Enzyme activities at the surface of intact Ehrlich tumor cells with albumin in the isotonic assay medium.
Glyceraldehyde 3-phosphate dehydrogenase and 3-phosphoglycerate kinase are, together with some other enzymes, present on the surface of intact Ehrlich tumor cells. Aldolase, on the contrary, represents cytoplasmic enzymes not present at all on the external surface, provided 2.5 percent of bovine albumin is included in the isotonic assay medium. A flux of aldolase from the cell interior to the cell exterior could be demonstrated in the absence of albumin. Therefore, any enzymatic activity monitored when keeping the Ehrlich tumor cells in the isotonic assay medium containing 2.5 percent albumin was considered to be primarily related to the outside of the plasma membrane. Of the total glyceraldehyde 3-phosphate dehydrogenase, 0.7 percent was located on the outer surface of the tumor cell, while the corresponding figure for 3-phospoglycerate kinase was 2.7 percent. Eighty percent of this surface-located 3-phosphoglycerate kinase was released into the assay medium during incubation, while the release of glyceraldehyde 3-phosphate dehydrogenase, at the same time, was minimal. A plasma membrane preparation of Ehrlich cells, mainly consisting of vesicles, showed the presence of 3-phosphoglycerate kinase but the absence of glyceraldehyde 3-phosphate dehydrogenase. Because of the vesicular nature of the membrane preparation, it was assumed that only one side of the membrane was exposed during assay. The specific binding properties of the two enzymes to the plasma membrane, as well as possible differences in their intramembranous location, are discussed.