B. As with other substrates, ASC-PMS markedly stimulates uptake of VG. However, as opposed to normal substrates, virtually all of the radiolabeled VG taken up is covalently bound to the membrane and is not released by unlabeled VG or by boiling. Furthermore, the rate of covalent labeling in the presence of ASC-PMS is markedly inhibited by DNP, a protonophore that dissipates the driving force for transport.