Effect of Zinc on Carbonic Anhydrase IX Activity in MDA‐MB‐231 Breast Cancer Cells

Abstract

Carbonic anhydrase IX (CAIX) is a zinc‐dependent, membrane‐bound glycoprotein that catalyzes the reversible hydration of carbon dioxide like other members of the carbonic anhydrase family: CO2 + H2O → H+ + HCO3−. CAIX is also considered a marker for hypoxic tumors (including those in breast cancers), a poor prognosticator for outcome, and associated with tumorigenesis. Based on previous studies with recombinant proteins that suggested that zinc activates CAIX independent of its role in catalysis, we sought to explore the effect of zinc on CAIX activity in a breast cancer model system, the triple negative MDA‐MB‐231 cell line. These cells express only CAIX and not other membrane‐bound CA family members. We directly analyzed CAIX activity in intact cells and membrane ghosts by using 18O exchange between CO2 and water as measured by membrane inlet mass spectrometry (MIMS). Using this system, we were unable to demonstrate any effect of zinc, over a wide range of concentrations, on hypoxia‐induced CAIX activity. Further, supplementing the growth medium with zinc did not alter either CAIX expression or activity in MDA‐MB‐231 cells. Thus, it appears that zinc is only involved in the catalytic process in a biological setting.