ABSTRACT
Toxoplasma gondii is an obligate intracellular protozoan parasite that grows within a vacuole in the cytoplasm of its host cell. Although overt disease is comparatively rare, infection is common in most mammals, including humans. The superficial similarity of T. gondii and viruses as obligate intracellular parasites led to various attempts to detect an antitoxoplasma effect of IFN. The critical tryptophan concentration for T. gondii is that which prevails within the vacuole in which the parasite is growing. Ethanol was used to extract the well-washed cells because one of the tryptophan metabolites, N-formylkynurenine, is decomposed by dilute acid. Since the tryptophan of medium was the only source of these metabolites, their final concentration in the medium could not have exceeded the initial tryptophan concentration. The high concentrations of tryptophan metabolites within the cell were then dissipated by diffusion across the plasma membrane.
