phase I reaction

of biotransformation
Enzymic modification of a substance by oxidation, reduction, hydrolysis, hydration, dehydrochlorination, or other reactions catalyzed by enzymes of the cytosol, of the endoplasmic reticulum (microsomal enzymes) or of other cell organelles.
Source:
PAC, 2004, 76, 1033 (Glossary of terms used in toxicokinetics (IUPAC Recommendations 2003)) on page 1064