Do neuroglobin and myoglobin protect Toxoplasma gondii from nitrosative stress?

Toxoplasma gondii is a Apicomplexa obligate intracellular protozoan parasite that infects up to a third of the world's population. In most humans infected with T. gondii, the disease toxoplasmosis is asymptomatic. However, T. gondii causes blindness, severe neurological disorders, hepatitis, and pneumonia in immunocompromised patients, and severe damage to the fetus. Here, we postulate that the colonization of the retina, heart, and skeletal muscle by T. gondii may reflect the role of neuroglobin (Ngb) and myoglobin (Mb) to protect the parasite from the toxoplasmacidal effects of nitric oxide (NO). This is based on the knowledge that Ngb and Mb catalyzes NO oxidation yielding the harmless nitrate. The postulated protective role of Ngb and Mb on the viability of T. gondii is reminiscent of that postulated for hemoglobin (Hb) and Mb in protecting intraerythrocytic Plasmodia and T. cruzi in cardiomyocytes, respectively, from the parasiticidal effect of NO. Therefore, undesirable pathogen protection by pseudo-enzymatic NO scavenging may represent a new unexpected function of members of the Hb superfamily.