Ethanol inhibition of reticulocyte protein synthesis: the role of haem

High performance liquid chromatographic studies of erythrocytes haem biosynthetic enzyme activities in alcoholic macrocytosis

The activities of 5-aminolaevulinate dehydrase (ALA-D), hydroxymethylbilane synthase (HMB-S), uroporphyrinogen III synthase (URO-S) and uroporphyrinogen decarboxylase (URO-D) in the erythrocytes of alcoholics were assayed and compared with those in controls and abstaining alcoholics. The HMB-S and URO-S activities in alcoholics were significantly higher than the controls and there were significant correlations between enzyme activities and mean corpuscular volume (MCV). Alcoholics abstaining from alcohol over a nine-month period showed a return to normal enzyme activities in parallel with the normalization of their MCV. Abstaining alcoholics thus all had normal HMB-S and URO-S activities. The ALA-D and URO-D activities of alcoholics were not significantly different from controls or from abstaining alcoholics. Alcohol appears to have a selective effect on the haem biosynthetic enzymes in the erythron.

Effect of isoniazid, a haem inhibitor, on globin chain synthesis in reticulocytes from non-thalassaemic and beta thalassaemic subjects

The effect of isonicotinic acid hydrazide (INH), a potent haem inhibitor, on globin chain synthesis was studied in reticulocytes from the following groups of patients: four non-thalassaemic patients (group i); five beta thalassaemia heterozygotes (group ii); three Hb S/beta thalassaemia heterozygotes (group iii); and two additional patients--one with homozygous beta thalassaemia and the other with thalassaemia intermedia (group iv). This was done to determine whether haem inhibitors depress alpha globin chain synthesis. The progressive increase of INH concentration (10-40 mmol l-1) in reticulocytes from a beta thalassaemia heterozygote resulted in a remarkable decrease of the alpha and beta chain synthesis, ranging from 80% to 97% and from 74% to 96% of control values, respectively, and in a gradual drop of alpha:beta ratio from 1.87 to 1.38. Furthermore, in the samples incubated with 40 mmol l-1 INH, a pronounced inhibition of globin chain synthesis 77 (19%) for alpha chain and 67 (27%) for beta or beta S chain) and a substantial drop of the alpha:beta or beta S ratio in samples with INH (median 1.16) compared with that in samples without INH (median 1.70) were observed. The inhibitory effect of INH was significantly or completely corrected by adding exogenous haem. It is suggested that haem inhibition and the resulting preferential diminution of alpha chain synthesis could provide a new approach to the treatment of homozygous beta thalassaemia with an excess of detrimental free alpha chain in erythroid cells.

Effects of acetaldehyde on human red cell metabolism: evidence for the formation of enzyme inhibitors

Since red cells transport and metabolize acetaldehyde in vivo, the effects of acetaldehyde on human red cell enzyme activities were studied. Incubation of intact red cells or undiluted red cell lysates at 37 degrees C for 4 h with 1-10 mmol/l acetaldehyde decreased only GOT, GPT and aldolase activities among the 26 enzymes tested. No inhibition occurred at 4 degrees C or when acetaldehyde was incubated with dilute hemolysates. Incubation of lysates with other reducing substrates or with acetate inhibited aldolase but not GOT or GPT. Preincubation of lysates with cyanate or fluoride markedly decreased acetaldehyde-mediated transaminase inhibition but not aldolase inhibition. Addition of pyridoxal phosphate, the vitamin B6 transaminase coenzyme, to GOT and GPT assay mixes did not reverse acetaldehyde-mediated transaminase inhibition. These findings suggest that acetaldehyde-mediated aldolase inhibition results from oxidation of acetaldehyde while transaminase inhibition results from nonoxidative acetaldehyde metabolism. When 100-200 mumol/l acetaldehyde is added to lysates at 2-h intervals and when lysates are incubated with ethanol, alcohol dehydrogenase and an NAD-regenerating system, enzyme inhibition occurs at acetaldehyde levels approaching those seen in vivo. Thus, the role of acetaldehyde-mediated enzyme inhibition in the toxicity of alcohol abuse warrants further study.

