Hexose-specific inhibition in vitro of human red cell Ca²⁺-ATPase activity

In a concentration-dependent manner (5.5-27.5 mmol/l), d-glucose incubated in vitro with human erythrocyte membranes at 37°C for 1 h inhibited membrane Ca²⁺-ATPase activity by up to 75%. The IC₅₀ was 11 mmol/l, l-Glucose was ineffective, as were 3-O-methylglucose, 2-deoxyglucose, sorbitol and myo-inositol. In contrast, d-fructose decreased Ca²⁺-ATPase activity nearly as effectively as d-glucose and mannose and galactose at 11 mmol/l were less than 50% as effective as d-glucose. Tunicamycin (12 pmol/l), but not 10 mmol/l aminoguanidine, progressively antagonized in vitro the d-glucose effect on the enzyme. Erythrocyte membrane Ca²⁺-ATPase activity may be regulated by glycosylation, rather than nonenzymatic glycation.