Oxygen uptake coupled with nitrite oxidation by washed cell suspensions of Nitrobacter ugilis was tested in the presence of copper, nickel, aluminum, uranyl, and molybdate ions. Copper ion was slightly stimulatory at low concentrations and strongly inhibitory at 17 mM. Molybdate ion showed either slight enhancement or no inhibition at all test concentrations. With the other test ions, inhibition of oxygen uptake was observed. Carbon dioxide fixation was generally more sensitive to metal ions than was oxygen uptake. Cytochrome reduction in washed cells of N. ugilis was demonstrated in the presence and absence of metal ions. Cytochrome uu3 and cytochrome ul (a-peak at 590 nm) were sensitive to aluminum, uranyl, and molybdate ions, whereas cytochrome c (/?-peak at 520 nm) was insensitive to test ions. All metal ions inhibited the reduction of cytochrome al (y-peak a t 438 nm). This inhibition decreased when the concentration of copper and molybdate was increased to 1.7 mM. Only aluminum and nickel ions demonstrated a concentration-dependent inhibition of cytochrome al (438 nm). The test ions blocked the activity of cytochrome c (a-peak at 550 nm). The inhibition of cytochrome c at 415 nm (7-peak) was also apparent.

The molar growth yield (YG) of Nitrobacter agilis was determined with the use of 14CO2 fixation and protein measurement. A low YG of 0.220 g dry weight per mole nitrite oxidized was obtained. Assuming 4 or 6 mol ATP utilized per NAD(P)+ reduced, 10.8 or 14.5% of the theoretical amount of ATP available is utilized for the observed YG.

The ;malic' enzyme (EC 1.1.1.40) has been purified (300-fold) from wheat germ and its abilities to catalyse the decarboxylation and the hydrogenation of oxaloacetic acid and oxaloacetate esters was studied. The free 1-carboxyl group is essential for the interaction of oxaloacetates and substituted oxaloacetates with the enzyme via cations. The free 4-carboxyl group is required for the decarboxylation but is not indispensable for the hydrogenation. At high concentrations, cations inhibit the enzymic hydrogenation of oxaloacetic acid but not that of 4-ethyl oxaloacetate. A plausible inhibitory mechanism is proposed.

Oxygen uptake coupled with nitrite oxidation by washed cell suspensions was not influenced by persulfate or tetrathionate. Carbon dioxide fixation was insensitive to tetrathionate, and in fact, an enhancement by tetrathionate was observed. Persulfate inhibited the fixation of carbon dioxide only at a high concentration (17 mM S,Oz-). Cytochrome reduction in washed cells of Nitrobucter ugilis was demonstrated in the presence and absence of sulfooxyanions. Cytochrome uu3 and cytochrome ul (a-peak at 590 nm) were sensitive to sulfooxyanions (sulfite, thiosulfate, metabisulfite, dithionate, persulfate, trithionate, and tetrathionate), whereas cytochrome c @-peak a t 520 nm) was insensitive to test ions. All sulfooxyanions inhibited the reduction of cytochrome a, (y-peak a t 438 nm). This inhibition decreased when the concentration of sulfooxyanions was increased to 17 mM. The reduction of cytochrome c at 550 nm (a-peak) was blocked by the test ions and the inhibition of cytochrome c at 415 nm (y-peak) was also apparent. Complete inhibition of cytochrome c (y-peak) was observed in the presence of 17 mM persulfate.

Several metal ions inhibited the oxygen uptake activity of Nitrobacter agilis, but their effects on the kinetic parameters of nitrite oxidation were mixed. Growth of Nitrobacter winogradskyi was inhibited by persulfate (>0.1 mM), tetrathionate (>0.5 mM), and trithionate (>5 mM). Oxygen uptake activity was, however, relatively insensitive to persulfate and tetrathionate ions.