Hypotonic medium xidase 一7 Figure 7-24 A Method For Labeling Proteins Exposed on One or oth Surfaces of a Membrane Vesicle. (a)When lactoperoxidase (LP) and 125) are present in the solution outside a membrane vesicle, LP catalyzes the labeling of membrane proteins exposed on the outer membrane surface (i.e. proteins A and B). If membrane vesicles are (b) first incubated in a hypotonic medium to make them permeable to LP and (c) then transferred to an isotonic solution containing 12I but no external LP proteins exposed on the inner membrane surface(i.e proteins B and C)become labeled