BLVRB

BLVRB is an NADPH-dependent monomeric oxidoreductase of the short-chain dehydrogenase/reductase (SDR) superfamily that catalyzes the reduction of biliverdin IXβ, IXγ, and IXδ tetrapyrroles as well as flavins via direct hydride transfer within an α/β dinucleotide binding fold active site (PMID:11224564, PMID:7929092). It physically interacts with heme oxygenase-1 to accept biliverdin product through a shared binding interface (PMID:12626517), and in red blood cells functions as an oxygen-sensitive Band 3 interactor whose Cys109 relays nitric oxide to GAPDH, coupling redox status to glycolytic regulation (PMID:41394639). Loss of BLVRB reductase activity—whether by the S111L mutation in humans or gene knockout in mice—causes reactive oxygen species accumulation that drives megakaryocyte-biased hematopoietic lineage commitment, enhanced thrombopoiesis, and defective stress erythropoiesis, establishing a non-redundant cytoprotective role in redox-regulated lineage partitioning (PMID:27207795, PMID:33359909). BLVRB also promotes NF-κB p65 phosphorylation and nuclear translocation, supporting hepatocellular carcinoma cell proliferation (PMID:26708147).

1. 1994 High

   Purification and kinetic characterization of BLVRB from human liver established it as a ~21 kDa NADPH-preferring monomeric reductase with strict specificity for non-IXα biliverdin isomers, defining its enzymatic identity as distinct from BVRA.

   Evidence Protein purification to homogeneity from human liver cytosol with Km determination and substrate specificity assays

   PMID:7929092

   Open questions at the time

   • Structural basis for IXα exclusion was unknown
   • Physiological significance of biliverdin IXβ reduction in vivo was unclear
   • No information on non-biliverdin substrates such as flavins
2. 1995 High

   Chromosomal mapping of BLVRB to 19q13.13–q13.2 anchored the gene locus for future genetic studies.

   Evidence FISH using a 0.76-kb cDNA probe on human chromosomes

   PMID:7656592

   Open questions at the time

   • No functional or disease-associated variants had been identified at this locus
3. 2001 High

   The 1.15 Å crystal structure of BLVRB with NADP+ and multiple ternary substrate complexes revealed the structural basis of catalysis—direct B-face hydride transfer from NADPH to the substrate positioned above the nicotinamide C4—and explained selectivity against biliverdin IXα through steric constraints at side-chain binding pockets.

   Evidence X-ray crystallography at 1.15 Å resolution with ternary complexes (mesobiliverdin IVα, biliverdin IXα, FMN, lumichrome)

   PMID:11224564

   Open questions at the time

   • No protein–protein interaction partners had been characterized
   • Cellular or in vivo function of the enzyme was unknown
4. 2003 High

   FRET and mutagenesis demonstrated that BLVRB physically binds heme oxygenase-1 with high affinity (Kd ~0.2 µM), competing with cytochrome P450 reductase for an overlapping surface, establishing a direct biliverdin channeling mechanism from HO-1 to BLVRB.

   Evidence FRET binding assays and site-directed mutagenesis of hHO-1 surface lysine/arginine residues

   PMID:12626517

   Open questions at the time

   • Functional consequence of disrupting HO-1–BLVRB interaction in cells was not tested
   • Whether biliverdin channeling is kinetically significant in vivo was unresolved
5. 2016 High

   Identification of the human loss-of-function variant S111L linked BLVRB reductase activity to a physiological redox pathway: defective biliverdin/flavin reduction causes ROS accumulation that biases hematopoietic progenitors toward megakaryocyte commitment and enhanced thrombopoiesis, while separately, knockdown/overexpression studies placed BLVRB upstream of NF-κB p65 phosphorylation in hepatocellular carcinoma cell growth.

   Evidence Platelet transcriptome sequencing and thrombocytosis cohort analysis with S111L functional characterization (Blood); siRNA knockdown, overexpression, and p65 phosphorylation assays in HCC cells (Cancer Science)

   PMID:26708147 PMID:27207795

   Open questions at the time

   • In vivo genetic validation in animal models was lacking for the hematopoietic phenotype
   • Mechanism linking BLVRB enzymatic activity to NF-κB p65 phosphorylation was not delineated
   • Whether the NF-κB effect depends on biliverdin reductase or flavin reductase activity was unclear
6. 2020 High

   Blvrb-knockout mice validated the non-redundant in vivo role: while steady-state hematopoiesis is normal, stress conditions reveal defective erythropoiesis with lipid peroxidation and a cell-autonomous megakaryocyte lineage bias, confirming BLVRB as a cytoprotective redox regulator of lineage partitioning under hematopoietic stress.

   Evidence Blvrb knockout mouse, stress hematopoiesis assays, lipid peroxidation measurement, multipotential progenitor colony assays

   PMID:33359909

   Open questions at the time

   • Specific lipid substrates of peroxidation and downstream effectors of lineage bias were not identified
   • Whether pharmacological BLVRB inhibition recapitulates the megakaryocyte bias was untested
7. 2025 High

   Selective small-molecule BLVRB inhibitors that stack within the active site against the nicotinamide ring recapitulated the megakaryocyte lineage bias in vitro and accelerated platelet recovery in mouse stress thrombopoiesis models, providing pharmacological proof-of-concept for therapeutic BLVRB targeting; concurrently, cross-linking interactomics in RBCs identified BLVRB as an oxygen-sensitive Band 3 interactor whose Cys109 trans-nitrosates GAPDH to regulate glycolysis.

   Evidence NMR and co-crystal structures with inhibitors plus in vivo oral dosing in mice (Nature Communications); deep cross-linking proteomics of ultra-pure RBCs, S-nitrosation assays, humanized mouse truncation model (preprint)

   PMID:40216753 PMID:41394639

   Open questions at the time

   • Band 3–BLVRB interaction and GAPDH trans-nitrosation findings are from a preprint and await peer review
   • Whether BLVRB inhibitors affect RBC glycolytic regulation is unexplored
   • Long-term safety of oral BLVRB inhibition on erythropoiesis has not been assessed

• The molecular mechanism linking BLVRB enzymatic product (reduced biliverdin/flavin) or ROS accumulation to specific signaling effectors that dictate megakaryocyte versus erythroid fate remains undefined, as does the relationship between the NF-κB signaling role and the hematopoietic lineage function.
• No direct signaling effector downstream of BLVRB-generated ROS has been identified in lineage commitment
• Whether the NF-κB and hematopoietic lineage functions share a common redox mechanism is unknown
• Structural basis for Band 3–BLVRB interaction has not been resolved