Biochemical characterization of biliverdins IXβ/δ generated by a selective heme oxygenase

The pro-oxidant effect of free heme (Fe2+-protoporphyrin IX) is neutralized by phylogenetically-conserved heme oxygenases (HMOX) that generate carbon monoxide, free ferrous iron, and biliverdin (BV) tetrapyrrole(s), with downstream BV reduction by non-redundant NADPH-dependent BV reductases (BLVRA and BLVRB) that retain isomer-restricted functional activity for bilirubin (BR) generation. Regioselectivity for the heme α-meso carbon resulting in predominant BV IXα generation is a defining characteristic of canonical HMOXs, thereby limiting generation and availability of BVs IXβ, IXδ, and IXγ as BLVRB substrates. We have now exploited the unique capacity of the Pseudomonas aeruginosa (P. aeruginosa) hemO/pigA gene for focused generation of isomeric BVs (IXβ and IXδ). A scalable system followed by isomeric separation yielded highly pure samples with predicted hydrogen-bonded structure(s) as documented by 1H NMR spectroscopy. Detailed kinetic studies established near-identical activity of BV IXβ and BV IXδ as BLVRB-selective substrates, with confirmation of an ordered sequential mechanism of BR/NADP+ dissociation. Halogenated xanthene-based compounds previously identified as BLVRB-targeted flavin reductase inhibitors displayed comparable inhibition parameters using BV IXβ as substrate, documenting common structural features of the cofactor/substrate-binding pocket. These data provide further insights into structure/activity mechanisms of isomeric BVs as BLVRB substrates, with potential applicability to further dissect redox-regulated functions in cytoprotection and hematopoiesis.