Differences in the inhibition of translation by cisplatin, transplatin, and certain related compounds.

The non-therapeutic cisplatin congeners transplatin and chloroethylenetriamine platinum (dien) inhibited translation to a similar extent as cisplatin did. The IC50 values were: cisplatin 23 microM, transplatin 54 microM, and dien 117 microM. Unlike certain heavy metal inhibitors of translation, the effect of neither cisplatin nor the congeners was reversed by 3':5'-cyclic adenosine monophosphate (cAMP). This suggests that the effect of these platinum compounds does not occur by the heavy metal mechanism. Polyribosomes and ribosomal subunits formed in transplatin-inhibited reactions differed from those in reactions inhibited by cisplatin. Specifically, large polyribosomes and complete 80S ribosomal subunits accumulated in the presence of transplatin. This indicates that while cisplatin slowed initiation of peptide synthesis, the trans-isomer slowed elongation. Substantive differences were not found between cisplatin and the monofunctional compound dien. This congener increased the non-peptidyl disintegrations per minute in the acid precipitates of assays containing [35S]methionine. The high background indicated that an interaction between the label and a precipitable component of the system was induced by dien. However, consumption of methionine by this interaction did not appear to be the cause of the inhibition. Although there may be differences in the mechanisms of the effects, the finding that the non-therapeutic congeners inhibit translation at similar concentrations as cisplatin suggests that this inhibition is not responsible for the anticancer effect. On the other hand, the possibility that decreased translation could play an important role in the toxicity of these compounds in certain quiescent cells cannot be ruled out.

Ascorbic acid: an endogenous inhibitor of isolated Na+,K+-ATPase.

During attempts to isolate and identify an endogenous ligand for the glycoside binding sites on Na+,K+-ATPase, bovine adrenal glands were found to contain a potent inhibitor of isolated Na+,K+-ATPase. The inhibitory principle was extracted from adrenal cortex, following homogenization in NaHCO3 solution and separation on a Sephadex G-10 column. The active principle was recovered from a fraction which eluted from the column after the 3H2O peak. The extract inhibited isolated Na+,K+-ATPase and the specific [3H]ouabain binding reaction. Sensitivity of the enzyme to the inhibitory action of the extract was species and tissue dependent; however, the pattern and the magnitude of the sensitivity were different from those of the digitalis glycosides. Moreover, the inhibitory principle failed to inhibit sodium pump activity, estimated from ouabain inhibitable 86Rb+ uptake by guinea pig brain slices. The activity of the extract to inhibit isolated Na+,K+-ATPase was stable under acidic condition but was lost rapidly at neutral pH, and could be eliminated by EDTA. In an acidic medium, the inhibitory principle had an absorption maximum at 244 nm which shifted to 264 nm and decayed rapidly at neutral pH. By using mass spectrometry, the principle was identified to be ascorbic acid, which has been shown previously to inhibit isolated Na+,K+-ATPase under appropriate conditions. Because ascorbic acid was incapable of inhibiting the sodium pump in intact cells, this inhibitor of the isolated enzyme does not appear to be the endogenous ligand which regulates sodium pump activity in vivo.

Cisplatin inhibits in vitro translation by preventing the formation of complete initiation complex.

We previously reported that mRNA loses the ability to direct in vitro peptide synthesis after incubation with cisplatin. The present study was designed to determine the step in translation that is affected. The rates of translation reactions inhibited by cisplatin were biphasic, having an initial rate comparable to that of the uninhibited reaction before decreasing. Analysis of cisplatin-inhibited reactions in sucrose density gradients showed a decrease in polyribosome formation. These results are consistent with an inhibition of the initiation step of protein synthesis. Individual steps in initiation were tested by analyzing the formation of ribosomal subunits in sucrose gradients that resolve the incomplete complexes. Cisplatin caused an accumulation of 48 S particles accompanied by a decreased amount of completed 80 S initiation complexes. Similar results were obtained in experiments utilizing radiolabeled methionine or mRNA. We conclude that cisplatin blocks the initiation of translation by preventing the joining of the 60 S ribosomal subunit to the 48 S preinitiation subunit.

Hemolysates from guinea-pig reticulocytes also efficiently translate added mRNA.

