The Role of Dipeptidases in Cells of Escherichia coli Strain K-12

Abstract

Abstract To obtain further information about the role of bacterial peptidases in vivo, the enzymic activity of Escherichia coli strain K-12 was studied. The substrates were four glycine-containing dipeptides in which the other amino acid was either l-leucine or l-phenylalanine, and the bacteria were harvested from cultures that varied in age from 24 hours (young cultures) to 144 hours (fully aged cultures). The specific enzymic activity of broken cell preparations toward a given substrate remains constant regardless of the age of the bacterial culture, but the relative activity (measured under the same conditions) increases in the following order: glycylleucine, glycylphenylalanine, leucylglycine, phenylalanylglycine. On the other hand, intact cells from young cultures show equal specific activity for the four substrates. This activity decreases in parallel during aging of the cultures, reaching a minimal level in about 72 hours when the pH of the culture is allowed to fall below about 6.0; at pH values nearer neutrality, the decrease in activity is less complete and occurs less rapidly. The peptidase activity of whole cell preparations is comparable to that of broken cell preparations only with glycylleucine as substrate; even with this substrate full enzymic activity is seen only with intact bacteria from young cultures. The enzymic activity of intact bacteria toward the other substrates represents only a fraction of the activity observed in broken cell preparations. The significance of these results is discussed, and it is concluded that the low specific activity of resting cells is not explicable in terms of the differential permeability of bacteria to the dipeptides.