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 muM, 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.