Detection of the changing substrate requirements of cultured animal cells by stoichiometric growth equations validated by enthalpy balances.

As part of an overall aim to base the feeding of substrates to cultured animal cells on their actual metabolic needs, we have developed a stoichiometric approach centred on the macronutrients in the medium. Heat flux records the overall metabolic activity and therefore was the sensitive indicator of changing metabolic requirement. Analyses were made of the experimental measurements on two engineered cell lines in batch culture, the 2C11-12 macrophage hybridoma cell capable of the respiratory burst and the CHO320 constitutively producing human interferon-gamma. The crux was to construct simplified stoichiometric equations for the growth reactions to represent metabolic activity as it changed with time. Beforehand, it was essential to select the appropriate components for the equations. The choice was then justified by constructing enthalpy balances in which the ratio of heat flux to enthalpy flux must be close to unity for validation. By combining the stoichiometric approach with heat flow measurements, it was shown both theoretically and experimentally that the set of stoichiometric coefficients constituting a validated growth equation has a one-to-one corresponding relationship to the metabolic activity of the average cell population. Thus, a strategy was established for feeding the cells at any one time with the correct ratio of the major substrates, glucose and glutamine, in response to metabolic requirements that change with time.