Methyl acceptors for protein methylase II from human-erythrocyte membrane

Novel Insights into Mercury Effects on Hemoglobin and Membrane Proteins in Human Erythrocytes

Mercury (Hg) is a global environmental pollutant that affects human and ecosystem health. With the aim of exploring the Hg-induced protein modifications, intact human erythrocytes were exposed to HgCl₂ (1-60 µM) and cytosolic and membrane proteins were analyzed by SDS-PAGE and AU-PAGE. A spectrofluorimetric assay for quantification of Reactive Oxygen Species (ROS) generation was also performed. Hg²⁺ exposure induces alterations in the electrophoretic profile of cytosolic proteins with a significant decrease in the intensity of the hemoglobin monomer, associated with the appearance of a 64 kDa band, identified as a mercurized tetrameric form. This protein decreases with increasing HgCl₂ concentrations and Hg-induced ROS formation. Moreover, it appears resistant to urea denaturation and it is only partially dissociated by exposure to dithiothreitol, likely due to additional protein-Hg interactions involved in aggregate formation. In addition, specific membrane proteins, including band 3 and cytoskeletal proteins 4.1 and 4.2, are affected by Hg²⁺-treatment. The findings reported provide new insights into the Hg-induced possible detrimental effects on erythrocyte physiology, mainly related to alterations in the oxygen binding capacity of hemoglobin as well as decreases in band 3-mediated anion exchange. Finally, modifications of cytoskeletal proteins 4.1 and 4.2 could contribute to the previously reported alteration in cell morphology.

Protein damage and methylation-mediated repair in the erythrocyte

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Mechanism of erythrocyte accumulation of methylation inhibitor S-adenosylhomocysteine in uremia

We have recently demonstrated that methyl esterification of erythrocyte membrane proteins, a reaction involved in recognition and repair of specifically damaged proteins, is impaired in uremia. This is accompanied by a significant increase in intracellular S-adenosylhomocysteine (AdoHcy), a potent inhibitor of methyltransferases. AdoHcy accumulation is normally prevented by its enzymatic hydrolysis to homocysteine (Hcy) and adenosine, a reversible reaction catalyzed by AdoHcy hydrolase. To assess the contribution that Hcy offers in the elevation of AdoHcy, we measured plasma and red blood cell Hcy, AdoHcy, adenosine, and S-adenosylmethionine (AdoMet) intracellular concentrations, as well as RBC AdoHcy hydrolase specific activity, in standard hemodialysis patients and normal subjects. Plasma and red blood cell Hcy levels are significantly higher in the dialysis group, and are positively correlated to AdoHcy levels. Adenosine and AdoMet levels, and AdoHcy hydrolase specific activity are not significantly different between the two groups. The enzymatic formation of labeled AdoHcy from Hcy and tracer adenosine appears to be significantly increased, in vitro, in erythrocytes from both control and uremic patients, when 50 microM Hcy (concentration comparable to plasma levels actually found in vivo in uremic patients) is added to the incubation medium. When erythrocytes from uremic patients are incubated in vitro in absence of Hcy, a significant reduction of intracellular AdoHcy is observed with time compared to identical samples incubated in presence of 50 microM Hcy, with a T1/2 of approximately 270 minutes. The results allow us to conclude that plasma and red cell Hcy levels actually found in uremia can be effectively responsible for the intracellular accumulation of the toxic compound AdoHcy.

Enzymatic methyl esterification of erythrocyte membrane proteins is impaired in chronic renal failure. Evidence for high levels of the natural inhibitor S-adenosylhomocysteine