The effect of ethanol on polypeptide chain initiation in reticulocyte lysates. Inhibition of recycling of initiation factor eIF-2

Using the reticulocyte cell-free system, we have investigated the mechanism by which ethanol inhibits the initiation of protein synthesis. Ethanol inhibited the formation of 40S-initiation complexes, and this effect correlated well with the inhibition by ethanol of overall peptide-chain initiation. Ethanol was a more potent inhibitor of translation at 37 degrees than at 30 degrees. The inhibition of peptide-chain initiation and 40S-initiation complex formation in reticulocyte lysates under other conditions is associated with increased phosphorylation of the alpha-subunit of protein synthesis initiation factor-2 (eIF-2 alpha) and the inhibition of recycling of this factor. Recycling of eIF-2 is mediated by another protein factor GEF (= guanine nucleotide-exchange factor). The addition of ethanol to reticulocyte lysates led to increased phosphorylation of eIF-2 alpha and to a decrease in the rate of exchange of guanine nucleotides bound to eIF-2. This second finding indicated that recycling of eIF-2 was impaired probably due to decreased availability of GEF. Using purified components it was found that ethanol inhibited the ability of GEF to stimulate eIF-2 and that this inhibition showed a similar temperature dependence to the effect of ethanol on overall protein synthesis. Taken together, these results suggest that ethanol leads to inhibition of peptide-chain initiation both through increased phosphorylation of eIF-2 alpha and by directly inhibiting the productive interaction of eIF-2 and GEF.

Protein synthesis in cell-free reticulocyte lysates on multi-hour incubation

Cell-free protein synthesis in rabbit reticulocyte lysate translation mixtures has been studied during multi-hour incubations. In an impaired lysate obtained from cells stored at 0 degrees C before lysis, and showing a low initial rate of synthesis, translation could be stimulated by a factor of 4 by including RNase inhibitor and additional ATP and GTP. In translation mixtures prepared from normal lysates, protein synthesis could be improved by approximately 50% by the addition of excess GTP. The observed increases in protein synthesis were obtained by improved maintenance of the initial rate of synthesis.

Differential effect of chronic ethanol administration on rates of protein synthesis on free and membrane-bound polysomes in vivo in rat liver during dependence development

Administration of ethanol thrice daily to rats in amounts sufficient to induce a high degree of physical dependence resulted in a 20% decrease in the rate of protein synthesis on liver membrane-bound polysomes in vivo after 3 days of treatment without affecting the rate on free polysomes. The inhibition was attributable to a decrease in the rate of polypeptide elongation as evidence by comparable decreases in nascent chain synthesis and completed protein release without any change in leucine uptake by liver. Chronic ethanol treatment did not affect the quantity or distribution of free and membrane-bound polysomes, the DNA concentration, or the weight of liver. The inhibition of protein synthesis on membrane-bound polysomes cannot, therefore, be readily ascribed to ethanol-induced nutritional deficiencies or to some nonspecific toxic effect of ethanol.

The effect of ethanol on heme and porphyrin synthesis

Despite the inherent difficulties in interpreting in vivo data on the effects of alcohol, it appears quite evident that ethanol itself alters the metabolism of the porphyrins and in so doing directly influences heme and hemoglobin synthesis. The mechanism of ethanol-induced iron overload remains elusive but, through observing the effects of phlebotomies and rechallenge with iron, the relationships of iron and alcohol to porphyria cutanea tarda are being more clearly defined. The control mechanisms of porphyrin synthesis by endogenous factors may be part of a more general red blood cell regulatory apparatus.

Acute ethanol ingestion and haem biosynthesis in healthy subjects

The effects of acute ethanol ingestion on the activities of the enzymes of haem biosynthesis in peripheral blood cells have been monitored in eight healthy subjects. The mitochondrial enzymes delta-aminolaevulinic acid (ALA) synthase, coproporphyrinogen oxidase and ferrochelatase were measured in leucocytes and the cytosolic enzymes ALA dehydratase, porphobilinogen (PBG) deaminase and uroporphyrinogen decarboxylase in erythrocytes. Ingestion of 1 . 316 mol ethanol resulted in increased activity of the rate-controlling enzymes ALA synthase and PBG deaminase and decreased activity of the other four enzymes. There was also increased urinary excretion of coproporphyrin. These observations may be relevant to the biochemical mechanisms involved in the ethanol-related conditions, sideroblastic anaemia, cutaneous hepatic porphyria and hepatic siderosis.

