In yeast cells genetically modified to express creatine kinase, the calculated free ADP/ATP ratio using 31P NMR was significantly higher than that measured directly in cell extracts.
NMR calculations may overestimate free ADP/ATP ratios in vivo; challenges assumptions and leaves open accurate energetics assessment.
Rabbit muscle creatine kinase has been introduced into the yeast Saccharomyces cerevisiae by transforming cells with a multicopy plasmid containing the coding sequence for the enzyme under the control of the yeast phosphoglycerate kinase promoter. The transformed cells showed creatine kinase activities similar to those found in mammalian heart muscle. 31P NMR measurements of the near-equilibrium concentrations of phosphocreatine and cellular pH together with measurements of the total extractable concentrations of phosphocreatine and creatine allowed calculation of the free ADP/ATP ratio in the cell. The calculated ratio of approximately 2 was considerably higher than the ratio of between 0.06 and 0.1 measured directly in cell extracts.
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Brindle et al. (1990) studied this question.
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