N

Ring G

Garbonyl G, ring G

Garbonyl G

G bound to O

NADPH, NADH

Aldehydes, ketones Garbonyl G Aldehydes, ketones Garbonyl G

aCytochrome P450.

b Reduced form of nicotinamide adenine dinucleotide phosphate. c Flavin-containing monooxygenase. dFlavin adenine dinucleotide. e Monoamine oxidase.

fXanthine oxidoreductase (xanthine oxidase and xanthine dehydrogenase). 9Aldo-keto reductase.

The organism's defense mechanism is completed by two additional classes of enzymes. The conjugating enzymes catalyze covalent bond formation between highly polar species (e.g., glucuronic acid) and a suitable functional group (usually hydroxyl) of the xenobiotic. The hydrolytic enzymes operate primarily on esters and amides, generating alcohols and acids or amines and acids, respectively. Both classes serve as mechanisms to enhance the elimination of highly lipoidal target molecules by increasing their water solubility through chemical modification.

The content of this chapter is focused on the metabolic reactions catalyzed by the enzyme systems listed in Table 1. Conjugation and hydrolytic reactions are the subjects of Chapter 5.06.

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