The presence of a specific and reversible inhibitor of δ-aminolevulinate synthetase in extracts of Rhodopseudomonas spheroides was evidenced. The inhibitor was very labile and lost its inhibitory activity fairly rapidly when the extracts were stored at 0 °, and because of this decrease of active inhibitor, the apparent activity of ALA synthetase in extracts increased by as much as 7-9 times, while the activities of δ-amino-levulinate dehydratase and succinyl-CoA synthetase did not appreciably change during the storage. The inhibitor was also sensitive to light. The inhibitor could be removed from extracts by simply passing the extracts through a column of Dowex-1 and by this way the activity of ALA synthetase in extracts was also increased by about 9 times. The inhibitor could be removed also by dialysis, ultrafiltration and gel filtration with Sephadex G-15 or G-25. Furthermore, fractions having the inhibitor activity could be prepared by fractionating fresh extracts with ethanol or acetic acid. The inhibition was reversible. The inhibitor differs from any of hemin, protoporphyrin and bacteriochlorophyl or its related compounds. It was suggested that the inhibitor may play an important role in tetrapyrrole synthesis in R. spheroides by controling the activity of ALA synthetase. © 1969.
