Effect of methionine sulfoximine and methionine sulfone on glutamate synthesis in Klebsiella aerogenes

At least two pathways exist in Klebsiella aerogenes for glutamate synthesis. A mutant blocked in one pathway due to the loss of glutamate dehydrogenase (gltD) does not require glutamate and has the same growth characteristics as the parent strain in most media; however, its growth is inhibited by the analogues methionine sulfoximine and methionine sulfone. Wild-type Klebsiella is resistant to 0.1 M methionine sulfoximine or methionine sulfone, whereas the gltD mutant is sensitive to 1 mM concentrations. Either glutamate or glutamine is effective in overcoming this inhibition. Activities of both glutamine synthetase and glutamate synthetase, two enzymes involved in the second pathway of glutamate synthesis, are inhibited by methionine sulfoximine and methionine sulfone. The primary effect of methionine sulfoximine appears to be the prevention of glutamine production necessary for subsequent glutamate synthesis via glutamate synthetase enzyme.

Regulation of pyruvate metabolism via pyruvate carboxylase in rat brain mitochondria

1. The fixation of CO(2) by pyruvate carboxylase in isolated rat brain mitochondria was investigated. 2. In the presence of pyruvate, ATP, inorganic phosphate and magnesium, rat brain mitochondria fixed H(14)CO(3) (-) into tricarboxylic acid-cycle intermediates at a rate of about 250nmol/30min per mg of protein. 3. Citrate and malate were the main radioactive products with citrate containing most of the radioactivity fixed. The observed rates of H(14)CO(3) (-) fixation and citrate formation correlated with the measured activities of pyruvate carboxylase and citrate synthase in the mitochondria. 4. The carboxylation of pyruvate by the mitochondria had an apparent K(m) for pyruvate of about 0.5mm. 5. Pyruvate carboxylation was inhibited by ADP and dinitrophenol. 6. Malate, succinate, fumarate and oxaloacetate inhibited the carboxylation of pyruvate whereas glutamate stimulated it. 7. The results suggest that the metabolism of pyruvate via pyruvate carboxylase in brain mitochondria is regulated, in part, by the intramitochondrial concentrations of pyruvate, oxaloacetate and the ATP:ADP ratio.

[The effect of hydroxyurea on DNA synthesis of embryonic rat cells in culture]

3H-thymidine incorporation by secondary cultures of embryonic rat cells and by cultures of a permanent rat cell line is strongly inhibited a few minutes after addition of hydroxyurea (HU) to the culture medium (2 × 10-3 ᴍ and 5 × 10-4 ᴍ, resp.); normal embryonic rat cells are considerably less sensitive to the drug than are permanent cells. About 76 percent of the population of a secondary culture can be synchronized at the beginning of the S-period, if the cells at first are accumulated in G1 phase by incubation in serum-free medium and subsequently are stimulated by the addition of ten percent calf serum in the presence of 2 × 10-3 ᴍ HU.

The size of DNA replicated in the presence of HU has been analysed by centrifugation in alkaline sucrose gradients. The results indicate that under these conditions DNA subunits are synthesized with sedimentation coefficients between 21S and 30S. These intermediates cannot be joined together, probably because a ligase is inhibited by HU.

Secondary culture incubated in medium-containing 1 × 10-4 ᴍ HU incorporate twice as much 3H-thymidine as do control cells in HU-free medium. This is due to an increased number of cells in the S phase, as was shown by autoradiography. The kinetics of thymidine incorporation indicate that the duration of G1 phase is shortened by small doses of HU