Carboxymethylation of membrane proteins of irreversibly sickled erythrocytes

Protein methylase II in five taxa from three phyla

Protein methylase II (protein O-methyltransferase, EC 2.1.1.24) was found in Dictyostelium discoideum amoebae, Astacus leptodactylus axonal, Locusta migratoroides neuronal, Torpedo marmorata electroplaque and Bos bovis stratial tissue and compared in both the soluble and particulate fractions. The intrinsic decay data of the methyl groups transferred onto proteins from Dictyostelium and Torpedo tissues were virtually identical. The short term kinetics of the methyl group transfer of all fractions and of all taxa investigated were non-linear and multiphasic. The particulate fractions displayed transient peaks at 1 min, 3 min, or both after the start of the reaction. The methyl group transfer was stimulated by the neurotoxins veratridine (VTx) and inhibited by veratridine plus tetrodotoxin (TTx) (axonal membrane vesicles of Astacus), stimulated transiently and in a biphasic manner by carbamoylcholine and phospholipase A2 (AChR-rich membrane vesicles of Torpedo), and stimulated transiently and biphasically by the adequate chemotactic stimulus cAMP (aggregation competent amoebae of Dictyostelium).

Stimulation of protein methylase II from Torpedo marmorata by cholinergic effectors

The enzymatic transfer of methyl groups mediated by protein methylase II onto proteins of the electroplaque tissue of Torpedo marmorata is described. The protein methylase II resides to the extent of 80% in the cytosol and 20% in the acetylcholine receptor-rich membrane. The kinetics of the methyl-group transfer are characteristically different in the cytosol and membrane fractions. The reaction is inhibited by phosphate with IC50 = 450 microM. The cholinergic effectors carbamoylcholine, flaxedil and alpha-bungarotoxin applied to the outside of the acetylcholine receptor-rich membrane vesicles stimulated the protein methylase II which is exclusively located inside the vesicles. The stimulation is biphasic and transient, yielding an increased initial velocity and a peak of activity at 2 min after the addition of the effector. The stimulation by carbamoylcholine is qualitatively similar to that elicited by the antagonist. In addition, the protein methylase II is stimulated transiently by phospholipase A2 with a time-course clearly different from that of the cholinergic effectors. We conclude that the conformational change in the receptor-protein elicited by cholinergic effectors is efficiently transduced to the cytoplasmic methylation sites.

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.

Morphology and fatty acid composition of reticulocytes from phenylhydrazine-treated rats

Reticulocytosis was induced in rats by injecting phenylhydrazine, a potent oxidizing agent. Red cell morphology was analyzed by scanning electron microscopy. The majority of red cells from rats given injections of phenylhydrazine were types 2 and 3 echinocytes. Stomatocytes were also observed, but pitted lobular reticulocytes were not detected. Echinocytes have not previously been observed in reticulocyte populations. In the reticulocytes, the relative levels of 16:1 and 18:1 were significantly greater than in erythrocytes. These differences in monoenoic acids may be due to the presence of endoplasmic reticulum, the site of desaturase activity in reticulocytes. Of all the fatty acids, the polyunsaturates are the most susceptible to attack during peroxidation. However, the polyunsaturated fatty acid composition of reticulocytes was similar both to that of erythrocytes and to reported values of young erythrocytes isolated by density. Therefore, it is unlikely that lipid peroxidation caused the formation of echinocytes.