The enzyme protein carboxyl methyltransferase type II has been recently shown to play a crucial role in the repair of damaged proteins. S-adenosylmethionine (AdoMet) is the methyl donor of the reaction, and its demethylated product, S-adenosylhomocysteine (AdoHcy), is the natural inhibitor of this reaction, as well as of most AdoMet-dependent methylations. We examined erythrocyte membrane protein methyl esterification in chronic renal failure (CRF) patients on conservative treatment or hemodialyzed to detect possible alterations of the methylation pattern, in a condition where a state of disrupted red blood cell function is present. We observed a significant reduction in membrane protein methyl esterification in both groups, compared to control. The decrease was particularly evident for cytoskeletal component ankyrin, which is known to be involved in membrane stability and integrity. Moreover, we observed a severalfold rise in AdoHcy levels, while AdoMet concentration was comparable to that detected in the control, resulting in a lower [AdoMet]/[AdoHcy] ratio (P < 0.001). Our findings show an impairment of this posttranslational modification of proteins, associated with high AdoHcy intracellular concentration in CRF. The data are consistent with the notion that, in CRF, structural damages accumulate in erythrocyte membrane proteins, and are not adequately repaired.

What is the function of protein carboxyl methylation?

The following functions of protein carboxyl methylation seem to be reasonably well established: Multiple, stoichiometric methylation of chemotactic receptors in bacteria at glutamyl residues serves as one (but not the only) adaptation mechanism of the transduction chain to constant background levels of chemotactic stimuli. Stoichiometric methylation of hormones and hormone carrier proteins plays a role in hormone storage and secretion by the pituitary gland. Substoichiometric methylation at D-aspartyl residues is involved in a repair mechanism of aged proteins. Stoichiometric methylation of calmodulin modulates the sensitivity of calmodulin-dependent processes to calcium. Research of the past 3 years has indicated that in order to demonstrate an involvement of methylation in the coupling of surface receptors to intracellular events three new criteria have to be met: (a) the cell should possess a protein carboxyl methylase with relatively narrow substrate specificity; (b) methylation should take place at L-amino acid residues; (c) the methyl accepting proteins should be methylated in a stoichiometric fashion.

Enzymatic basis for the calcium-induced decrease of membrane protein methyl esterification in intact erythrocytes. Evidence for an impairment of S-adenosylmethionine synthesis

The effect of Ca2+ loading, induced by the ionophore A23187, on methyl esterification of membrane proteins (i.e. bands 2.1, 3, 4.1 and 4.5) has been investigated in intact human erythrocytes. When the cells were incubated with L-[methyl-3H]methionine, 40 microM CaCl2 and 10 microM A23187 induce a 50% inhibition of membrane protein methyl esterification. This effect is selectively due to the increased intracellular Ca2+ concentration, as it is antagonized by 10 mM EGTA, and other divalent cations such as Mn2+ do not exert any inhibition. In order to clarify the mechanism(s) of the reported inhibition, the various events involved in the methyl esterification process in vivo were analyzed. L-Methionine uptake as well as protein methylase II activity are not directly affected by altered intracellular Ca2+ concentrations. Conversely in the Ca2+-loaded erythrocytes the conversion of [3H]methionine into [3H]AdoMet, catalyzed by AdoMet synthetase, decreases up to 25%. When the undialyzed erythrocyte cytosolic fraction is assayed in vitro for AdoMet synthetase the activity of the enzyme from the CaCl2/A23187-treated erythrocytes is significantly lower than the control, up to 5 mM ATP. This result suggests that in the Ca2+-loaded erythrocytes the ATP intracellular concentration is significantly lowered. The direct evaluation of ATP intracellular concentration, by HPLC, confirms a significant drop of ATP level, as a consequence of the Ca2+ loading. The removal of Ca2+ from the cells quantitatively restores both the AdoMet synthesis and the methyl esterification levels. The possible role of altered ATP intracellular concentrations as a regulatory factor in the AdoMet-dependent reactions as well as in post-translational protein methylation related to the ageing process is also discussed.

