β-Glucuronidase at the gut-liver interface: Structural selectivity, enterohepatic reactivation, and the pharmacomicrobiomic case for precision inhibition.

β-Glucuronidase (β-G, EC 3.2.1.31) reverses phase II glucuronidation, and microbial β-G at the gut-liver interface reactivates biliary and circulating conjugates, closing enterohepatic loops for drugs, estrogens, bile acids, and bilirubin. Human GUSB buries catalytic glutamates in a deep pocket, whereas bacterial glycoside hydrolase family 2 (GH2) enzymes expose an open gorge with mobile loops and, in a flavin mononucleotide (FMN)-binding subclass, an allosteric site (enabling > 104-fold selective inhibition). This review surveys biochemical, microbiological, and translational literature with quantitative emphasis, advancing four positions: β-G activity follows a homeostatic-to-pathological continuum; interindividual gus operon and estrobolome variation necessitates pharmacomicrobiomic stratification; the estrobolome links gut β-G to female endocrine and hepatic health (breast cancer, endometriosis, polycystic ovary syndrome (PCOS), and metabolic dysfunction-associated steatotic liver disease (MASLD); and β-G is both a target to inhibit and a tool to exploit (e.g., SN-38 antibody-drug conjugates). Selective inhibitors (entropy-driven competitive, covalent-allosteric, natural products) are chemically mature, but efficacy is gated by microbial phenotype, not chemistry. We analyze structural determinants of selectivity, four substrate classes dictating distinct therapeutic responses, diagnostic platforms spanning bulk chromogenic assays to activity-based protein profiling, and a broadening chemotype space. We argue that combining fecal β-G phenotyping with genotype-informed, mechanistically matched inhibition is the most realistic near-term path to clinical benefit, while warning against indiscriminate suppression that may disrupt barrier-protective β-G activity.