"Fire burn and cauldron bubble" (W. Shakespeare): what the calorimetric-respirometric (CR) ratio does for our understanding of cells?

In this paper, it has been shown that the existence of anaerobic pathways u nder aerobic: conditions is not always due to poor environmental conditions . There are two sources of lactate in cultured cells, oxidative glutaminoly sis and reductive glycolysis. Only the latter is measured in the CR ratio a s a value more negative than the oxycaloric equivalent for the relevant cat abolic substrate. The validity of the value for the CR ratio can be determi ned by Mayer's enthalpy balance method. Highly exothermic ratios are no ref lection of thermodynamic efficiency but in many cases indicate the need for ATP not supplied by, or insufficiently available from, oxidative phosphory lation. For other types of cell grown in culture, a highly exothermic CR ra tio is due to the fact that there are not sufficient quantities of the appr opriate anabolic precursors in the culture medium. Then, biosynthetic precu rsors must be constructed from substrates using the catabolic pathways and this leads to the reduction of pyruvate in order to conserve NAD(+). An on- line measurement of the CR ratio would monitor cell growth and could be use d to control fed-batch cultures. (C) 2000 Elsevier Science B.V. All rights reserved.