Tryptophan synthase has been purified about 100-fold from Saccharomyces cerevisiae. Further-more two tryptophan synthase inactivating factors have been purified about 1000-fold from baker's yeast. The following findings demonstrate that the inactivating factors are enzymes: (1) they can be purified with typical fractionation methods for enzymes, (2) they are heat labile, (3) inactivation of tryptophan synthase follows for more than one hour a first-order reaction, (4) the rate of inactivation increases about 3-fold from 20 °C to 30. With hydroxyapatite column chromatography two fractions with tryptophan synthase inactivating activity can be separated (inactivases I and II). Inactivase I was purified 1200-fold, inactivase II 1900-fold. Inactivase I inactivates the apo-enzymes of tryptophan synthase and threonine dehydratase, but not of aspartate transaminase. Inactivase II inactiviates the apo-enzymes of tryptophan synthase and aspartate transaminase but not of threonine dehydratase. Neither inactivase inactivates any of three non-pyridoxal enzymes tested (alcoholdehydrogenase, hexokinase, glucose-6-phosphate dehydrogenase). There are also differences in the sensitivity of the two inactivases to pyridoxal phosphate and a mixture of pyridoxal phosphate and serine. In the exponential growth phase of a yeast culture no tryptophan synthase inactivating activity is found in the crude extract. whereas in the stationary phase high inactivating activity is observed. A biological significance of this observation is discussed. Some evidence that the chemical mechanism of the inactivation reaction is a proteolytic “splitting” of the substrate enzyme is reported.
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Katsunuma et al. (1972) studied this question.