Mechanisms of Gut Microbial Enzyme-Mediated Metabolism of Glucuronide Conjugates and Drug Biotransformation

Glucuronidation is an important phase II metabolic pathway in the clearance of drugs and endogenous metabolites, but its biological consequences do not necessarily terminate once the host conjugation reaction is completed. After large amounts of glucuronide conjugates enter the intestinal lumen through biliary excretion or intestinal epithelial transport, they may undergo deconjugation under the action of gut microbiota-derived enzyme systems, thereby re-releasing parent drugs, active metabolites, or intermediates with toxic potential, and further altering the extent of enterohepatic circulation, local exposure levels, systemic pharmacokinetic processes, and the spectrum of adverse reactions. Therefore, gut microbial enzyme-mediated deconjugation is an important reprocessing step after host conjugative metabolism and also a key entry point for understanding interindividual variability in drug response, microbiota-dependent toxicity, and co-metabolic regulation.

Keywords: gut microbiota; glucuronide conjugates; beta-glucuronidase; drug biotransformation; enterohepatic circulation; pharmacokinetics; toxicity reactivation; microbial metabolism

1. Formation, Transport, and Intestinal Re-Exposure of Glucuronide Conjugates

1.1 Glucuronidation is an important component of the host clearance system

(1) The core function of glucuronidation is to increase hydrophilicity and promote excretion

After drugs and their phase I metabolites are catalyzed by UDP-glucuronosyltransferases, they form glucuronide conjugates, which generally exhibit increased polarity, enhanced water solubility, and reduced membrane permeability. These changes facilitate excretion through bile, urine, or intestinal epithelial transport systems and are therefore often regarded as a typical detoxification and inactivation process.

(2) Conjugation does not necessarily mean terminal inactivation

For some compounds, glucuronidation does significantly reduce the binding capacity of the parent drug to its target; however, for other drugs or endogenous metabolites, this step more often represents transient inactivation and cross-organ redistribution rather than the true end of the metabolic chain. Once these conjugates enter the intestinal lumen and are hydrolyzed again by microbial enzymes, their pharmacological activity, toxic potential, and local exposure patterns may all be redefined.

1.2 The intestinal phase is a critical node at which the fate of glucuronide conjugates diverges

(1) Biliary excretion delivers large amounts of conjugates into the intestinal lumen

Many glucuronide conjugates with relatively large molecular weights, high polarity, or specific transport characteristics are more likely to be excreted into the intestine through bile rather than cleared directly by the kidney. After entering the intestinal lumen, the subsequent fate of these compounds is jointly influenced by host transport processes and the gut microbial enzymatic network.

(2) The intestinal lumen is an important compartment for conjugate reprocessing

In the intestine, glucuronide conjugates may further undergo multiple processes, including deconjugation, hydrolysis, reabsorption, reconjugation, rereduction, dehydroxylation, demethylation, or fermentative utilization. Therefore, the intestine is not a passive terminal site of drug metabolism, but rather a reprocessing compartment formed by the coupling of host and microbial metabolism.

2. Key Enzyme Systems Involved in Glucuronide Conjugate Metabolism

2.1 Host-side enzymes determine how conjugates are formed

(1) The UGT family are the core executors of glucuronidation

The UGT family determines whether a drug enters the glucuronidation pathway, where conjugation occurs, and how efficient the process is. Among them, UGT1A1, UGT1A9, and UGT2B7 are of representative importance in the metabolism of many drugs. Differences in the expression of distinct UGT subtypes determine whether individuals differ in their ability to form conjugates from the same drug.

(2) Phase I metabolic enzymes influence whether subsequent glucuronidation can occur

Before undergoing glucuronidation, many drugs must first be converted by CYP enzymes, esterases, or reductases into hydroxylated, carboxylated, or de-esterified products that are more suitable for conjugation. Therefore, the formation of glucuronide conjugates is not solely the result of UGT activity, but rather the product of the sequential action of phase I and phase II metabolism.

2.2 Microbial enzymes determine whether conjugates can be deconjugated and further transformed

(1) Beta-glucuronidase is the most critical deconjugating enzyme

This enzyme can directly hydrolyze the glucuronide bond and release the original aglycone, making it the core functional enzyme in the intestinal reactivation process. For irinotecan, mycophenolic acid, and certain nonsteroidal anti-inflammatory drugs, this step often directly determines whether re-exposure and toxicity reactivation occur.

