Irreversible Inhibition of [3H]Glycine Transport into Channel Catfish Erythrocytes by Thiol Group Modifiers

Abstract

Erythrocytes can take up amino acids from the blood by using a variety of transport systems. GLYT is a key transport protein in the plasma membrane responsible for the Na²⁺-dependent uptake of glycine needed for glutathione biosynthesis. Certain cysteine-specific compounds, particularly mercuric chloride and 5,5′-dithiobis(2-nitrobenzoate), irreversibly inhibited the [³H]glycine transport via GLYT by red blood cells isolated from channel catfish. Bimolecular rate constants (k₂) of 0.556 (mmol/l)^–1 min^–1 and 0.032 (mmol/l)^–1 min^–1, respectively, were calculated for the two inhibitors. Addition of 2-mercaptoethanol 1 min after the initiation of inactivation by mercuric chloride stopped further inactivation, but did not reverse the inhibition. The presence of glycine, but not Na⁺ ions, during the preincubation of the cells with each inhibitor markedly reduced the degree of inhibition. Thus cysteinyl residues within the transport protein appear to be vital for the binding and uptake of glycine by channel catfish erythrocytes.