Activation of bovine and chicken liver dihydrofolate reductases and its relationship to a specific cysteine residue in their NH2-terminal amino acid sequences.

The enzymatic activity of bovine liver dihydrofolate reductase is activated approximately 1.5- to 2.5-fold on treatment with organic mercurials. In contrast to the almost instantaneous reaction and high degree of activation (approximately 10-fold) observed with chicken liver dihydrofolate reductase, the beef liver enzyme requires relatively specific conditions of pH, temperature, preincubation times, and presence of substrate to exhibit this degree of activation. It is also demonstrated that both chicken liver and beef liver dihydrofolate reductases (and perhaps all of the animal dihydrofolate reductases) contain a single sulfhydryl group within the 11 or so amino acids of the NH2-terminal sequence and that this is the site of mercurial interaction and activation. On the other hand, this sulfhydryl group and the characteristic activation does not occur in the corresponding bacterial reductases. It is suggested that although the reactive sulfhydryl group may be characteristic of animal dihydrofolate reductases, it is not directly required for substrate binding or the catalytic mechanism per se.