Enzymic modification of a substance by oxidation, reduction, hydrolysis, hydration, dehydrochlorination or other reactions catalysed by enzymes of the cytosol, of the endoplasmic reticulum (microsomal enzymes) or of other cell organelles. Broader Term: biotransformation. Related term: cytochrome P-420, cytochrome P-448, cytochrome P-450,microsome, phase 2 reaction, phase 3 reaction