The influence of progressive growth on the specific catalase activity of human diploid cell strains. III. Experiments bearing on the mechanism responsible for change in specific activity

It was shown previously that the specific catalase activity of human diploid cell strains falls immediately after subculture and then progressively rises in an exponential fashion. In this paper evidence is presented suggesting that the rise in catalase activity cannot be due to an accumulation within the cell of a small molecule which enhances enzyme activity in cell‐free extracts. It is also shown that activity per cell, as well as per unit cell protein, rises as the culture grows. The rate of fall of specific catalase activity immediately after subculture is greater if the cells are at a low population density than if they are at a high one. The rate of fall can be made more sharp by increasing the frequency with which the cultures are fed. It is shown that used medium, which has previously been incubated with cultured cells of the same strain, does not significantly change either the rate of fall of specific catalase activity following subculture, or the rate of its subsequent rise. It is postulated, as one possibility, that the cells liberate into the medium an enhancer of cell catalase activity which is highly labile. The steady state concentration of this enhancer in the medium might be expected to increase as the culture grew, but to decrease when the cells are subcultured into fresh medium or when the frequency of feedings is increased.