(2) Subsequent microbial enzymes can further reshape the fate of the released aglycone

Deconjugation is only the first step. After release, the aglycone may continue to undergo the action of microbially derived reductases, dehydroxylases, demethylases, azoreductases, or lyases, thereby generating new microbiota-derived metabolites. The actual outcome of drug biotransformation usually depends on the coordinated action of deconjugating enzymes and downstream transforming enzymes, rather than on a single enzymatic step.

Table 1. Key enzymes involved in gut microbial enzyme-mediated metabolism of glucuronide conjugates

Enzyme class	Representative enzyme/family	Main reaction step	Significance for drug biotransformation
Host phase I metabolic enzymes	CYP family	Activation of drug precursors or structural modification	Provide conjugatable sites for subsequent glucuronidation
Host hydrolases	Carboxylesterases	Prodrug activation, ester bond hydrolysis	Generate active metabolites that can be further conjugated or reactivated
Host phase II metabolic enzymes	UGT1A1	Glucuronidation	Forms representative conjugates such as SN-38G
Host phase II metabolic enzymes	UGT1A9	Glucuronidation	Participates in the metabolism of many phenolic hydroxyl- and carboxyl-containing drugs
Host phase II metabolic enzymes	UGT2B7	Glucuronidation	Participates in the conjugative metabolism of many drugs and endogenous molecules
Microbial deconjugating enzymes	Beta-glucuronidase	Deconjugation hydrolysis	Releases parent drugs or active/toxic aglycones
Microbial reductases	Nitroreductases, carbonyl reductases, etc.	Downstream aglycone modification	Alter the activity and stability of the released molecules
Microbial dehydroxylases	Aromatic dehydroxylation-related enzyme systems	Deep transformation of aromatic aglycones	Reshape the terminal metabolic profile of the released products
Microbial demethylases	O-demethylation-related enzyme systems	Removal of aromatic methoxy groups	Generate new active or inactive derivatives
Microbial lyases	Aromatic ring-cleavage-related enzyme systems	Further degradation of small molecules	Promote conversion of drugs into terminal microbial metabolites

3. Pharmacokinetic and Toxicological Consequences of Deconjugative Metabolism

3.1 Enterohepatic circulation is one of the most important systemic consequences of deconjugative metabolism

(1) Reabsorption after deconjugation can markedly prolong in vivo drug exposure

When glucuronide conjugates are hydrolyzed in the intestine and release parent molecules that are more readily absorbable, the drug can re-enter the portal vein and systemic circulation through the intestinal epithelium, thereby forming a typical enterohepatic circulation process. This process can prolong the apparent half-life, maintain plasma drug concentrations, and increase overall exposure.

(2) Secondary absorption reshapes the kinetic structure of exposure

Enterohepatic circulation often causes drug concentration-time curves to exhibit kinetic features such as double peaks, multiple peaks, or prolongation of the terminal elimination phase. Therefore, microbiota-mediated deconjugation does not merely increase total exposure; more importantly, it can remodel the kinetic structure of exposure, thereby influencing dosing frequency, duration of action, and assessment of accumulation risk.

3.2 Deconjugation can cause high local exposure

(1) The intestinal mucosa is the first site exposed to rereleased active molecules

In many cases, molecules released after deconjugation are not immediately and completely absorbed, but instead first establish a state of high local exposure in the intestinal lumen and on the intestinal mucosal surface. If the molecule itself has cytotoxic, irritative, or pro-inflammatory properties, local toxicity often emerges before systemic effects.

(2) Local re-exposure often constitutes the key layer in toxicity interpretation

Some drugs may not show prominent systemic toxicity, yet after deconjugation of their glucuronide conjugates in the intestine, they may cause severe diarrhea, mucosal injury, or enteritis-like reactions. Therefore, research should not be confined to plasma drug concentrations alone, but should also analyze the process of local rerelease within the intestinal lumen.

Table 2. Major consequences of gut microbial enzyme-mediated drug biotransformation of glucuronide conjugates

Transformation type	Direct result	Pharmacokinetic impact	Pharmacodynamic/toxicological impact
Reabsorption after deconjugation	Parent drug re-enters circulation	Prolonged half-life, increased AUC	May prolong drug efficacy, but may also increase systemic toxicity
Local accumulation after deconjugation	Active or toxic molecules rise locally in the intestinal lumen	High local exposure	May cause intestinal mucosal injury, inflammation, and diarrhea
Reoxidation/reconjugation after deconjugation	Re-entry into host metabolic circulation	Increased metabolic cycling complexity	Alters exposure duration and interindividual variability
Further microbial transformation after deconjugation	Generation of novel microbiota-derived metabolites	Reshapes the terminal metabolic profile	May alter activity, toxicity, or target spectrum
Reduced deconjugation capacity	More complete clearance of conjugates	Weakened enterohepatic circulation	May reduce toxicity, but may also shorten the duration of drug action

