The heterotrophic growth of Thiobacillus acidophilus was inhibited by branched-chain amino acids; valine, isoleucine, and leucine. The inhibition by valine and leucine were partially reversed by isoleucine, and the inhibition by isoleucine was partially reversed by valine. Inhibitions by methionine or threonine were partially reversed when both amino acids were present in the growth medium. Inhibition by tyrosine was increased by phenylalanine or tryptophan. Cystine completely inhibited growth. Other amino acids tested produced little or no inhibition.Acetohydroxy acid synthetase (AHAS) activity was demonstrated in crude extracts of T. acidophilus. In crude extracts the optimum pH was 8.5 with a shift to 9.0 in the presence of valine. Valine was the only branched-chain amino acid which inhibited the AHAS activity. The presence of only one peak of AHAS activity upon centrifugation in linear glycerol density gradients demonstrated that the AHAS activity sediments as one component.
