THIOL-GROUP CONTROL OF SODIUM-LITHIUM COUNTERTRANSPORT KINETICS IN UREMIA - EVIDENCE OF A MEMBRANE ABNORMALITY AFFECTED BY HEMODIALYSIS

Uraemia affects erythrocyte metabolism and membrane function but no co nsistent effect on Na/Li countertransport (CT) has been reported. We r eport only small differences in Na/Li CT at 150 mmol/l Na over haemodi alysis, but major differences in other properties of Na/Li CT. The K-m for external sodium and V-max both increased during haemodialysis but the V-max/K-m ratio, which was greater than normal, was not affected. The thiol reagent, N-ethylmaleimide (NEM), which causes a decrease in K-m and V-max in normal subjects, had no effect on K-m in the predial ysis erythrocytes. After haemodialysis, the sensitivity of Na/Li CT to NEM was improved. The changes in Na/Li CT kinetics were not related t o changes in membrane lipid fluidity or plasma lipids. These observati ons suggest that uraemia affects a thiol group that controls Na/Li CT kinetics and that haemodialysis temporarily improves this aspect of me mbrane function.