1. 1. In the catalytic reaction at 37° of d- and l-amino-acid oxidases with d-alanine and l-leucine, respectively, as substrate, benzoate and its derivatives act as competitive inhibitors. The Hammett plots show no straight-line relationship between the K i and the σ values of the different substituents. 2. 2. With d-amino-acid oxidase ( d-amino acid:O 2) oxidoreductase (deaminating), (EC 1.4.3.3) at 10° in the 1/ v vs. i/[ d-alanine] plot, the lines obtained in the presence and in the absence of inhibitor (benzoate or its derivatives, with the exception of the nitro-substituents), intercept at finite [ d-alanine]. This was also the case with d-methionine as substrate. At 25° with d-methionine substrate inhibition is found at relatively high concentrations, which is not observed with d-alanine. This substrate inhibition can be abolished by benzoate and ATP, but not by m- or p-nitrobenzoate. ATP acts as non-competitive inhibitor, but only inhibits the reaction partially. At 37° the inhibition by ATP is abolished by benzoate. Titration of d-amino-acid oxidase with benzoate showed that, to obtain 50% saturation, 2–3 times more benzoate is necessary in the presence of an excess of ATP than in its absence. ATP itself does not induce spectral shifts. It is concluded that two forms of d-amino-acid oxidase are present, a low-temperature and a high-temperature conformation, which have about the same activation energies, but differ in activity, due to different transition probabilities Δ S ≠ = 0.7 e.u.). Δ H for this conformational change is 55 000 cal·mole −1; Δ S = 185 e.u.. It can be shown that benzoate and its derivatives have more affinity for the high-temperature conformation, while ATP has more affinity for the low-temperature form; m- and p-nitrobenzoate have equal affinity for both forms. 3. 3. With l-amino-acid oxidase l-amino-acid:O 2 oxidoreductase (deaminating), EC 1.4.3.2 it was found that ATP and pyrophosphate influence the catalytic oxidation reaction. The linear Arrhenius plot obtained with l-leucine as substrate is converted by the addition of pyrophosphate into a Z-shaped curve. The substrate inhibition in the presence of l-leucine is abolished by pyrophosphate at low temperature. In the presence of l-valine as substrate a Z-shaped curve is obtained, of which, as in the case of l-leucine, the high-temperature and low-temperature parts are parallel. In the presence of pyrophosphate the curve shows one break. As in the case with d-amino- acid oxidase, this enzyme also exists in two conformations with different activities and entropies of activation ( Δ S ≠ = 1.8–2.3 e.u.), but with the same activation energies. From the transition it can be calculated that Δ H = 46 000 cal·mole −1 and Δ S = 145 e.u. for the conformational transition from the low- to the high-temperature form. 4. 4. The general phenomenon of non-linear Arrhenius plots of enzyme-catalyzed reaction is discussed in connection with these observations.