleotides were incorporated into RNA in a DNA-dependent process, as indicated by the inhibition by actinomycin D. In addition, CTP and ATP were incorporated into the CCA end of mitochondrial tRNA. ATP was also incorporated into an unidentified acid-insoluble compound, which hydrolysed in alkali to a product that was not ATP, ADP or 5'- or 2(3')-AMP. Atractyloside inhibited the incorporation of ATP into RNA with 50% inhibition at 2-3nmol/mg of protein. The [(3)H]UTP-labelled RNA had peaks of 16S and 13S characteristic of mitochondrial rRNA. In addition a peak at 20-21S was observed as well as heterogeneous RNA sedimenting throughout the gradient. The synthesis of all these species was inhibited by actinomycin D, indicating that rat liver mitochondrial DNA codes for mitochondrial rRNA as well as other as yet unidentified species.