4. Representative Drug and Endogenous Molecule Scenarios

4.1 Irinotecan-related toxicity is a classic model of “host conjugation-microbial deconjugation-local reactivation”

(1) SN-38G is not a truly safe endpoint

The active irinotecan metabolite SN-38 is glucuronidated by the host to form SN-38G, which originally facilitates inactivation and excretion; however, after entering the intestine, it can be hydrolyzed again by microbial beta-glucuronidase to release SN-38, resulting in renewed exposure of the intestinal mucosa.

(2) Local reactivation directly drives delayed diarrhea

This process constitutes an important mechanistic basis for irinotecan-related delayed diarrhea and intestinal mucosal injury, and it has therefore become a representative model for studies of strategies targeting microbial deconjugating enzymes.

4.2 Mycophenolic acid-related gastrointestinal toxicity indicates that immunosuppressants are likewise affected by microbial deconjugation

(1) Glucuronide metabolites of mycophenolic acid can rerelease active mycophenolic acid

After glucuronide conjugates formed during mycophenolic acid metabolism enter the intestine, they can be converted back to mycophenolic acid under the action of microbial beta-glucuronidase, thereby participating in enterohepatic circulation and altering the exposure profile.

(2) Increased microbial enzyme activity can simultaneously amplify therapeutic exposure and gastrointestinal intolerance risk

The activity of microbial beta-glucuronidase is mechanistically linked to mycophenolic acid-related gastrointestinal intolerance, indicating that adverse effects of immunosuppressants are not determined solely by host metabolism.

4.3 Intestinal injury related to nonsteroidal anti-inflammatory drugs may likewise be associated with deconjugation

(1) Acyl glucuronide metabolites of certain NSAIDs can enter enterohepatic circulation

Represented by diclofenac, some drugs form glucuronide-related metabolites that can enter the intestine through bile and participate in recirculation under the influence of the microbiota.

(2) There is a clear mechanistic link between deconjugation and intestinal injury

Inhibition of intestinal beta-glucuronidase activity can attenuate diclofenac-related enterohepatic circulation and reduce enteropathy-like injury, suggesting that this process influences not only pharmacokinetics but also toxic expression.

4.4 Hormones and bile-related endogenous molecules are also important subjects within this mechanistic framework

(1) Estrogen-related glucuronide conjugates can be regulated by microbial deconjugation

Estradiol, estrone, and their related conjugates may be influenced by microbial deconjugation in the intestine, thereby altering the extent of reabsorption and systemic hormonal homeostasis. This process is relevant not only to endocrine balance, but also to the formation of exposure differences in hormone-based drugs.

(2) Bilirubin and other bile-related substrates help broaden the scope of this field

The conjugation-deconjugation processes of bilirubin and certain bile-related metabolites indicate that this mechanism is not limited to exogenous drugs, but also involves reprocessing of endogenous host molecules in the intestine. Therefore, intestinal deconjugation has broader significance in metabolic regulation.

