Studies on the regulation of the three enzymes of the Leloir pathway in cultured mammalian cells. II. A search for quantitative interrelationships between the three enzyme activities.

Studies on a normal human diploid cell strain revealed that the specific activity of the cell protein, for each of the three enzymes of the Leloir pathway, changed significantly as the cells grew. The kinetics of change in specific activity varied according to the enzyme being studied, and the kinetics for each enzyme varied from experiment to experiment. Within each experiment, there was no consistent correlation between specific activity for any one enzyme and specific activity for the other two. The ratios between the specific activities did not tend to remain constant as the absolute levels of specific activity changed. Hence, the activities did not behave coordinately. The kinetics of change in these ratios varied from experiment to experiment. The failure of galactose to stimulate increased cellular activity for the three enzymes (shown in the preceding paper), and the absence of a coordinate relationship between the activities, represent a striking difference between the behavior of these enzymes in human diploid cell strains and their behavior in E. coli.