Reteive,d forpublication 26 October. 1977 Addition of 0.1% casein hydrolysate to a iniiiimal growth medium decreased membrane-bound transhydrogenase activity in Escherichia coli by about 80%. Of the amino acids added individually to the growth medium, only leucine and, toa lesser extent, methionine and alanine were effective. a-Ketoisocaproateand leucine-containing'peptides repressed the activity, and leucine also repressed activity in adenyl cyclase-deficient and relaxed strains. Derepression of transhy