Abnormal membrane protein methylation and merocyanine 540 fluorescence in sickle erythrocyte membranes

Sickle cell erythrocytes exhibit reduced carboxyl methylation of membrane proteins compared to normal erythrocytes. This altered methylation in sickle membrane proteins is also observable when extracted membranes, both intact and alkali treated, were used as substrates for the homologous protein methylase II (S-adenosylmethionine:protein-carboxyl O-methyltransferase, EC. 2.1.1.24). However, when glycophorin A, one of the major methyl acceptors in both membranes, was extracted by lithium diiodosalicylate and used as the methyl acceptor, the proteins from both membranes were methylated equally, suggesting an involvement of membrane structure in membrane-bound protein methylation. Merocyanine 540 (MC-540), a fluorescent probe, was used to determine if the membranes differed in organization. Incubation of both normal and sickle erythrocytes membranes with MC-540 produced a marked increase in extrinsic fluorescence, reflecting a relatively nonpolar environment for the dye bound to the membranes. The fluorescence from sickle cell ghosts was only 87% as intense as that from normal ghosts, while the actual amount of MC-540 associated with sickle cell membranes was only 62% of normal. These data suggest that differences exist in the distribution of surface charges on these plasma membranes. These results are consistent with the hypothesis that abnormal levels of membrane protein methylation observed in sickle erythrocytes may be a result of abnormal membrane organization characteristic to sickle cell anemia.

Differential membrane protein carboxyl-methylation of intact human erythrocytes by exogenous methyl donors

The patterns of membrane protein carboxyl-methylation by protein methylase II (S-adenosylmethionine: protein-carboxyl O-methyltransferase, EC 2.1.1.24) in intact human erythrocytes were shown to differ markedly whether the methyl donor, S-adenosyl-L-[methyl-3H]methionine, was supplied exogenously or formed intracellularly via exogenously added L-[methyl-3H]methionine. The differences include the following. (1) The methylation of cytoskeletal components (band 2.1 and 4.1) occurs only in the case of the L-[methyl-3H]methionine-labelled cells. (2) The methionine-mediated methylation was much less sensitive to S-adenosyl-L-homocysteine inhibition than the adenosylmethionine-mediated methylation (22% versus 95% inhibition at 10 microM). (3) The membrane protein methylation mediated by exogenous adenosylmethionine and methionine differed markedly in their alkali labilities; at pH 6.0, 30% of the adenosylmethionine-mediated protein methyl esters were hydrolysed after 30 min (37 degrees C) while the methionine-mediated esters were stable. At pH 7.4, the respective labilities were 60% and 30% for the 30 min incubation. To explain these results, a possible involvement of cytoskeletal structure associated with the intact erythrocyte is discussed.

Further investigation of methylation on sickle erythrocyte membranes

The methylation of erythrocyte membrane proteins has been investigated with fractionated reversible and irreversible sickle erythrocytes to better understand conflicting results obtained from two laboratories (Green and Kalra (6), Ro et al. (1). When subpopulations of intact erythrocytes obtained by two different separation methods (33% bovine serum albumin and Stractan II gradient centrifugations) were incubated with L-[methyl-3H] methionine at pH 7.2 and 37 degrees C, membranes from both reversible and irreversible sickle erythrocyte populations showed about half the [3H]methyl group incorporation than that observed in normal erythrocytes. In addition, this difference in the level of methylation between normal and sickle cells was maintained during the entire course of a 2-hr incubation utilizing S-adenosyl-L-[methyl-3H]methionine, the immediate in vivo methyl donor.

Endogenous substrates for protein carboxyl methyltransferase in cytosolic fractions of bovine brain

A method of polyacrylamide gel electrophoresis utilizing the discontinuous pH-stacking gel format, the cationic detergent cetylpyridinium chloride, and an acidic buffer system has been applied to detection of specific substrates for protein carboxyl methyltransferase (PCM, EC 2.1.1.24) in cytosol fractions of bovine cerebral cortex. This electrophoresis system produces a high-resolution separation of proteins while preventing spontaneous hydrolysis of protein carboxyl methyl esters. Separation occurs largely on the basis of molecular weight. By running polyacrylamide gels at 4 degrees C or 25 degrees C, it was possible to demonstrate that any specific methyl-accepting protein is modified to form a labile methyl ester rather than the more stable N-derivative. Using this system, we have found that partially purified fractions of PCM contain a variety of endogenous methyl-accepting proteins. The apparent specificity of these substrates varies widely; some apparently abundant proteins show little or no methylation, while other apparently less abundant proteins exhibit a relatively high degree of methylation. One protein, with an apparent Mr of 46,000, exhibited an exceptional degree of methylation. Two distinct classes of protein carboxyl methyl esters could be distinguished by their differing susceptibility to nonenzymatic hydrolysis. The possible relevance of our findings to the recent suggestion that PCM specifically methylates abnormal D-aspartyl residues in age-racemized proteins is considered.