Ethanol inhibition of rabbit reticulocyte haem synthesis at the level of delta-aminolaevulinic acid synthetase

Ethanol inhibition of rabbit reticulocyte synthesis occurs as a result of a decrease in haem synthesis. The present study therefore was undertaken in order to localize the inhibitory site of ethanol on the haem biosynthetic pathway. Ethanol (0.05--0.15 M) inhibition of reticulocyte protein synthesis was prevented by simultaneous incubation with 0.025--1 mM delta-aminolaevulinic acid (ALA). Ethanol inhibited both 14C-glycine and 14C-ALA incorporation into haem. However, the extent of haem formation with 14C-ALA as substrate in the presence of ethanol was still equal to that when 14C-glycine was used. These data suggest that ethanol inhibits the haem synthetic pathway at several loci, but that the decrease in haem synthesis, responsible for the decrease in protein synthesis, is due to the inhibition at the rate-limiting enzyme, delta-aminolaevulinic acid synthetase (ALA-S). To confirm this, ALA-S activity was then directly measured in intact reticulocytes, and it was shown that ethanol indeed inhibited its activity. The inhibition of ALA-S was prevented by 10(-4) M dibutyryl cyclic AMP (db cAMP) or theophylline, agents which elevate intracellular cAMP and which have previously been shown to prevent and reverse ethanol inhibition of haem and protein synthesis. Thus, it appears that cAMP protects against ethanol toxicity by preventing inhibition of ALA-S.

Hematologic and oncologic implications of alcoholism

Hematologic complications of alcoholism are common. Folic acid deficiency can lead to pancytopenia. Iron-deficiency anemia, various hemolytic states, and abnormalities in both function and number of platelets can occur. There is a great deal of interest in the adverse effects of alcohol on pyridoxine (vitamin B6) and folic acid metabolism. Malignant disease often complicates alcoholism and should always be suspected.

Benzene inhibition of in vitro rabbit reticulocyte haem synthesis at delta aminolaevulinic acid synthetase: reversal of benzene toxicity by pyridoxine

Benzene (0.113 M) inhibited haem and protein synthesis in rabbit reticulocytes. This inhibition of haem synthesis was found when L-2-[14C]-glycine was used as the radioactive precursor. However, when 4-[14C]delta-aminolaevulinic acid (ALA) was used, there was no significant inhibition. Since ALA measures the haem synthetic pathway beyond the enzyme delta-aminolaevulinic acid synthetase (ALA synthetase), these results suggest that benzene inhibits haem synthesis at or before ALA synthetase. This was confirmed by demonstrating that 1 mM ALA both protected against and reversed the benzene inhibition of reticulocyte protein synthesis. In addition, 1 mM pyridoxine both protected against and reversed the benzene inhibition of reticulocyte protein synthesis. In addition, ImM pyridoxine both protected against and reversed the benzene inhibition of reticulocyte haem and protein synthesis. These results indicate that benzene (or a metabolite) either competes with pyridoxal phosphate at ALA synthetase or competes with pyridoxine for pyridoxal phosphokinase. These results are discussed in terms of their implications for the possible roles of ALA synthetase and the haemin-controlled repressor in benzene-induced aplastic anaemia.

Ferritin and sideroblastic anaemias: inhibition of protein synthesis by protease contaminants in commercial preparations of ferritin

Commercial preparations of ferritin inhibited reticulocyte-lysate cell-free protein synthesis and disaggregated polyribosomes to monoribosomes and ribosomal subunits. These effects were prevented by addition of reduced glutathione (GSH) to the incubation medium, but ferritin did not lower GSH concentration in the lysates. The more purified the ferritin preparation, the less inhibition of protein synthesis was observed. These data suggested that the effect was due to a contamination of the ferritin with proteolytic activity. In confirmation of this proposal we demonstrated that there was protease activity in both the 2X and 5X crystalized ferritin preparations, with 2.5 times greater activity in the 2X preparation. The proteolytic activity in ferritin was inhibited by incubation with the protease inhibitor tosyl lysine chloromethyl ketone (TLCK). When an amount of trypsin equivalent to the protease activity of the ferritin was added to the incubation mixture, similar effects on protein synthesis and the ribosome-polyribosome component were found. Both GSH and TLCK prevented these effects of trypsin. These data suggest that the previously reported effect of ferritin on reticulocyte cell-free protein synthesis was due to contamination of the ferritin by a protease. It appears that ferritin does not play a direct role in the pathogenesis of sideroblastic anaemias.