A 40-year-old man presents with fever 8 months after returning from Ethiopia. He had no fever at the time of return and took no prophylaxis. Blood film shows P. ovale parasites. He has normal G6PD activity. After treatment of the acute episode with chloroquine, primaquine 0.25 mg base/kg/day for 14 days is commenced for radical cure. What is the mechanism of action of primaquine against P. ovale?