5. Commonly Used Products in Studies of Gut Microbial Enzyme-Mediated Glucuronide Conjugates

5.1 Enzymatic Products for Research on Glucuronide Biotransformation

Catalog No.	Product Name	Grade and Purity	Mechanistic Step	Research Direction / Intended Use
H1439464	Human CYP1A2,High-Reductase	—	Host phase I metabolism	Suitable for studying oxidation of aromatic drug precursors and for evaluating structural modification steps before glucuronidation.
H1439014	Human CYP2C9,High-Reductase+b5	—	Host phase I metabolism	Suitable for studying oxidative metabolism of common drugs such as NSAIDs, and can be linked with model compounds such as diclofenac, naproxen, and ibuprofen to analyze the “oxidation–conjugation–intestinal re-exposure” pathway.
H1439228	Human CYP2E1,High-Reductase+b5	—	Host phase I metabolism	Suitable for studies of small-molecule drug activation and oxidative metabolism, and can serve as a supplementary upstream model before glucuronidation.
H1439237	Human CYP3A4,High-Reductase+b5	—	Host phase I metabolism	Suitable for broad-spectrum drug metabolism studies and is one of the most representative CYP tools for constructing in vitro systems of “host metabolism–conjugate formation–intestinal reprocessing.”
H1439017	Human CYP3A5,High-Reductase +b5	—	Host phase I metabolism	Suitable for comparison with CYP3A4 to analyze how differences in precursor generation under different CYP backgrounds affect subsequent glucuronidation potential.
M1515861	Micro Carboxylesterase (CarE) Activity Assay Kit (1-Naphthyl acetate, Micro Method)	BioReagent	Host hydrolysis / prodrug activation	Suitable for studies related to activation of prodrugs such as irinotecan, and can be used to connect the classic mechanistic model of “CarE generation of active metabolites–UGT conjugation–microbial deconjugation and reactivation.”
C1515984	Carboxylesterase (CarE) Activity Assay Kit (1-Naphthyl Acetate, Colorimetric Method)	BioReagent	Host hydrolysis / prodrug activation	Suitable for routine-throughput CarE activity determination and can serve as a basic tool for evaluating prodrug activation capacity and for subsequent SN-38/SN-38G studies.
G755230	β-Glucuronidase from abalone	Purified, aqueous solution, β-glucuronidase 150,000-250,000 units/mL, β-glucuronidase≥20,000,000 units/g protein	Microbial deconjugation	Suitable for high-activity liquid enzyme systems and can be directly used for hydrolysis of glucuronide conjugates and evaluation of deconjugation efficiency.
G755235	β-Glucuronidase	Bioactive,Recombinant,ActiBioPure™,High Performance,EnzymoPure™,≥5000 U/mg enzyme powder; ≥30000 U/mg protein	Microbial deconjugation	Suitable for constructing standardized recombinant enzyme deconjugation models, facilitating substrate comparison, kinetic analysis, and inhibition studies.
G755246	β-Glucuronidase from Escherichia coli	ActiBioPure™, Bioactive, High Performance, EnzymoPure™, ≥10,000 U/mg protein	Microbial deconjugation	Suitable for routine enzymology experiments, substrate screening, and comparison of hydrolytic capacity toward different glucuronide conjugates.
G755240	β-Glucuronidase from Escherichia coli	Type VII-A, lyophilized powder, 5,000,000-20,000,000 units/g protein, pH 6.8 (30 min assay)	Microbial deconjugation	Suitable for deconjugation experiments requiring high activity and a stable liquid system, and can be used for mechanistic validation and kinetic studies.
G755237	β-Glucuronidase from Escherichia coli	Recombinant, ≥20,000,000 units/g protein, expressed in E. coli, aqueous glycerol solution	Microbial deconjugation	Suitable for rapid modeling in liquid enzyme systems and for in vitro hydrolysis of glucuronide conjugates and condition optimization.
G755228	β-Glucuronidase from limpets (Patella vulgata)	aqueous glycerol solution,≥5,000,000 units/g protein, pH 6.8 (biuret)	Microbial deconjugation	Suitable for comparing activities of β-glucuronidases from different sources and for studying substrate-source preferences.
G755241	β-Glucuronidase from limpets (Patella vulgata)	ActiBioPure™, Bioactive, BioPerformance Certified, EnzymoPure™, 1000-5000 U/mg protein	Microbial deconjugation	Suitable for source-difference comparison and routine hydrolysis experiments, and can serve as a supplementary non-bacterial enzyme model.
G755251	β-Glucuronidase from limpets (Patella vulgata)	aqueous solution,≥85,000 units/mL	Microbial deconjugation	Suitable for lyophilized enzyme formulation scenarios and for deconjugation experiments requiring long-term storage followed by reconstitution.
G755226	β-Glucuronidase from Helix aspersa (garden snail)	aqueous solution	Microbial deconjugation	Suitable for functional comparison of β-glucuronidases from different animal sources, expanding the range of deconjugation model sources.
G755232	β-Glucuronidase from bovine liver	Type L-II, lyophilized powder, 1,000,000-3,000,000 units/g solid	Microbial deconjugation	Suitable for comparative studies of deconjugation capacity between host-derived and microbially derived enzymes.
G755231	β-Glucuronidase from Helix pomatia	Type HA-4	Microbial deconjugation	Suitable for deconjugation validation in liquid enzyme systems and for analyzing activity differences among enzymes from different sources.
G755255	β-Glucuronidase from Helix pomatia	Type B-3,≥2,000,000 units/g solid	Microbial deconjugation	Suitable for substrate hydrolysis and rapid mechanistic validation in relatively high-activity liquid systems.
G755253	β-Glucuronidase from Helix pomatia	Type H-3AF, aqueous solution,≥60,000 units/mL	Microbial deconjugation	Suitable for reconstruction of glucuronide deconjugation reactions and comparison of source-dependent differences.
G755250	β-Glucuronidase from Helix pomatia	Type HP-2, aqueous solution,≥100,000 units/mL	Microbial deconjugation	Suitable for mechanistic studies under powdered enzyme system conditions and with partially purified enzyme preparations.
G755247	β-Glucuronidase from Helix pomatia	Type H-2, aqueous solution,≥85,000 units/mL	Microbial deconjugation	Suitable for substrate screening and stability studies using lyophilized enzyme preparations.
G755243	β-Glucuronidase from Helix pomatia	Type H-1, partially purified powder,≥300,000 units/g solid	Microbial deconjugation	Suitable for routine liquid enzyme hydrolysis systems and deconjugation experiments with good reproducibility.
M755239	MS β-Glucuronidase	Type H-5, lyophilized powder,≥400,000 units/g solid	Microbial deconjugation / sample preparation	Suitable for automated pretreatment and mass-spectrometry sample hydrolysis workflows, and can be used to establish analytical workflows after deconjugation of glucuronide conjugates.

