Some properties of an SH group essential for choline transport in human erythrocytes.

1. The choline transport system in human erythrocytes is inhibited by N-ethylmaleimide (NEM), cystamine and p-chloromercuribenzene sulphonic acid (PCMBS).2. External choline increases the rate of inhibition by NEM and cystamine but decreases the rate of inhibition by PCMBS. Intracellular choline has the opposite effect.3. Competitive inhibitors of choline transport that are not themselves transported protect the carrier against all three thiol reagents.4. Some thiol reagents with a very low lipid solubility do not inhibit choline transport.5. The transport inhibition by cystamine is reversed by various reducing agents.6. Cystamine protects the transport system against NEM and PCMBS.7. It is suggested that NEM and cystamine react with an SH group of the transport system and that this SH group is more reactive when the carrier is facing inside. PCMBS penetrates erythrocytes only very slowly and is assumed to react preferentially with the outward facing carrier.8. The reactive SH group seems to be located in a lipophilic environment.