ERYTHROCYTE-MEMBRANE THIOL PROTEINS ASSOCIATED WITH CHANGES IN THE KINETICS OF NA LI COUNTERTRANSPORT - A POSSIBLE MOLECULAR EXPLANATION OFCHANGES IN DISEASE/

Background Abnormal erythrocyte Na/Li countertransport is associated w ith diseases such as essential hypertension and diabetic renal disease . Although it seems unlikely that Na/Li countertransport contributes t o any disease process, it may be abnormal because of a change in the c ell membrane that is part of the disease process. Methods We have show n that Na/Li countertransport kinetics are modified by two types of th iol group. One of these, which we have called 'type 1', is rapidly alk ylated by N-ethylmaleimide to give a kinetic pattern similar to that i n the above diseases. Results At pH 6 and 2 degrees C, both N-ethylmal eimide and iodoacetamide cause the K-m of Na/Li countertransport to de crease to completion in 300 s, with 78% (SEM 6%) of the decrease occur ring in 30 s. Using these reaction conditions, N-ethylmaleimide reacte d with a unique thiol group on a 33-kD protein, blocking its subsequen t reaction with biotin maleimide. This 33-kD protein was present in ra bbit erythrocytes, which have high levels of Na/Li countertransport, b ut absent from rat erythrocytes, which have no Na/Li countertransport. Iodoacetyl biotin labelled a 60-kD protein that was specifically bloc ked by iodoacetamide. Conclusion We suggest that these proteins are me mbers of a cluster of membrane proteins that can modify Na/Li countert ransport and may have a functional role in the disease processes.