INHIBITION OF ANAEROBIC PHOSPHATE RELEASE BY NITRIC-OXIDE IN ACTIVATED-SLUDGE

Activated sludge not containing significant numbers of denitrifying, p olyphosphate [poly(P)]-accumulating bacteria was grown in a fill-and-d raw system and exposed to alternating anaerobic and aerobic periods. D uring the aerobic period, poly(P) accumulated up to 100 mg of P . g of (dry) weight, When portions of the sludge were incubated anaerobicall y in the presence of acetate, 80 to 90% of the intracellular poly(P) w as degraded and released as orthophosphate, Degradation of poly(P) was mainly catalyzed by the concerted action of polyphosphate:AMP phospho transferase and adenylate kinase, resulting in ATP formation. In the p resence of 0.3 mM nitric oxide (NO) in the liquid-phase release of pho sphate, uptake of acetate, formation of poly-beta-hydroxybutyrate, uti lization of glycogen, and formation of ATP were severely inhibited or completely abolished. In cell extracts of the sludge, adenylate kinase activity was completely inhibited by 0.15 mM NO. The nature of this i nhibition was probably noncompetitive, similar to that with hog adenyl ate kinase, Activated sludge polyphosphate glucokinase was also comple tely inhibited by 0.15 mM NO. It is concluded that the inhibitory effe ct of NO on acetate-mediated phosphate release by the sludge used in t his study is due to the inhibition of adenylate kinase in the phosphat e-releasing organisms. The inhibitory effect of nitrate and nitrite on phosphate release is probably due to their conversion to NO. The lack of any inhibitory effect of NO on adenylate kinase of the poly(P)-acc umulating Acinetobacter johnsonii 210A suggests that this type of orga nism is not involved in the enhanced biological phosphate removal by t he sludges used.