SALICYLATE INHIBITS CITRULLINE SYNTHESIS IN MITOCHONDRIA: IMPLICATIONS FOR REY'S SYNDROME

Abstract

In Reye's Syndrome (RS) there is a general decrease in the specific activities of mitochondrial enzymes, including two of the urea cycle, carbamyl phosphate synthetass and ornithine carbamyl-transferase. These two enzymes catalyze the synthesis of citrul-line (CIT) from ornithine, NH3, and CO2; the functional impairment of this reaction sequence probably contributes to hyperammonemia in RS. Salicylate (SA) has been implicated as a synergistic factor in the pathogenesis of RS. In this study we investigated whether SA inhibits CIT synthesis in isolated rat liver mitochondria. CIT synthesis was assayed at 37°C by measuring 14CO2 fixation in the presence of ornithine and NH3 (ABB 178:19,1977). SA inhibited CIT formation (μmol/hr/mg mito protein) as follows:Gentisic acid, salicyluric acid and acetaminophen had no effect at 5 mM. CIT synthesis is dependent on matrix ATP: a mechanism of inhibition was suggested by further experiments showing that SA uncouples oxidative phosphorylation and decreases the rate of ATP synthesis. Matrix ATP/ADP ratios were decreased by SA as follows: 1.15±.04 (O SA); 0.90±.21 (0.25 mM SA); 0.63±.09 (0.5 mM SA); 0.11±.04 (1 mM SA). Conclusion: SA is a potent inhibitor of CIT synthesis at physiological concentrations, probably as a result of its uncoupling action which lowers matrix ATP. The implication for RS is that SA might inhibit urea cycle function beyond the already compromised level. (NIH 14936)