| Abstract: | We have evaluated four techniques for labelling the surface proteins of cultured mammalian cells. The techniques are: (a) the lactoperoxidase system; (b) the pyridoxal phosphate-[3H]borohydride system; (c) the [3H]4,4'-diisothiocyano-2,2'-dihydrostilbene disulfonate sysem and (d) the galactose oxidase-[3H]borohydride system. The subcellular distribution of radiolabel produced by these technics has been evaluated by autoradiography at the light microscope level and by cellular fractionation. We find that while all four systems label the surface membranes in the majority of the cell population, they also heavily label internal sites in a small subpopulation of nonviable cells. The contribution of the internally labelled cells to further biochemical analysis may represent a severe problem in investigations which rely solely on surface labels for the study of plasma membrane organization. |