1. Lysate is prepared from guinea-pig reticulocytes and its translation activity is compared both quantitatively and qualitatively with that found in rabbit reticulocyte lysates. 2. Guinea-pigs are comparatively resistant to acetylphenylhydrazine, a drug used to induce reticulocytosis in rabbits. 3. The characteristics and conditions for maximal translation activity in guinea-pig reticulocyte lysates are quite similar to those described for rabbit reticulocyte systems with the possible exception of the components necessary for providing high energy phosphate bonds. 4. Both rabbit globin and brome mosaic virus messenger RNAs direct protein synthesis in the guinea pig translation system with activity similar to that found in the rabbit system. 5. Translation products synthesized in the guinea-pig and rabbit systems and fluorographed following separation on sodium dodecyl sulfate-polyacrylamide demonstrate similar electrophoretic patterns and intensities.

Catalytic activity of administered gulonolactone oxidase polyethylene glycol conjugates.

Scurvy in guinea pigs provides a convenient model of inborn metabolic disease for the investigation of enzyme therapy protocols. Gulonolactone oxidase, the enzyme in ascorbic acid biosynthesis that is missing from the scurvy-prone species, was modified by attachment of polyethylene glycol. The catalytic properties of this enzyme were affected little by the modification. Intravenous injection of this modified form of the enzyme elicited ascorbic acid synthesis in a dose-dependent manner. The modified enzyme was stabilized to incubation at 37 degrees C but was not protected from inactivation by trypsin. The circulating half-life of enzyme activity was not prolonged by this modification. Further, attachment of polyethylene glycol did neither abolish the enzyme's ability to react with preformed antibodies nor eliminate its immunogenicity.

Tirilazad mesylate protects vitamins C and E in brain ischemia-reperfusion injury.

Brain concentrations of the antioxidant vitamins C and E decreased following unilateral carotid occlusion and reperfusion for 2 or 24 h in gerbils. Administration of the 21-aminosteroid inhibitor of lipid peroxidation, tirilazad mesylate (U74006F), prevented the decrease in level of both of these vitamins following 2 h of reperfusion. After 24 h of reperfusion, however, alpha-tocopherol (vitamin E) continued to be protected, but ascorbic acid (vitamin C) showed a pronounced decrease in content. The changes in concentrations of these vitamins are consistent with U74006F acting to inhibit peroxidation in the CNS by scavenging of lipid peroxyl radicals and suggest that, in the presence of this agent, injury-induced depletion of ascorbic acid may occur without irreversible tissue damage.

Glutaraldehyde-reacted immunoprecipitates of L-gulonolactone oxidase are suitable for administration to guinea pigs.

The potential pharmacologic use of enzymes has long been considered. Practical applications, however, have been limited by the toxicity and allergic response to administered foreign proteins. A simple in vitro modification that allows the intraperitoneal administration of large doses of L-gulonolactone oxidase to guinea pigs is described. The enzyme is precipitated by guinea pig antisera and reacted with glutaraldehyde (0.125%). The product is comparatively nontoxic in guinea pigs. Administration of this enzyme enables guinea pigs to synthesize ascorbic acid. Success of this approach may depend on reinforcement by the bifunctional reagent of the enzyme-antibody complex.

The effect of certain vitamin deficiencies on hepatic drug metabolism.

There is increasing evidence that the liver microsomal drug metabolizing system is affected by various vitamins such as ascorbic acid, riboflavin, and alpha-tocopherol. In regard to ascorbic acid deficiency there is a decrease in the quantity of hepatic microsomal electron transport components such as cytochrome P-450 and NADPH-cytochrome P-450 reductase, as well as decreases in a variety of drug enzyme reactions such as N-demethylation, O-demethylation, and steroid hydroxylation. In addition, young animals given high supplements of vitamin C have increased quantities of electron transport components and overall drug metabolism activities. Kinetic studies indicate no change in the apparent Km of N-demethylase, O-demethylase or hydroxylase for drug substrates in animals depleted or given high amounts of the vitamin. However, there are qualitative changes in both type I and II substrate-cytochrome P-450 binding. Ascorbic acid is not involved in microsomal lipid peroxidation or in any qualitative or quantitative change in phosphatidylcholine. Replenishing vitamin C-deficient animals with ascorbic acid required 3 to 7 days for the electron transport components and drug metabolism activities to return to normal levels. Induction with phenobarbital and 3-methylcholanthrene is not impaired in the deficient animal since drug metabolism activities are induced to the same extent as normal controls; however, the administration of delta-aminolevulinic acid, a precursor of heme synthesis, to deficient animals caused an increase in the quantity of cytochrome P-450. The effects of riboflavin deficiency on electron transport components and drug metabolism activities have been noted only in adult animals after prolonged periods of deficiency. Decreases in drug metabolism activities occur with both type I (aminopyrine and ethylmorphine) and type II (aniline) substrates. As was found with ascorbic acid deficiency, drug enzyme induction occurred to the same extent with phenobarbital in deficient and normal animals. In addition, it required from 10 to 15 days for the drug metabolism activities to return to normal levels when deficient animals were replenished with riboflavin. The effect of vitamin E on drug metabolism is specific in N-demethylase activities decrease while O-demethylase activities are not affected in the deficient state. This vitamin differs from ascorbic acid and riboflavin in that several laboratories have reported no quantitative decrease in cytochrome P-450, although there are some reports that it and delta-aminolevulinic acid dehydratase are lowered quantity of cytochrome in E-deficient animals. The effect of vitamin E, if any, on the P-450 is unresolved; an important question that requires further clarification. As with ascorbic acid there is no difference in the apparent Km of N-demethylase enzymes for varous substrates and the protective effect of vitamin E does not appear to be one of an antioxidant inhibiting microsomal lipid peroxidation.

