The use of 6-labeled glucose to assess futile cycling in Escherichia coli.

To assess the "futile cycle" fructose-6-P leads to fructose-1,6-P2 leads to fructose-6-P in Escherichia coli we have grown the cells on [6-14C]glucose and determined label in the 1-position of glucose obtained from glycogen. In a variety of strains, including a wild type and a mutant without fructose diphosphatase, 1-position labeling was negligible. But there was little label in the 1-position of fructose-1,6-P2 either, which shows that hexose diphosphate and triose-P are not in equilibrium in this organism. Therefore, the lack of 1-position labeling in glycogen does not necessarily indicate lack of futile cycling. One strain, however, a temperature-sensitive glyceraldehyde-3-P dehydrogenase mutant grown at permissive temperature, gave substantial labeling of the 1-position of fructose-1,6-P2. In this strain 1-position labeling in glycogen was low, indicating minimal futile cycling.