MODIFICATION OF ERYTHROCYTE NA+ LI+ COUNTERTRANSPORT KINETICS BY 2 TYPES OF THIOL-GROUP/

Erythrocyte Na+/Li+ countertransport activity is decreased by reagents that react with thiol groups. An understanding of the role of these g roups in control of Na+/Li+ countertransport may help to explain its a ssociation with disease states. The effect of thiol reactive agents on the kinetic parameters of Na+/Li+ countertransport has not previously been described. In choline medium, N-ethylmaleimide (NEM) and iodoace tamide (IAamide) cause a rapid decrease of about 40% in K-m for extern al sodium (K-m(So)) that is complete in 10 s with a much smaller chang e in V-max and an increase in the V-max/K-m ratio. In Na medium, NEM a nd IAamide both cause a rapid decrease in K-m(So) and V-max. With NEM the partial reduction in V-max is complete in 100 s although the NEM i s sufficient to reduce V-max up to 15 min. With IAamide the decrease i n V-max is initially slower but it continues apparently towards comple te inhibition. These results indicate at least two types of thiol grou p controlling Na+/Li+ countertransport kinetics. The type 1 thiol reac tion is Na independent and causes an increase in the apparent rate con stant for Na association with the unloaded carrier so that V-max/K-m r ises and K-m(So) decreases. The type 2 thiol reaction is facilitated b y Na at the outside ion-binding site and causes a decrease in V-max, p ossibly by total blockage of carriers with IAamide but by a different mechanism with NEM such as reduced turnover rate.