1. The synthesis of N-maleoylmethionine sulphone (MMS), a membrane-impermeant protein-labelling reagent, is described. Radioactively labelled MMS can be readily prepared at high specific radioactivity from [35S]methionine. 2. The permeability of the erythrocyte membrane to the reagent was assessed by determining the extent of inactivation of glyceraldehyde 3-phosphate dehydrogenase after treatment of erythrocytes with MMS. Some inactivation of this enzyme was found when high concentrations (20mM) of the compound were used, but this could be prevented by pretreatment of the erythrocytes with 4,4‘-di-isothiocyanatostilbene-2,2’-disulphonic acid, suggesting that MMS slowly enters the cells via the anion-transport system. 3. Treatment of erythrocytes with [35S]MMS resulted in the labelling of six major components. Labelling of erythrocyte membranes resulted in the intense labelling of many additional components. 4. MMS inhibited erythrocyte glucose transport. Cytochalasin b protected glucose transport against inactivation by MMS. Labelling experiments in erythrocytes in the presence and in the absence of cytochalasin b showed that the cytochalasin b-protected material was a broad band in the band-4.5 region.
