Reduced erythrocyte membrane protein methylation in sickle cell anemia

The methylation of erythrocyte membrane components in sickle cell anemia has been studied and found to differ considerably from that of normal erythrocytes. When sickle erythrocytes were incubated under physiological conditions (pH = 7.4, 37 degrees C) in the presence of L-[methyl-3H]methionine or S-adenosyl-L-[methyl-3H] methionine, a 50% decrease in the protein-carboxyl methylation was observed compared to the normal erythrocyte. This reduction in degree of methylation was reflected in all of the major methylated protein bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since this methylation is catalyzed by the cytosolic protein methylase II (S-adenosylmethionine:protein-carboxyl O-methyltransferase, EC 2.1.1.24), the in vitro substrate capability of sickle erythrocyte ghosts and pH 11 treated ghosts were tested by incubation with purified protein methylase II and S-adenosyl-L-[methyl-14C]methionine. Both types of sickle cell ghost preparations showed the same 50% decrease in methylation as was seen in the intact cells. Since it was also shown that the protein methylase II and methyl acceptor membrane protein levels in the sickle erythrocytes are the same as the normal control, the data suggest that the observed reduced methylation may be due to an altered membrane conformation. The methylation of phospholipid was also studied and found to be decreased in sickle cell erythrocytes. However, this methylation was extremely minor in comparison to protein-carboxyl methylation which represents the bulk of the membrane methylation.