Further characterization of the erythrocyte membrane protein abnormality in megaloblastic anemia.

Erythrocyte membranes obtained from patients with severe megaloblastic anemia contain spectrin as indicated by double immunodiffusion against anti-spectrin raised in rabbits. These membranes have normal protein patterns determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate after solubilization at 100 degrees C for 3 min in SDS. When incubation is carried out at 37 degrees C for 3 hours, however, the membrane protein patterns become grossly abnormal, lacking spectrin, band 3 and having several diffuse smaller bands. Cross-linking of these membranes with SH oxidizing agents, such as diamide prevented this dissociation phenomenon. The SH group content of megaloblastic membranes, both from severe and mild megaloblastic anemia, was significantly higher than that of control membranes. This was also confirmed by the greater incorporation of 3H-N-ethylmalemide into red cell membranes from mild and severe megaloblastic anemia than controls. Incorporation of this probe was greatest into the band 3 region. These findings indicate that the erythrocyte membrane proteins in megaloblastic anemia have an intrinsic abnormality which renders them more susceptible to degradation by their endogenous proteases.

Membrane proteins synthesized by human reticulocytes and their precursors.

Membrane protein synthesis in human immature erythroid cells was studied by incubating the cells with 35S-methionine in vitro. The radioactive precursor amino acid was incorporated into membrane protein in a linear fashion for approximately 60 min, after which there was only a slight increase in incorporation. Intracellular protein synthesis, in contrast, was linear for periods up to 2 h. Analysis of isolated membranes by polyacrylamide gel electrophoresis in sodium dodecyl sulphate showed that peripheral blood reticulocytes synthesized two proteins in the 4.5 region (MW=50-60000 D) and a third protein coinciding with band 6 (glyceraldehyde-3-phosphate dehydrogenase). Separation of reticulocytes into different age groups on stractan II gradients showed more immature reticulocytes synthesize a wider range of membrane proteins, extending from bands 4.1 to 8. When nucleated red cells were present in the incubations, synthesis of band 3 was also observed. Earlier erythroid precursor present in erythropoietic BFU-E cultures synthesized spectrin in addition to the other membrane proteins. The data indicate that human red cell membrane protein synthesis follows a programmed pattern and that as the erythroid elements mature they lose the capacity to synthesize certain membrane proteins.