Glucuronidation Pathways of 5- and 7-Hydroxypropranolol: Determination of Glucuronide Structures and Enzyme Selectivity

Propranolol, a non-selective beta-blocker medication, has been utilized in the treatment of cardiovascular diseases for several decades. Its hydroxynaphthyl metabolites have been recognized to possess varying degrees of beta-blocker activity due to the unaltered side-chain. This study achieved the successful separation and identification of diastereomeric glucuronic metabolites derived from 4-, 5-, and 7-hydroxypropranolol (4-OHP, 5-OHP, and 7-OHP) in human urine. Subsequently, reaction phenotyping of 5- and 7-hydroxypropranolol by different uridine 5'-diphospho-glucuronosyltransferases (UGTs) was carried out, with a comparison to the glucuronidation of 4-hydroxypropranolol (4-OHP). Among the 19 UGT enzymes examined, UGT1A1, UGT1A3, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2A1, and UGT2A2 were found to be involved in the glucuronidation of 5-OHP. Furthermore, UGT1A6 exhibited glucuronidation activity towards 7-OHP, along with the aforementioned eight UGTs. Results obtained by glucuronidation of corresponding methoxypropranolols and MS/MS analysis of 1,2-dimethylimidazole-4-sulfonyl (DMIS) derivatives of hydroxypropranolol glucuronides suggest that both the aromatic and aliphatic hydroxy groups of the hydroxypropranolols may be glucuronidated in vitro. However, the analysis of human urine samples collected after the administration of propranolol leads us to conclude that aromatic-linked glucuronidation is the preferred pathway under physiological conditions.

Exploring the Chemical Space of Proluciferins as Probe Substrates for Human Cytochrome P450 Enzymes

We report the synthesis of 21 new proluciferin compounds that bear a small aliphatic ether group connected to the 6' hydroxy function of firefly luciferin and either contain an acid or methyl ester function at the dihydrothiazole ring. Each of these compounds was found to be a substrate for some members of the human CYP1 and CYP3 families; a total of 92 new enzyme-substrate pairs were identified. In a screen of the whole human P450 complement (CYPome) with three selected proluciferin acid substrates, another 13 enzyme-substrate pairs were detected, which involve enzymes belonging to the CYP2, CYP4, CYP7, CYP21, and CYP27 families. All in all, we identified new probe substrates for members of seven out of 18 human CYP families.

Complete Reaction Phenotyping of Propranolol and 4-Hydroxypropranolol with the 19 Enzymes of the Human UGT1 and UGT2 Families

Propranolol is a competitive non-selective beta-receptor antagonist that is available on the market as a racemic mixture. In the present study, glucuronidation of propranolol and its equipotent phase I metabolite 4-hydroxypropranolol by all 19 members of the human UGT1 and UGT2 families was monitored. UGT1A7, UGT1A9, UGT1A10 and UGT2A1 were found to glucuronidate propranolol, with UGT1A7, UGT1A9 and UGT2A1 mainly acting on (S)-propranolol, while UGT1A10 displays the opposite stereoselectivity. UGT1A7, UGT1A9 and UGT2A1 were also found to glucuronidate 4-hydroxypropranolol. In contrast to propranolol, 4-hydroxypropranolol was found to be glucuronidated by UGT1A8 but not by UGT1A10. Additional biotransformations with 4-methoxypropanolol demonstrated different regioselectivities of these UGTs with respect to the aliphatic and aromatic hydroxy groups of the substrate. Modeling and molecular docking studies were performed to explain the stereoselective glucuronidation of the substrates under study.

Functional Expression of All Human Sulfotransferases in Fission Yeast, Assay Development, and Structural Models for Isoforms SULT4A1 and SULT6B1

Cytosolic sulfotransferases (SULTs) catalyze phase II (conjugation) reactions of drugs and endogenous compounds. A complete set of recombinant fission yeast strains each expressing one of the 14 human SULTs was generated, including SULT4A1 and SULT6B1. Sulfation of test substrates by whole-cell biotransformation was successfully demonstrated for all enzymes for which substrates were previously known. The results proved that the intracellular production of the cofactor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) necessary for SULT activity in fission yeast is sufficiently high to support metabolite production. A modified variant of sulfotransferase assay was also developed that employs permeabilized fission yeast cells (enzyme bags). Using this approach, SULT4A1-dependent sulfation of 1-naphthol was observed. Additionally, a new and convenient SULT activity assay is presented. It is based on the sulfation of a proluciferin compound, which was catalyzed by SULT1E1, SULT2A1, SULT4A1, and SULT6B1. For the latter two enzymes this study represents the first demonstration of their enzymatic functionality. Furthermore, the first catalytically competent homology models for SULT4A1 and SULT6B1 in complex with PAPS are reported. Through mechanistic molecular modeling driven by substrate docking, we pinned down the increased activity levels of these two isoforms to optimized substrate binding.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

