THE ROLE(S) OF ADENYLATE KINASE AND THE ADENYLATE CARRIER IN THE REGULATION OF PLANT MITOCHONDRIAL RESPIRATORY ACTIVITY

The effect of inhibitors of the ATP synthase, the adenylate carrier and adenylate kinase on the respiratory rate, steady-state membrane potential and mitochondrial ATP levels have been investigated in potato and pea leaf mitochondria. Under ADP-limited conditions, it was found that the addition of oligomycin, aurovertin or efrapeptin increased the membrane potential and decreased the respiratory rate, implying that ATP synthesis was occurring prior to inhibition. Molybdate, NEM or vandate had no effect on the ATP synthesised, suggesting that ADP-regeneration required for continued phosphorylation was not due to contaminating ATPases or phosphatases. ATP levels were significantly reduced by carboxyatractyloside (CAT) and increased by P1,P5-di(adenosine-5') pentaphosphate (Ap5A). The respiratory rate could be stimulated by the addition of AMP and the stimulated rate was sensitive to oligomycin and aurovertin. Preincubation with CAT or Ap5A abolished AMP stimulation of NADH oxidation. It is suggested that respiration can sustain a limited but significant net formation of ATP, even in the absence of any added ADP. A model involving the combined activities of the adenylate carrier, adenylate kinase and the ATP synthase is proposed to account for the ATP synthesised under these conditions. Furthermore, it is suggested that the cycling of mitochondrial ADP and ATP via this model may represent a major regulatory influence on the activity of mitochondrial respiration under conditions of ADP-limitation - a condition likely to reflect the in vivo situation in plant cells.