The rate of reduction of terminal acceptors (nitrate, nitrite, and oxygen) in anaerobically grown cells of Paracoccus denitrijicans increased on permeabilization of cytoplasmic membrane. It was proved that under aerobic conditions the increase of the rate of nitrate reduction was caused by: (i) the abolishment of the permeability barrier for nitrate, (ii) the enhancement of the influx of redox equivalents to the respiratory chain due to the stimulation of succinate dehydrogenase reaction, and (iii) the inhibition of electron flow to oxygen by endogenously formed nitrite.Nitrite inhibits oxygen reduction by its interaction with the terminal part of the respiratory chain (I5o = 15 pM) localized at the inner aspect of the cytoplasmic membrane. The distribution of nitrite between intact cells and the suspension medium follows the Nernst equation for monovalent anion. The possible physiological consequences of the low intracellular nitrite concentration are discussed.Bacteria of Paracoccus denitrificans growing anaerobically in the presence of nitrate form a respiratory chain enabling the utilization of four terminal acceptors: NO;, NO;, NzO, and 0 2 [1,2]. In the presence of more than one acceptor a preferential reduction of one of them can be observed; simultaneously the reduction of other acceptors is inhibited. Thus, the addition of oxygen results in an immediate block of nitrate consumption [3]; similarly, the reduction of nitrate is partly inhibited even by the utilization of endogenously originating or externally added NO; and N20 [4]. It has been proved by several independent methods that the above phenomena depend on the redox state of the components of the respiratory chain in the branching sites [4-61 and that they can be described as the competition of terminal branches for a constant limited influx of redox equivalents from dehydrogenases [4].There are, however, experimental results indicating that this simple description of the control of electron fluxes to the individual terminal acceptors need not be sufficient. It has actually been found that membrane vesicles [3] and/or cells treated with the detergent Triton X-100 [7] are capable of reducing nitrate even in the presence of oxygen. The reason of this behaviour has not been definitely elucidated yet (cf. discussion in [S]). The objective of the present paper, therefore was to clarify the connections between the permeability of the cytoplasmic membrane of P. denitrqicans and the rate of reduction of the terminal acceptors of 0 2 , NO;, and NO;. The results obtained give evidence of hitherto not considered regulatory mechanism. On the basis of it a more realistic concept of the control of denitrification processes in the bacterium P. denitrificans can be presented.Abbreviation. CF30PhzC(CN)2, carbonyl cyanide p-trifluoromethoxyphenylhydrazone.Enzymes. Nitrate reductase (EC 1.7.99.4) ; nitrite reductase (cytochrome cdt ) (EC 1.7.2.1 ) ; succinate dehydrogenase (EC 1.3.99.1 ). MATERIALS AND METHODS Growth of bacteriaParacoccus denitrificans (NCIB 8944) w...