SULFHYDRYL-GROUPS ESSENTIAL FOR THE VOLUME-SENSITIVE RELEASE OF TAURINE FROM ASTROCYTES

The volume-sensitive [H-3]taurine efflux from cultured astrocytes was found to be strongly inhibited by the sulfhydryl group-modifying reage nt N-ethylmaleimide (NEM). This maleimide inhibited taurine efflux evo ked by 50% hyposmotic solutions with an IC50 of 54 mu M. The inhibitio n by NEM followed pseudo-first order reaction kinetics. A double log p lot of the pseudo-first order constant against NEM concentration gave a linear relationship with a slope of 1.2. The data are consistent wit h a simple bimolecular reaction mechanism in which one molecule of NEM reacts with one sulfhydryl group per transport unit. The membrane-imp ermeant maleimide derivative 5-eosin maleimide did not affect the volu me-stimulated taurine efflux. The sulfhydryl-modifying mercurial reage nts mersalyl and p-chloro mercuribenzenesulfonate (0.5-1 mM) increased [H-3]taurine efflux under isosmotic conditions and concomitantly decr eased the hyposmolarity-evoked efflux. The results demonstrate an esse ntial requirement for sulfhydryl groups for the volume-sensitive tauri ne efflux.