Severe cytochrome c oxidase inhibition in vivo does not induce a pyrimidine deficiency; neuroprotective action of oral uridine prodrug PN401 requires supraphysiological levels of uridine

It has been hypothesized that mitochondrial respiratory chain dysfunction leads to a pyrimidine deficiency since the pyrimidine biosynthetic enzyme dihydroorotate dehydrogenase is coupled to the electron transport chain. The uridine prodrug triacetyluridine (PN401) is neuroprotective in several models of neurodegenerative disease involving respiratory chain toxins. Therefore, the therapeutic effects of PN401 might involve the correction of a pyrimidine deficiency secondary to respiratory chain impairment. We infused mice with the cytochrome c oxidase inhibitor azide, which inhibited brain complex IV activity. Chronic infusion of azide for 2 or 14 days induced significant toxicity and mortality but did not cause a pyrimidine deficit in the brain. In contrast, the pyrimidine synthesis inhibitor N-phosphonoacetyl- l-aspartate (PALA) produced a pyrimidine deficit with minimal mortality. Treatment with 6% PN401 decreased mortality and cerebrocortical apoptosis caused by azide. Previously, we found that optimal neuroprotection against mitochondrial complex II inhibition required 4–6% PN401. PN401 at 1, 3, 6 and 10% in chow induced nonlinear increases in plasma uridine with 6% PN401 elevating plasma uridine up to 80 μM, and these higher micromolar uridine levels were also required for neuroprotection in chemical hypoxia models in vitro. Our results indicate that severe complex IV inhibition in vivo does not lead to a pyrimidine deficiency, and therefore the protective effect of PN401 in the azide toxin model is not mediated through the correction of a pyrimidine deficiency. Furthermore, supraphysiological levels of uridine are required to produce optimal protective effects in disorders involving impairment of mitochondrial respiratory complex II or IV.