Improved methods for investigating the external redox potential in hybridoma cell culture

Because of the interest in understanding and optimizing secretion of proteins from mammalian cells, reliable and more reproducible methods are needed to monitor the external redox potential of animal cells in suspension culture. An improved off-line method was established that greatly reduces the typically long response time of redox electrodes in cell culture media and improves the standardization of redox probes. In addition, the dependence of medium redox potential on dissolved oxygen concentrations and pH was investigated using cell-free medium. Off-line as well as on-line redox potential measurements were then applied to spinner or bioreactor cultures of murine hybridoma cells. Serum containing or protein-free medium were used. The time dependence of the experimentally determined external redox potential was found to be affected not only by oxygen, pH, and medium composition. but to a significant extent by the rate of generation of reductants by hybridoma cells. The observed specific rate of medium reduction by generation of reductants (ΔmV h(-1) viable cell(-1)) decreased during exponential growth while cell number increased from 2×10(5) viable cells ml(-1) to 3.5×10(6) viable cells ml(-1). This rate, however, was essentially constant at -7.3 mV h(-1)±3.7 mV h(-1) per 10(10) viable cells during growth under conditions of constant dissolved oxygen tension and constant pH. Using these observations, the quantity of reductants synthesized and secreted into the medium by viable hybridoma cells was estimated to be approximately 1.3 mole h(-1) per 10(10) viable hybridoma cells. The time course of specific monoclonal antibody secretion rate did not correlate with changes in the external oxidation/reduction potential in either serum containing or protein-free medium.