Phase I Reactions: Oxidation of Aliphatic and Aromatic Carbon-Containing Systems

Phase I biotransformation primarily involves microsomal enzymes that catalyze oxidative reactions, using molecular oxygen and NADPH. Notably, only one oxygen atom is incorporated into the metabolite.

Aromatic carbons, such as those in phenobarbital, undergo oxidation to form intermediate epoxides, which then rearrange to form metabolites like hydroxyphenobarbital, with reduced pharmacological activity.

Aliphatic carbon atoms, as seen in valproic acid, undergo terminal oxidation to yield hydroxy metabolites.

Benzylic and allylic carbon atoms are oxidized to carbinols, which are further oxidized to carbonyl compounds and acids, as observed in tolbutamide and hexobarbital metabolism.

The alpha carbons of carbonyl or imine groups are readily hydroxylated, as seen in diazepam.

Alicyclic carbon atoms, like those in minoxidil, are typically hydroxylated at the C-3 or C-4 positions.

These reactions are vital and can significantly impact the pharmacological activity of the resulting metabolites.