[Absorption, metabolism and excretion of nicotine in humans]

Nicotine is an alkaloid present in many plants of Solanaceae family. The levorotatory enantiomer (S) is a naturally occurring form. Nicotine enters the human body as a component of tobacco smoke. In alkaline environment the rate of nicotine permeation through biological membranes is increased. Almost 90% of nicotine absorbed by the body is metabolized in the liver. Nicotine may also be metabolized in the kidneys, lungs, brain, and respiratory epithelium membranes. The nicotine undergoes many transformations. Key role in the metabolism of nicotine is played by cytochrome P450 oxidases (mainly CYP2A6). Apart from them, UDP-glucuronosyltransferases, cytosolic aldehyde oxidase, amine N-methyltransferase, and flavin-containing monooxygenase 3 are involved in the decomposition of nicotine. Six major metabolites of nicotine have been identified. One of the most important metabolite is cotinine, from which is formed of trans-3'-hydroxycotinine--the compound which is excreted in the largest amount within the urine. The rate of nicotine metabolism is affected by diversified activity of polymorphic enzymes involved in this process, diet, gender and physiological condition of the organism.