Rapid and convenient biotransformation procedure for human drug metabolizing enzymes using permeabilized fission yeast cells

The development of convenient assays for the in vitro study of drug metabolizing enzymes (DMEs) such as cytochromes P450 (CYPs) and UDP-glucuronosyltransferases (UGTs) greatly facilitates metabolism studies of candidate drug compounds and other xenobiotics. We have developed and optimized an experimental approach that combines the advantages of recombinant expression in yeast with a microsomal-like biotransformation and thus allows for rapid and convenient enzymatic assays. Recombinant strains of the fission yeast Schizosaccharomyces pombe have previously been demonstrated to functionally express human CYPs and UGTs. Permeabilization of such cells with Triton X-100 results in the formation of enzyme bags, which can be used as biocatalysts. This protocol describes the preparation of such enzyme bags (3 h) and their application in enzyme activity assays (4 h) utilizing either pro-luminescent substrates and luminescence measurements or non-luminescent substrates and liquid chromatography coupled to mass spectrometry (LC-MS). Both applications provide practical tools for investigating CYP and UGT reactions in vitro without the need for additional sophisticated instrumentation or expertise.

Screening of the whole human cytochrome P450 complement (CYPome) with enzyme bag cocktails

We have previously introduced the use of permeabilized fission yeast cells (enzyme bags) that recombinantly express full-length CYPs for drug metabolism studies. Such enzyme bags are cells with pores that function as enzymes in situ. They can easily be prepared without a need for ultracentrifugation and may be used in similar protocols as microsomes. In this study we report the preparation of enzyme bag cocktails that permit the testing of multiple CYPs in a single enzyme bag reaction. Moreover, we established a convenient testing scheme that permits a rapid screen of all human CYPs for activity towards any given candidate substrate. An important aspect of this approach is the reduction of individual CYP test assays. If a cocktail containing many CYPs tests negative, it follows that all CYPs included in that cocktail need not be tested individually, thus saving time and resources. The new protocol was validated using two probe substrates.

Drug and Chemical Glucosidation by Control Supersomes and Membranes from Spodoptera frugiperda (Sf) 9 Cells: Implications for the Apparent Glucuronidation of Xenobiotics by UDP-glucuronosyltransferase 1A5

Accumulating evidence indicates that several human UDP-glucuronosyltransferase (UGT) enzymes catalyze both glucuronidation and glucosidation reactions. Baculovirus-infected insect cells [Trichoplusia ni and Spodoptera frugiperda (Sf9)] are used widely for the expression of recombinant human UGT enzymes. Following the observation that control Supersomes (c-SUP) express a native enzyme capable of glucosidating morphine, we characterized the glucosidation of a series of aglycones with a hydroxyl (aliphatic or phenolic), carboxylic acid, or amine functional group by c-SUP and membranes from uninfected Sf9 cells. Although both enzyme sources glucosidated the phenolic substrates investigated, albeit with differing activities, differences were observed in the selectivities of the native UDP-glucosyltransferases toward aliphatic alcohols, carboxylic acids, and amines. For example, zidovudine was solely glucosidated by c-SUP. By contrast, c-SUP lacked activity toward the amines lamotrigine and trifluoperazine and did not form the acyl glucoside of mycophenolic acid, reactions all catalyzed by uninfected Sf9 membranes. Glucosidation intrinsic clearances were high for several substrates, notably 1-hydroxypyrene (∼1400-1900 µl/min⋅mg). The results underscore the importance of including control cell membranes in the investigation of drug and chemical glucosidation by UGT enzymes expressed in T. ni (High-Five) and Sf9 cells. In a coincident study, we observed that UGT1A5 expressed in Sf9, human embryonic kidney 293T, and COS7 cells lacked glucuronidation activity toward prototypic phenolic substrates. However, Sf9 cells expressing UGT1A5 glucosidated 1-hydroxypyrene with UDP-glucuronic acid as the cofactor, presumably due to the presence of UDP-glucose as an impurity. Artifactual glucosidation may explain, at least in part, a previous report of phenolic glucuronidation by UGT1A5.