5.2 Substrates and Tool Reagents for Research on Glucuronide Biotransformation

Name	CAS No.	Experimental step	Key use	Use notes
D-Glucaric acid-1,4-lactone monohydrate	61278-30-6	Enzyme inhibition studies	Used as a classical beta-glucuronidase inhibitor to validate the contribution of deconjugation to drug reactivation and intestinal toxicity	Suitable as a positive inhibitory control and mechanistic blocking tool
4-Nitrophenyl beta-D-glucuronide	10344-94-2	Enzyme activity assay	Used as a chromogenic substrate to measure beta-glucuronidase activity	Suitable for initial screening in crude microbial enzyme preparations, fecal bacterial suspensions, and purified enzyme systems
4-Methylumbelliferyl-beta-D-glucuronide	6160-80-1	Fluorometric enzyme activity assay	Used as a fluorescent substrate for highly sensitive detection of beta-glucuronidase activity	Suitable for microsamples and kinetic analysis
D-Glucuronic acid	6556-12-3	Product/metabolite analysis	Used as a deconjugation product or control substrate for method development and metabolic pathway validation	Suitable for LC-MS/MS or enzymatic control systems
4-Nitrophenol	100-02-7	Chromogenic detection	Used as the readout product standard after hydrolysis of 4-nitrophenyl beta-D-glucuronide	Suitable for standard curves and enzyme activity quantification
4-Methylumbelliferone	90-33-5	Fluorescent detection	Used as the fluorescent product standard after hydrolysis of 4-methylumbelliferyl-beta-D-glucuronide	Suitable for fluorometric activity quantification
Acetaminophen	103-90-2	Typical glucuronidated drug model	Used to study the intestinal remetabolic potential of drugs with a high proportion of glucuronidation	Suitable for comparison with sulfation pathways
Diclofenac sodium	15307-79-6	NSAID enterohepatic circulation studies	Used to analyze the relationship between glucuronide-related metabolite reflux and intestinal injury	Suitable for integrated pharmacokinetic-intestinal toxicity analysis
Indomethacin	53-86-1	NSAID-related control studies	Used for extended studies of acyl glucuronide-related intestinal injury and microbial involvement	More suitable for comparative studies among nonsteroidal anti-inflammatory drugs
Naproxen	22204-53-1	NSAID model	Used to analyze glucuronidation and recirculation of aromatic acid drugs	Suitable for pharmacokinetic time-course and biliary excretion studies
Ibuprofen	15687-27-1	NSAID model	Used to compare differences in conjugation and intestinal re-exposure among different arylpropionic acid drugs	Suitable for comparison with diclofenac and naproxen
Irinotecan hydrochloride trihydrate	136572-09-3	Representative drug model	Used to study the classic mechanism of “host glucuronidation-intestinal deconjugation-local reactivation”	Suitable for establishing irinotecan-related intestinal toxicity models
SN-38	86639-52-3	Toxicity reactivation studies	Used as the active irinotecan metabolite to construct models of local toxicity and re-exposure after deconjugation	Suitable for studies of intestinal toxicity and reactivation mechanisms
Mycophenolic acid	24280-93-1	Microbiota-dependent exposure studies	Used to study deconjugation of glucuronide metabolites of mycophenolic acid and their intestinal toxic consequences	Suitable for studies of gastrointestinal adverse reactions related to immunosuppressants
Ezetimibe	163222-33-1	Enterohepatic circulation studies	Used to analyze re-exposure and maintenance of absorption after extensive glucuronidation	Suitable for studies of intestinal transport and sustained exposure
Raloxifene hydrochloride	82640-04-8	Conjugation-deconjugation cycling studies	Used to study microbial reactivation phenomena of highly conjugated compounds with prominent enterohepatic circulation	Suitable as a supplementary model for highly conjugated drugs
Estradiol	50-28-2	Endogenous substrate model	Used to study microbial re-editing of glucuronide conjugates of hormone-like compounds	Suitable for host-microbiota co-metabolism research
Estrone	53-16-7	Endogenous substrate model	Used to compare deconjugation differences among different estrogen substrates	Suitable for studies of hormonal homeostasis and microbiota relationships
Ethinyl estradiol	57-63-6	Pharmaceutical hormone model	Used to study the problem of intestinal re-exposure of exogenous hormone drugs	Suitable for studies of oral hormonal formulations
Bilirubin	635-65-4	Endogenous glucuronidation-related model	Used to analyze bilirubin-related conjugation metabolism and the deconjugation process	More suitable for bile metabolism and microbiota interaction studies
Baicalin	21967-41-9	Natural product glucuronide conjugate model	Used to study deconjugation and reabsorption of conjugates derived from natural medicines	Suitable for in vivo transformation studies of traditional Chinese medicine
Baicalein	491-67-8	Deconjugated product model	Used as the aglycone after deconjugation of baicalin to study reabsorption and restoration of activity	Suitable for paired aglycone-conjugate studies
Glycyrrhizic acid	1405-86-3	Natural product model containing glucuronic acid structures	Used to study hydrolysis of glucuronic acid-related structures in natural ingredients by gut microbiota	Suitable for intestinal transformation studies of natural medicines
Glycyrrhetinic acid	471-53-4	Post-deglycosylation active product model	Used as the transformed product related to glycyrrhizic acid to analyze functional changes after deconjugation	Suitable for natural product reactivation studies
Scutellarin	27740-01-8	Natural flavonoid glycoside model	Used to study deglycosylation, reabsorption, and retransformation of flavonoid glycosides in the intestine	Suitable for studies of plant-derived metabolism
Scutellarein	529-53-3	Aglycone model	Used as the product after deconjugation of flavonoids to study exposure differences after structural remodeling	Suitable for paired comparison with flavonoid glycosides
Puerarin	3681-99-0	Extended model for natural product glycosides and gut microbial transformation	Used to study the transformation pathways of plant-derived glycosylated compounds by the gut microbiota	Suitable for microbiota-natural product interaction studies
Genistein	446-72-0	Flavonoid aglycone model	Used to analyze absorption and restoration of function after deconjugation of flavonoid structures	Suitable for studies of estrogen-like natural products
Daidzein	486-66-8	Microbial retransformation studies	Used to study further reduction and structural remodeling of flavonoid compounds by the microbiota	Suitable for analysis of downstream microbial transformation of aglycones

