Ammonium Chloride Inhibits Pyruvate Oxidation in Rat Liver Mitochondria: A Possible Cause of Fatty Liver in Reye's Syndrome and Urea Cycle Defects

1. Earlier studies with liver slices showed that inhibition by NH⁺₄ of the oxidation of palmitate to CO₂ was greater than total oxidation, whereas salicylate exerted a stronger inhibitory effect on the latter. We have now investigated the effects of NH₄Cl and salicylate on ADP-induced O₂ consumption by mitochondria (State 3 rate) respiring on pyruvate, and oxidation of [1-¹⁴C]- and [2-¹⁴C]-pyruvate to¹⁴CO₂.

2. The rate of State 3 respiration was inhibited and plateaued at 45% with 10 mmol/l NH₄Cl.

3. Oxidation of [1-¹⁴C]pyruvate was not significantly affected by either NH₄Cl or salicylate. Oxidation of [2-¹⁴C]pyruvate was strongly inhibited and plateaued at 70% with 1 mmol/1 NH₄Cl (IC₅₀ = 0.125 mmol/1). ADP (1 mmol/l) increased the rate of decarboxylation of [2-¹⁴C] pyruvate but the extent of NH₄Cl inhibition was not affected. Salicylate had a slight activating effect in the absence or presence of NH₄Cl.

4. These results indicate that NH₄Cl inhibits the oxidative metabolism of acetyl-CoA in the tricarboxylic acid cycle. Therefore, inhibition of fatty acid oxidation to acetyl-CoA as well as its further oxidative metabolism occurring under hyperammonaemia (>0.1 mmol-1.49 mmol/l in Reye's syndrome patients) may be one of the causes of fatty acidaemia.

5. The cumulative inhibitory effects of NH⁺₄ and fatty acyl derivatives on mitochondrial oxidative metabolism and production of ATP, as well as the uncoupling effects of salicylate, may contribute to some of the pathophysiology observed in patients with Reye's syndrome, and enzyme defects of the urea cycle.