Aqueous two-phase partition and detergent precipitation of a drug-responsive NADH oxidase from the HeLa cell surface

The partitioning behaviour of a drug (capsaicin)-responsive NADH oxidase (tNOX) activity released from HeLa ceIls by low pH treatment followed by heat and proteinase K was determined. When partitioned in a standard 6.4% PEG 3350/6.4% dextran T-500 two-phase system, the bulk of the tNOX activity was in the dextran-rich lower phase. The activity was inhibited by and bound to the triazine dye, Cibacron blue. Affinity partition, where the Cibacron blue was coupled to amino PEG 5000 and added to the first two-phase separation step, resulted in the partitioning of activity to the upper PEG phase. A second partition with PEG-salts resulted in the release of the tNOX from the Cibacron blue–amino PEG enriched phase into the salt-enriched lower phase. The phase-purified protein exhibited anomalous behavior and tended to multimerize in sodium dodecyl sulphate (SDS) prior to SDS-polyacrylamide gel electrophoresis (PAGE). Multimerization appeared to be enhanced by PEG. The multimerization was enhanced with the reduced protein in the presence of detergent prior to SDS–PAGE. In addition, the activity was precipitated by PEG 8000 at concentrations between 6 and 30% by weight. In the presence of or after exposure to PEG 3350 or PEG 8000, the protein could not be detected by Western blot analysis after SDS–PAGE suggesting that the protein failed to enter the gel even though other HeLa cell surface proteins were unaffected. The anomalous multimerization behavior has thus far precluded the use of phase partition as a practical purification step for the oxidase.