Organic osmolytes betaine, sorbitol and inositol are potent inhibitors of erythrocyte membrane ATPases

Organic osmolytes are used in animal and plant cells to adapt to hyper- and hypoosmolar stress. We used our RBC-membrane model to investigate the effects of the osmolytes betaine, sorbitol and myo-inositol on Na +/K +-ATPase, Ca 2+-ATPase and calmodulin-stimulated Ca 2+-ATPase (CaM). Our results show that betaine inhibited ATPases by more than 61%: Na +/K +-ATPase (75 ± 5.9 vs 27 ± 2.2), Ca 2+-ATPase (236 ± 18.9 vs 62 ± 4.9), and CaM (450 ± 18 vs 174 ± 6.9) (μM pi/min/mg protein, control (0 μM betaine) vs 100 μmol/L betaine). Sorbitol (100 μmol/L) inhibited the Ca 2+-ATPases by 41% (126 ± 7.6 vs 74 ± 4.4) and CaM by 42% (253 ± 17.7 vs 147 ± 10.3). Inositol (100 μmol/L) inhibited Na +/K +-ATPase strongest (37 ± 1.9 vs 20 ± 1.0; 47% inhibition) while it showed a lesser effect on the Ca 2+-ATPases (136 ± 6.8 vs 102 ± 5.1; 25% inhibition). All osmolytes inhibited RBC membrane ATPases at concentrations above 50 μmol/L, which corresponds to high normal physiologic range for organic osmolytes in serum. Furthermore, the presence of osmolytes (250 μmol/L) decreased hypoosmotic stress induced hemolysis by 42%. Together these data indicate an important regulatory role of organic osmolytes on human RBC membrane ATPases and a protective function of osmolytes in RBCs against hypoosmotic stress.