REGULATION OF ASPARTATE FAMILY AMINO ACID BIOSYNTHESIS IN BREVIDACTERIUM FLAVUM .I. INHIBITION BY AMINO ACIDS OF ENZYMES IN THREONINE BIOSYNTHESIS

1. Effect of 13 amino acids on enzymes in the pathway from aspartate to threonine in Brevibacterium flavum were estimated.2. Simultaneous addition of threonine and lysine (10 mM each) caused 56% inhibition of aspartate kinase [EC 2.7.2.4], whereas addition of either amino acid did not cause significant inhibitory effect.3. L-Aspartate-β-semialdehyde dehydrogenase [EC 1.2.1.11] was scarcely inhibited by amino acids of aspartate family. Serine inhibited the activity by about 40% at a concentration of 10 mM.4. Homoserine dehydrogenase [EC 1.1.1.3] was competitively inhibited by threonine and by isoleucine. 3.3 mM of threonine inhibited the activity completely in the forward and reverse reaction, and 10 mM of isoleucine inhibited the activity by about 90% in the forward reaction, and by about 60% in the reverse reaction.5. Homoserine kinase [EC 2.7.1.39] was inhibited by threonine in the extent of 60% at 30 mM. Other amino acids of aspartate family showed slightly inhibitory effect at 10 mM.6. Threonine synthase [EC 4.2.99.2] was scarcely inhibited by amino acids of aspartate family. Cysteine and glutathione showed 100% inhibition at 10 mM revealed by measurement of the determination of phosphohomoserine disappearance. © 1968 by the Journal of Biochemistry.