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.
PAC, 2004, 76, 1033. 'Glossary of terms used in toxicokinetics (IUPAC Recommendations 2003)' on page 1064 (