Xenon is a noble gas used as an anesthetic agent. Xenon can associate with amino acid side chains of the active site of enzymes such as serine proteinases (including elastases and collagenases); these enzymes can form a specific binding cavity for single xenon atom without inducing major changes in protein structure. It has been demonstrated that xenon binds within the heme cavity of cytochrome P-450 monooxygenases and is capable of inhibiting the catalytic activity of some enzymes in vitro. Current evidence suggests that inhibition of N-methyl-D-aspartate (NMDA) receptor signaling is the mechanism by which xenon induces anesthesia. Although approved for use in anesthesia in Russia in 2000, it was unavailable for use in western countries until October 2005 when approval for its use was granted in Germany.