Occurrence and Some Properties of Two Threonine Dehydratases in Euglena gracilis

SUMMARY: Two threonine dehydratases, I and II, were found to occur in Euglena gracilis and each enzyme was purified partially. Like the biosynthetic threonine dehydratases of other organisms, dehydratase I was feedback-inhibited by isoleucine and not affected by adenylates. Dehydratase II was not influenced by branched-chain amino acids and adenylates. Occurrence of dehydratase II, with a much higher activity than dehydratase I, suggests that dehydration of threonine is not the key point in the regulation of isoleucine biosynthesis in E. gracilis.