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The value of the adenylate energy charge, [(adenosine triphosphate) + 1/2 (adenosine diphosphate)]/[(adenosine triphosphate) + (adenosine diphosphate) + (adenosine monophosphate)], in Escherichia coli cells during growth is about 0.8. During the stationary phase after cessation of growth, or during starvation in carbon-limited cultures, the energy charge declines slowly to a value of about 0.5, and then falls more rapidly. During the slow decline in energy charge, all the cells are capable of forming colonies, but a rapid fall in viability coincides with the steep drop in energy charge. These results suggest that growth can occur only at energy charge values above about 0.8, that viability is maintained at values between 0.8 and 0.5, and that cells die at values below 0.5. Tabulation of adenylate concentrations previously reported for various organisms and tissues supports the prediction, based on enzyme kinetic observations in vitro, that the energy charge is stabilized near 0.85 in intact metabolizing cells of a wide variety of types.

Modulation of enzyme behavior by variation in the concentration of specific metabolites has not been previously reviewed in this series. For the first review of a ten-year-old field, the admonition of the editors to attempt an appraisal of the present status of the field rather than an annotated bibliography seems particularly apt. This article is limited, with few ex ceptions, to discussion of in vitro systems, where the effect of a metabolite modifier on a specified enzyme can be demonstrated directly. Two essentially separate, although functionally related, subjects are reviewed: the kinetics of regulatory enzymes, with models and concepts that have been proposed, and the possible metabolic significance to be attributed to the properties of such enzymes.Parts of the area covered here have been discussed, usually along with enzyme repression, in several reviews or summary articles (1-6). Enzyme activity in an intact cell must obviously be affected by many factors besides metabolite modulation. In vivo effects of spatial, permeability, and transport relationships, and of changes in such parameters as pH, temperature, ionic strength, and the concentrations of individual ions have been reviewed by Grisolia (7).

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