Increased methyl esterification of membrane proteins in aged red-blood cells. Preferential esterification of ankyrin and band-4.1 cytoskeletal proteins

The enzymatic carboxyl methyl esterification of erythrocyte membrane proteins has been investigated in three different age-related fractions of human erythrocytes. When erythrocytes of different mean age, separated by density gradient centrifugation, were incubated under physiological conditions (pH 7.4, 37 degrees C) in the presence of L-[methyl-3H]methionine, the precursor in vivo of the methyl donor S-adenosylmethionine, a fourfold increase in membrane-protein carboxyl methylation was observed in the oldest cells compared with the youngest ones. The identification of methylated species, based on comigration of radioactivity with proteins stained with Coomassie blue, analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, shows, in all cell fractions, a pattern similar to that reported for unfractionated erythrocytes. However in the membrane of the oldest erythrocytes the increase in methylation of the cytoskeletal proteins, bands 2.1 and 4.1, appears to be significantly more marked compared with that observed in the other methylated polypeptides. Furthermore the turnover rate of incorporated [3H]methyl groups in the membrane proteins of the oldest cells markedly increases during cell ageing. Particularly in band 4.1 the age-related increase in methyl esterification is accompanied by a significant reduction of the half-life of methyl esters. The activity of cytoplasmic protein methylase II does not change during cell ageing, while the isolated ghosts from erythrocytes of different age show an age-related increased ability to act as methyl-accepting substrates, when incubated in presence of purified protein methylase II and methyl-labelled S-adenosylmethionine, therefore the relevance of membrane structure in determining membrane protein methylation levels can be postulated. Finally the possible correlation of this posttranslational protein modification with erythrocyte ageing is discussed.

Purification of protein methylase II from human erythrocytes

Protein methylase II (S-adenosylmethionine:protein-carboxyl O-methyltransferase, EC. 2.1.1.24) which methyl esterifies free carboxyl groups of protein substrate using S-adenosyl-L-methionine as the methyl donor, has been purified from human erythrocytes approximately 13000-fold with a yield of 12%. The purified enzyme was over 95% pure as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A bulk of hemoglobin present in the erythrocyte lysate, which severely limited the use of affinity chromatography for purification, was effectively removed by ammonium sulfate precipitation and by the subsequent salt washing of the precipitates followed by molecular sieve chromatography on Sephadex G-75. This preparation can be further purified by affinity chromatography, in which S-adenosyl-L-homocysteine is covalently linked to Sepharose-4B, followed by Sephadex G-75 chromatography to yield an enzyme with an activity of 27000 pmol methyl group transferred/mg/min at 37 degrees C.

Protein carboxymethylation during in vitro culture of human peripheral blood monocytes and pulmonary alveolar macrophages

Protein carboxymethylase (PCM) activity was evaluated for long-term in vitro cultures of human peripheral blood monocytes and pulmonary alveolar macrophages. Both cell types exhibited increases in endogenous (without addition of the exogenous substrate, gelatin) and total (with gelatin) enzyme activity with increased time in culture. Monocytes developed increased activity after a 5-day lag period; three-to four-fold increases over day 1 values in both total and endogenous specific activity occurred. In contrast, PCM activity increased for pulmonary alveolar macrophage (PAM) without a detectable lag period. Although the increase in endogenous activity of 10--14-day PAM culture was similar to comparable age monocyte cultures, total enzyme activity increased only two-fold above day 1 values. The observation of changes in PCM endogenous specific activity in monocyte cultures may reflect alterations in enzyme activity and/or levels of endogenous methyl-acceptor proteins.