The activity of glutaminase per milligram of protein increased threefold after cultured
human diploid fibroblasts were subcultured in fresh medium. The maximum activity was reached
after 2 days of growth and decreased once the cells reached confluency. The increase of
glutaminase activity was independent of the glutamine concentration between 0.2 and 2.0 mmol/1.
In contrast, the specific activity of glutamate dehydrogenase was independent both of the
glutamine concentration and the growth phase of the cultured cells. These results indicate that
glutaminase, the first enzyme involved in the utilization of glutamine as an energy source, is
elevated in rapidly dividing human diploid fibroblasts.
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