THIOL-GROUP MODULATION OF SODIUM-LITHIUM COUNTERTRANSPORT KINETICS INDIABETIC NEPHROPATHY

Abnormal erythrocyte sodium-lithium countertransport (Na-Li CT) activi ty, traditionally measured at a single sodium concentration of 140 mmo l . l(-1) (V-140), may represent an inherited risk marker for diabetic nephropathy. The membrane defect underlying this association is poorl y understood, though modulation by key protein thiol groups appears to be important in essential hypertension. To improve understanding of t his abnormality, Na-Li CT kinetics in untreated erythrocytes and after thiol group alkylation with N-ethylmaleimide were investigated in 18 subjects with diabetic nephropathy, 20 normoalbuminuric insulin-depend ent diabetic (IDDM) subjects and 18 non-diabetic individuals. Using th e traditional assay, V-140 was similar in subjects with diabetic nephr opathy compared to IDDM control subjects vs 0.311 (0.152-0.475) (0.247 (0.111-0.498) mmol Li . h(-1) . l erythrocytes(-1)). Kinetic paramete rs were abnormal in subjects with diabetic nephropathy compared with d iabetic and non-diabetic control subjects, with both V-max (maximal Na -Li CT activity) (0.454 (0.257-0.963) vs 0.338 (0.183-0.972) vs 0.332 (0.213-0.603) mmol Li . h(-1) . 1 erythrocytes(-1), p < 0.05), and V-m ax/K-m(So) ratio, reflecting ion association (6.03 (2.3-9.6) vs 4.73 ( 2.0-10.4) vs 4.48 (1.5-7.1), p < 0.01), significantly higher. N-ethylm aleimide decreased K-m(So) and V-max abolishing differences in V-max/K -m(So) ratio between groups (2.45 (1.18-4.21) vs 2.23 (0.96-4.3) vs 2. 44 (1.4-3.7), but enhancing the differences in V-max (0.186 (0.090-0.3 15) vs 0.120 (0.051-0.256) vs 0.128 (0.080-0.206) mmol Li . h(-1) . l erythrocytes(-1), p < 0.0001). Of subjects with diabetic nephropathy, 78% were outside the 75th percentile of the non-diabetic control subje cts when V-max and V-max/K-m(So) ratio were combined, compared to 20 % of the normoalbuminuric control subjects. We conclude that the tradit ional assay, V-140, is poor at detecting individuals with diabetic nep hropathy. Study of the kinetic parameters of the transporter, includin g thiol group modulation, suggests that increased ion association, V-m ax/K-m(So) ratio may represent the inherited defect and improves ident ification of subjects with diabetic nephropathy.