H

Figure 18.6 Conjugation (phase II) biotransformation reactions.

Notice the sulfhydryl (— SH) group on the cysteine. It is this that is bonded to the foreign molecule in the first step. For example, consider the reaction of glutathione with 1-chloro-2,4-dinitrobenzene shown in Figure 18.7a. The second step is removal of the two free amino acids leaving a thiol ester with cysteine. This is followed by the third and final step in Figure 18.7b, a reaction with acetyl-CoA to produce 2,4-dinitrophe-nyl mercapturic acid. This mechanism is effective with electrophilic compounds. It is an important detoxification mechanism for elimination of carcinogenic epoxides produced by the monooxygenases.

Cl + Y-Glu-CySH-Gly glutathione

2,4-dinitrochlorobenzene

Cl + Y-Glu-CySH-Gly glutathione

2,4-dinitrochlorobenzene

Y-Glu

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