Further Characterization of the Human Adrenal‐derived P‐GIycoprotein Recognized by Monoclonal Antibody MRK 16 Reacting with Only Human P‐Glycoprotein

This paper describes further characterization of the 170–180‐kDa glycoprotein (P‐glycoprotein) recognized by the monoclonal antibody MRK 16 in the human adrenal. By electron microscopy, P‐glycoprotein was observed in the adrenal cell membranes. However, MRK 16‐defined P‐glycoprotein was not found in cow, pig, horse, monkey or rabbit adrenal, indicating that MRK 16 recognizes the non‐homologous part of P‐glycoprotein of various species. Eleven out of 16 adrenal tumors including 4 cases of primary aldosteronism and 7 cases of Cushing syndrome were intensely stained with MRK 16, whereas pheochromocytoma, non‐functioning adrenocortical adenoma with no associated increase of serum adrenal‐derived hormones and myolipoma of the adrenal were not. Finally, P‐glycoprotein‐MRK 16‐protein A‐Sepharose complex derived from human adrenal possessed marked ATPase activity. Taken together, these data suggest that P‐glycoprotein may play a physiological role in the human adrenal.