The oxidation-reduction potential of the glucose oxidase from Aspergillus niger was determined by titration with leucosafranine in the pH range 4.5–7.5. The results are compared with the oxidation-reduction potential values reported by Ke1 for FAD. Possibilities for the binding between the apoprotein and the prosthetic group are discussed.
1. Kynurenate hydroxylase (kynurenate, NAD(P)H:O2 oxidoreductase (hydroxylating), EC 1.14.1.3), an enzyme which catalyzes the conversion of kynurenate to kynurenate-7,8-dihydrodiol, has been purified from Pseudomonas.
2. The partially purified enzyme preparation is stimulated 6–8 fold by the addition of ferrous ion. It is strongly inhibited by and α,α′-dipyridyl and its activity is specifically restored by the addition of ferrous ion.
3. Flavin nucleotides were shown to participate in the kynurenate hydroxylase reaction. After acid ammonium sulfate treatment of the enzyme, it is active only upon the addition of flavin nucleotides.
4. The enzyme is unstable but is fairly well protected from inactivation by some reducing agents, heated extracts, or in an atmosphere of nitrogen. The enzyme, once inactivated by oxygen, can be reactivated almost to the original level by the addition of sodium borohydride. The effects of various reducing agents and conditions required for this reactivation have been studied. required for this reactivation have been studied.
1. Homoserine dehydrogenase I (l-homoserine:NADP+ oxidoreductase, EC 1.1.1.3) of Escherichia coli is inactivated with apparent first-order kinetics by exposure at pH 9 in Tris buffer. This inactivation is accompanied by desensitization of the enzyme towards threonine.
2. L-Aspartate and ATP, the substrates of the associated activity, β-aspartokinase I (ATP:L-aspartate 4-phosphotransferase, EC 2.7.2.4) protect against the desensitization of homoserine dehydrogenase.
A system is described in which glutaminine synthetase (EC 6.3.1.2) from Escherichia coli B is irreversibly inactivated by incubation with ATP, Mg2+ and gluatmine or glutamate in vitro. Under the conditions chose, the glutamyl transferase activity does not change significantly.
Mechanisms are discussed for the in activation of glutamine synthetase in vitro as well as the relationship of this process to the inactivation of the enzyme by NH4+ in intact cells.
Fluorocitrate was investigated as an inhibitor of the citric acid cycle which could make possible the separate study of β-oxidation of fatty acids bound to albumin using isolated rat-liver mitochondria. Fluorocitrate was shown to inhibit completely not only the CO2 formation but also the incorporation of label from fatty acids into citrate in agreement with previous work. The reaction products identified were acetoacetate and β-hydroxybutyrate. No evidence for interference of fluorocitrate with β-oxidation or with phosphorylation coupled to this oxidation was found. Acetoacetate formation from fatty acids was rapid and initially linear.
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