The calorimetric-respirometric ratio is an on-line marker of enthalpy efficiency of yeast cells growing on a non-fermentable carbon source

Although on-line calorimetry has been widely used to detect transitions in global metabolic activity during the growth of microorganisms, the relation ships between oxygen consumption flux and heat production are poorly docume nted. In this work, we developed a respirometric and calorimetric approach to determine the enthalpy efficiency of respiration-linked energy transform ation of isolated yeast mitochondria and yeast cells under growing and rest ing conditions. On isolated mitochondria, the analysis of different phospho rylating and non-phosphorylating steady states clearly showed that the simu ltaneous measurements of heat production and oxygen consumption rates can l ead to the determination of both the enthalpy efficiency and the ATP/O yiel d of oxidative phosphorylation. However, these determinations were made pos sible only when the net enthalpy change associated with the phosphorylating system was different from zero. On whole yeast cells, it is shown that the simultaneous steady state measurements of the heat production and oxygen c onsumption rates allow the enthalpy growth efficiency (i.e, the amount of e nergy conserved as biomass compared to the energy utilised for complete cat abolism plus anabolism) to be assessed. This method is based on the compari son between the calorimetric-respirometric ratio (CR ratio) determined unde r growth versus resting conditions during a purely aerobic metabolism. Ther efore, in contrast to the enthalpy balance approach, this method does not r ely on the exhaustive and tedious determinations of the metabolites and ele mental composition of biomass. Thus, experiments can be performed in the pr esence of non-limiting amounts of carbon substrate, an approach which has b een successfully applied to slow growing cells such as yeast cells expressi ng wild-type or a mutant rat uncoupling protein-1. (C) 2001 Elsevier Scienc e B.V. All rights reserved.