Kinetic studies of l-valine and glycyl- l-valine uptake by Leuconostoc mesenteroides

Abstract

When nongrowing but actively respiring cell suspensions of Leuconostoc mesenteroides were incubated under appropriate conditions with l-valine-1-C 14, rapid uptake of the labeled amino acid occurred. This accumulation of valine-1-C 14 was much greater from glycyl- l-valine-1-C 14 than from the free amino acid form. By following the kinetics of valine uptake from the two sources, l-valine and glycyl- l-valine, specific uptake systems were found, and these systems followed Michaelis-Menten kinetics for enzymes. The apparent association constant, K̄, for valine uptake from glycyl- l-valine was 3.26 × 10 3 and was significantly greater than the K̄ (1.60 × 10 3) observed for l-valine. Subsequent kinetic studies with combinations of labeled and unlabeled amino acids and peptides showed that the system which accumulated valine from glycyl- l-valine or other l-valine dipeptides was independent of the one operative with the free amino acid. These uptake systems were energy dependent as shown by the need for glucose as an energy source and by the effect of 2,4-dinitrophenol, which inhibited the uptake of the valine. Previously unrecognized was the stimulatory effect of unlabeled glycyl- l-valine and other l-valine dipeptides on the uptake of l-valine-1-C 14. In the kinetic studies, K̄ for the uptake of l-valine-1-C 14 was increased from 1.60 × 10 3 to 3.25 × 10 3 by the presence of unlabeled glycyl- l-valine. d-Valine peptides did not cause the same effect nor did other peptides which did not contain valine. Protein synthesis during the uptake studies did not appear to be necessary or associated with the uptake processes because chloramphenicol did not affect the uptake of valine or the stimulatory effect of the l-valine dipeptides.