Identification and partial purification of the erythrocyte L-lactate transporter

1. Intact erythrocytes were incubated with 100 microM-4,4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS), a concentration sufficient to inhibit lactate transport irreversibly by 65%. DIDS-labelled proteins were detected by immunoblotting of erythrocyte membrane proteins with an anti-DIDS antibody. Labelled polypeptides of 35-45 kDa in rat erythrocytes, and of 40-50 kDa in rabbit and guinea pig erythrocytes, were detected by this technique. In human erythrocytes, which have 10-fold less transport activity, no labelled polypeptide in this molecular mass range was detected. 2. Labelling of these 35-50 kDa polypeptides was decreased markedly in the presence of the specific inhibitors of lactate transport alpha-cyano-4-hydroxycinnamate and 4,4'-dibenzamidostilbene-2,2'-disulphonate (DBDS), which compete with DIDS for binding to the transporter. However, the weakly bound inhibitor 4,4'-dinitrostilbene-2,2'-disulphonate (DNDS) afforded little protection against labelling by DIDS. 3. The lactate transporter from rat erythrocytes was solubilized with decanoyl-N-methyl glucamide (MEGA-10) and partially purified by Mono-Q anion-exchange chromatography, with transport activity eluting at 0.1-0.15 M-NaCl. The 35-45 kDa DIDS-labelled polypeptide from rat erythrocytes was eluted in the same peak of protein as lactate transporter activity during Mono-Q chromatography. 4. These observations provide strong evidence that the lactate transporter is a polypeptide of 35-45 kDa in rat erythrocytes and of 40-50 kDa in rabbit and guinea pig erythrocytes.