Observations on the β-galactosidase activity ofBact.lactis aerogenes(Aerobacter aerogenes)

Abstract

When fully adapted cells ofBact.lactis aerogenes(Aerobacter aerogenes) are grown in an aerated medium containing lactose as sole carbon source, the activity of β-galactosidase per unit mass is constant during the greater part of the logarithmic phase, but rises rapidly towards the end, reaching a maximum soon after the onset of the stationary phase. Subsequently the activity as determined with intact cells falls slowly, while that determined with cells whose permeability has been increased by shaking with benzene remains almost constant for some hours. On re-inoculation of the cells into a new medium, the activity falls rapidly to the steady value characteristic of the logarithmic phase. The pH/activity curve of the enzyme shows that the changes are not associated with a compensation effect by which decreasing specific activity at the lower pH late in the growth cycle is balanced by an increase in the amount of enzyme formed. The rise, moreover, occurs in cultures in which the pH is kept constant. Measurements on cultures grown (a) in a stream of nitrogen, where the activity is higher throughout; and (b) a stream of oxygen, where the rise in activity does not occur at all, strongly suggest that oxygen starvation is a major cause of the rise. More lactose is metabolized in compensation for its less efficient utilization during the last phase where growth has become partially anaerobic. The enzyme activity per cell varies in a somewhat different way from the activity per unit mass, but there is little evidence to suggest that the activity per cell is a more fundamental quantity than the activity per unit mass.