Ascorbic acid and hepatic drug metabolism.

Previous in vivo studies indicate that hepatic microsomal drug metabolism decreases in ascorbic acid deficiency and is augmented when high supplements of the vitamin are given to guinea pigs. Kinetic studies with O-demethylase indicate no significant change in the apparent Km of p-nitroanisole in normal, ascorbic acid-deficient animals, or in animals given high supplements of ascorbic acid. The decrease in drug metabolism activity caused by ascorbic acid deficiency is not due to increased lipid peroxidation, nor was phosphatidyl choline significantly altered quantitatively or qualitatively in microsomes from ascorbic acid-deficient animals. Microsomal cytochrome P-450 prepared from ascorbic acid-deficient livers is less stable to sonication, dialysis and treatment with metal chelators. The decrease in cytochrome P-450 and O-demethylase activity associated with dialysis could be prevented by the addition of ascorbic acid. The molar ratio of microsomal ascorbic acid to cytochrome P-450 (plus P-420) is in the order of 2:1. This ratio is maintained during ascorbic acid deficiency in liver and adrenal tissue, during dialysis, on storage and with a partial purification of the cytochrome, which suggests a close association between ascorbic acid and the cytochrome. In addition, ascorbic acid protects cytochrome P-450 and aniline hydroxy lase activity from inhibition by ferrous iron chelators such as alpha, alpha'-dipyridyl. The chelator binds to cytochrome P-450 and prevents formation of the reduced cytochrome P-450-CO spectrum; it in turn gives a reduced spectrum with the cytochrome at 450 nm. These studies suggest that there is an interaction between ascorbic acid and cytochrome P-450 involving the reduced form of the heme iron.

Treatment of a metabolic disease, scurvy, by administration of the missing enzyme.

Reaction of immunoprecipitated L-gulonolactone oxidase with glutaraldehyde allows multiple administrations of large amounts of this enzyme extracted from either chicken or rats to guinea pigs. L-Gulonolactone oxidase converts L-gulonolactone to ascorbic acid, and its absence from guinea pigs and primates results in their requirement for this vitamin. By administration of this enzyme guinea pigs are able to survive on an ascorbic-acid-deficient regimen.

Heterologous immunoprecipitates also have potential for therapeutic use.

Potential therapeutic usefulness of administered enzymes is limited by toxicity and allergenicity. To overcome these problems we are using scurvy to test various enzyme modifications that may be suitable for therapy. L-Gulonolactone oxidase, which catalyzes the final step in ascorbic acid biosynthesis, is immunoprecipitated with specific antisera from rabbits and then cross-linked with glutaraldehyde. The modified enzyme retains activity sufficient to elicit ascorbic acid synthesis in scorbutic guinea pigs. Intraperitoneal injection of this altered enzyme to animals supplemented with L-gulonolactone increases plasma concentrations of the vitamin. Importantly, multiple doses of the complex are tolerated. Therefore, it is possible to prolong survival time of animals fed an ascorbic acid-deficient diet by this enzyme replacement therapy. This procedure can also be applied to other enzymes that have potential therapeutic use. Serum cholinesterase and asparaginase both retain activity after this modification and are tolerated in single or in weekly repeated injections. Following three or four weekly injections, an anaphylactic reaction to serum but not to enzyme can be elicited if they are injected intravascularly. We conclude that the stability of the immobilized foreign enzyme is a critical factor in lessening the toxicity to multiple injections of these foreign proteins.