6. Common Misconceptions in Research on Gut Microbial Enzyme-Mediated Transformation of Glucuronide Conjugates

6.1 Common misconceptions

(1) Treating glucuronidation simply as terminal inactivation

This understanding overlooks the reactivation process during the intestinal phase and also underestimates the importance of enterohepatic circulation, local toxicity, and interindividual variability.

(2) Reducing microbial effects to simply “whether beta-glucuronidase is present”

The true outcome of drug biotransformation depends not only on whether the enzyme is present, but also on its activity level, substrate compatibility, microbial network structure, local transport, and reabsorption capacity. Therefore, the mere presence or absence of a single enzyme is not sufficient to replace a complete mechanistic analysis.

In essence, gut microbial enzyme-mediated metabolism of glucuronide conjugates is a re-editing process that occurs after host clearance metabolism. It may prolong effective drug exposure, cause local toxicity reactivation, and further reshape the terminal metabolic profile through additional microbial biotransformation. Research on this process should not remain at the level of the general statement that “the microbiota affects drug metabolism,” but should instead be further grounded in the continuous mechanistic chain composed of specific conjugates, specific deconjugating enzymes, specific reabsorption processes, and specific pharmacodynamic and toxicological consequences. Only by establishing a closed-loop explanatory framework linking host metabolism, microbial enzymology, and pharmacokinetics can the rules governing gut microbiota-mediated drug biotransformation be understood more accurately.

For more related articles, please see below:

[1] Intestinal flora testing experiments