IF a structure-based, acidophile-informed active-site variant of Chromobacterium sp. DS-1 cholesterol oxidase — re-engineered via site-directed mutagenesis of its FAD-binding and catalytic histidine/aspartate residues using sequence alignments with orthologs from confirmed acidophilic soil bacteria (e.g., Acidithiobacillus spp. that naturally operate at pH 4.0–5.5) to shift the enzyme's pH optimum from neutral toward pH 4.5–5.0, then expressed in glyco-engineered CHO or HEK293 cells to acquire M6P glycosylation, and administered at 0.5, 5, and 50 nM for 72 hours — is applied to PMA-differentiated THP-1 macrophages pre-loaded with 25 μg/mL 7-ketocholesterol (7KC) complexed with methyl-β-cyclodextrin (MβCD) for 24 hours,

THEN at least one dose will produce ≥40% reduction in intracellular 7KC versus vehicle control (quantified by LC-MS/MS with d7-7KC deuterated internal standard), confirmed lysosomal colocalization by LAMP1 immunofluorescence, and restoration of efferocytic capacity to ≥70% of non-loaded macrophage baseline (measured by pHrodo-labeled apoptotic Jurkat cell uptake assay), whereas parallel treatment with an identically produced, non-M6P-tagged variant and a catalytically inactive point mutant (active-site His/Asp → Ala substitutions) will each fail to reduce 7KC by ≥20% or restore efferocytosis to threshold, respectively,

BECAUSE the following causal chain connects intervention to outcome:

1. Receptor-mediated lysosomal delivery: The M6P glycosylation tag on the recombinant enzyme binds the cation-independent mannose-6-phosphate receptor (CI-MPR) on macrophage surfaces, triggering clathrin-mediated endocytosis and trafficking to the late endosome/lysosome compartment, a pathway extensively validated for ERT proteins in macrophage models. (M6P-tagged enzyme delivery to lysosomes via CI-MPR)[Zhu et al., J Biol Chem 2005; Vaso et al., Mol Ther 2010, as cited in Evidence Set]

2. Critical pH-activity gap in current candidates: Wild-type Chromobacterium sp. DS-1 cholesterol oxidase and Rhodococcus jostii orthologs display peak catalytic activity at near-neutral pH; the Evidence Set explicitly identifies "structure-activity relationships of these enzymes are predominantly characterized at neutral pH, leaving a gap in mutagenesis data required to optimize active-site residues for the acidic lysosomal environment (pH 4.5–5.0)" as a primary unresolved problem. (SAR gap for lysosomal pH optimization)[Evidence Set, Synthesis of Findings and Gap Analysis section]

3. Acidophile-informed mutagenesis restores activity at lysosomal pH [SPECULATIVE]: Guided by structural alignment with orthologs from Acidithiobacillus thiooxidans or similar acidophilic organisms whose sterol-metabolizing oxidoreductases are naturally adapted to pH 4–5, substitution of surface-exposed acidic residues and repositioning of proton relay networks within the FAD-binding domain is predicted to lower the pKa of key catalytic residues (His → more ...

SENS category: LysoSENS