Transport of α-aminoisobutyric acid by Streptococcus pyogenes and its derived L-form

Abstract

The authors studied the uptake of α-aminoisobutyric acid (AIB) in S. pyogenes and its physiologically isotonic L-form. S. pyogenes cells starved for glucose or treated with carbonyl cyanide-m-chlorophenyl hydrazone accumulated limited amounts of AIB. A high apparent K(m) value characterized the glucose-independent transport of AIB. The rate and extent of AIB accumulation significantly increased in the presence of glucose. Two saturable transport components with distinct apparent K(m) values characterized glycolysis-coupled transport of AIB. A biphasic Lineweaver-Burk plot was also obtained for L-alanine transport by glycolyzing S. pyogenes cells. AIB seems to share a common transport system(s) with glycine, L- and D-alanine, L-serine, and L-valine. This was shown by the competitive inhibition of AIB uptake by these compounds and their ability to induce competitive exchange efflux of accumulated AIB. About 30% of the AIB uptake was not inhibited by a saturating amount of L-valine, indicating the existence of more than one system for AIB transport. p-Chloromercuribenzoate markedly inhibited the accumulation of AIB by both glycolyzing and glucose-starved cells. In contrast, carbonyl cyanide-m-chlorophenyl hydrazone affected only metabolism-dependent uptake of AIB, which was also sensitive to dinitrophenol, N-ethylmaleimide, iodoacetate, fluoride (NaF), arsenate, and N,N'-dicyclohexycarbodiimide. These results are interpreted according to the chemiosmotic theory of Mitchell, whereby a proton motive force constitutes the driving force for AIB accumulation. AIB was not accumulated by the L-form. However, a temporary accumulation of AIB by a counterflow mechanism and a saturable system with a low apparent affinity were demonstrated for AIB transport by this organism. The authors suggest that a deficiency in the coupling of energy to AIB transport is responsible for the apparent lack of active AIB accumulation by the L-form.