To determine the role of the kynurenine pathway in rhodoquinone and de novo NAD+ biosynthesis and whether NAD+ rescue pathways are essential in parasitic worms (helminths).
We demonstrate that rhodoquinone, the key electron transporter used by helminths under hypoxia, derives from the tryptophan catabolism even in the presence of a minimal kynurenine pathway. We show that of the kynurenine pathway genes only the kynureninase and tryptophan/indoleamine dioxygenases are essential for rhodoquinone biosynthesis. Metabolic labeling with tryptophan revealed that the lack of the formamidase and kynurenine monooxygenase genes did not preclude rhodoquinone biosynthesis in the flatworm Mesocestoides corti. In contrast, a minimal kynurenine pathway prevented de novo NAD+ biosynthesis, as revealed by metabolic labeling in M. corti, which also lacks the 3-hydroxyanthranilate 3,4-dioxygenase gene. Our results indicate that most helminths depend solely on NAD+ rescue pathways, and some lineages rely exclusively on the nicotinamide salvage pathway. Importantly, the inhibition of the NAD+ recycling enzyme nicotinamide phosphoribosyltransferase with FK866 led cultured M. corti to death.
We use comparative genomics of more than 100 hundred helminth genomes, metabolic labeling, HPLC-MS targeted metabolomics, and enzyme inhibitors to define pathways that lead to rhodoquinone and NAD+ biosynthesis in helminths. We identified the essential enzymes of these pathways in helminth lineages, revealing new potential pharmacological targets for helminthiasis.
Our results demonstrate that a minimal kynurenine pathway was evolutionary maintained for rhodoquinone and not for de novo NAD+ biosynthesis in helminths, and shed light on the essentiality of NAD+ rescue pathways in helminths.