Conjugation With Glucuronic Acid

Phase II reaction

CONJUGATION WITH GLUCURONIC ACID

Also called as glucuronidation, it is the most common and most important phase II reaction for several reasons:

1.        Readily available source of conjugating moiety, D-glucuronic acid which is derived from D-glucose.

2.        Several functional groups viz. alcohols, acids, amines, etc. can combine easily with D-glucuronic acid.

3.        Quantitatively, conjugation with D-glucuronic acid occurs to a high degree.

4.        All mammals have the common ability to produce glucuronides,

5.        The free carboxyl function of glucuronic acid has a pK_a in the range 3.5 to 4.0 and hence ionisable at both plasma and urine pH thereby greatly increasing the water solubility of the conjugated substrate.

6.        The glucuronidation enzymes are in close association with the microsomal mixed function oxidases, the major phase I drug metabolising enzyme system; thus, a rapid conjugation of phase I metabolites is possible.

7.        Lastly, glucuronidation can take place in most body tissues since the glucuronic acid donor, UDPGA is produced in processes related to glycogen synthesis and thus, will never be deficient unlike those involved in other phase II reactions.

Glucuronide formation occurs in 2 steps –

1. Synthesis of an activated coenzyme uridine-5'-diphospho-α -D-glucuronic acid (UDPGA) from UDP-glucose (UDPG). The coenzyme UDPGA acts as the donor of glucuronic acid. UDPG is synthesized by interaction of α-D-glucose-1-phosphate with uridine triphosphate (UTP).

2. Transfer of the glucuronyl moiety from UDPGA to the substrate RXH in presence of enzyme UDP-glucuronyl transferase to form the conjugate. In this step, the - configuration of glucuronic acid undergoes inversion and thus, the resulting product is – β-D-glucuronide (also called as glucosiduronic acid or glucopyranosiduronic acid conjugate).

The steps involved in glucuronide synthesis are depicted below:

where X = O, COO, NH or S.

An example of glucuronidation of benzoic acid is shown below.

A large number of functional groups are capable of forming oxygen, nitrogen and sulphur glucuronides. Carbon glucuronides have also been detected in a few cases.

Oxygen or O-Glucuronides

Xenobiotics with hydroxyl and/or carboxyl functions form O-glucuronides.

1. Hydroxyl Compounds: These form ether glucuronides. Several examples of such compounds are given below.

Aliphatic alcohols e.g. chloramphenicol, trichloroethanol

Alicyclic alcohols e.g. hydroxylated hexobarbital

Arenols (phenols) e.g. morphine, paracetamol

Benzylic alcohols e.g. methyl phenyl carbinol

Enols e.g. 4-hydroxy coumarin

N-hydroxyl amines e.g. N-hydroxy dapsone

N-hydroxyl amides e.g. N-hydroxy-2-acetyl aminofluorine

2. Carboxyl Compounds: These form ester glucuronides

Aryl acids e.g. salicylic acid

Arylalkyl acids e.g. fenoprofen

Nitrogen or N-Glucuronides

Xenobiotics with amine, amide and sulphonamide functions form N-glucuronides.

Aliphatic 2^o amines e.g. desipramine

Aliphatic 3^o amines e.g. tripelennamine

Nonaromatic 3o heterocyclic amines e.g. cyproheptadiene

Amides e.g. meprobamate

Sulphonamides e.g. sulphadimethoxine

Sulphur or S-Glucuronides

Thiols (SH) form thioether glucuronides e.g. thiophenol.

Carbon or C-Glucuronides

Xenobiotics with nucleophilic carbon atoms such as phenylbutazone form C-glucuronides.

Certain endogenous compounds such as steroids, bilirubin, catechols and thyroxine also form glucuronides.