Purified brush borders, prepared fro newborn pig intestine, were incubated in the presence of 203Hg-labelled p-chloromercuribenzenesulphonic acid and the membrane proteins later separated by polyacrylamide-gel electrophoresis. The presence of either D-glucose or phlorrhizin, during a preliminary incubation in non-radioactive p-chloromercuribenzenesulphonic acid, increased the subsequent binding of the 203Hg-labelled compound to a protein of molecular weight 31500. This increase appeared to be specific for the low-molecular-weight protein, provided that the concentration of protecting agent used corresponded to that used to produce a biological response in the intact tissue. These results are discussed in relation to the known properties of other presumptive sugar carriers isolated from different membranes.
