PRINCIPLES OF TOXICOLOGY - Biology East Borneo

58 BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATIONAbbreviations: a = major absorption sites; e = excretion sites; f = filtration sites; m = major metabolism site; p = metabolic product;s = secretion sites; x = xenobiotic.Figure 3.1 Diagram of major sites of xenobiotic absorption, metabolism, and excretion.tion from the renal tubule following glomerular filtration or active secretion (f and s, respectively, inFigures 3.1 and 3.2) and from the gastrointestinal tract following biliary secretion. Biotransformationalso decreases the entry of xenobiotics into cells of all organs and makes them more suitable forsecretion by active transport mechanisms into the bile and urine. Active secretion requires both energyand a carrier protein and is capable of forcing molecules up a chemical gradient. Of the carriermolecules, those that recognize and transport organic acids have particular importance for drugconjugates since they can carry glucuronides, sulfate esters, and amino acid conjugates. While