Na,K-ATPase α Subunit-like Immunoreactive Proteins in Human Body Fluids

Na,K-ATPase was recently considered a target for breast cancer treatment since its potent inhibitors ouabain and related digitalis compounds that were reported circulating in body fluids before, were found to possess potent anti-breast cancer activity. Based on this finding, it is hypothesized that Na,K-ATPase derivatives could be circulating in the body and potential candidate biomarkers for early disease detection. Western blotting analysis was employed to investigate the possible presence of Na,K-ATPase-like proteins in cerebrospinal fluid (CSF) and plasma using previously prepared antisera against Na,K-ATPase α subunit and its formic acid-derived fragments. Results from this analysis show that human Na,K-ATPase α subunit-like immunoreactive proteins with apparent molecular weights of 97, 68, 66, 57 and 55 kDa in the CSF, as well as 68, 66, 57 and 55 kDa in the plasma have been identified. By combining immunoaffininty binding and electroelution, these immunoreactive proteins were purified from human body fluids. This is the first report of the presence in the body fluids of immunoreactive proteins cross-reacted with anti-human Na,K-ATPase catalytic α subunit antibodies. These results suggest that these proteins are probably Na,K-ATPase derivatives resulting from degradation and released in vivo during cellular metabolism.