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6-monoacetylmorphine (6-MAM) is one of 2 active metabolites of heroin (diacetylmorphine), the other being the much less active 3-monoacetylmorphine (3-MAM).
Along with morphine, 6-MAM is rapidly created from heroin in the body, and is excreted in the urine. Since 6-MAM is a unique metabolite to heroin, its presence in the urine confirms that heroin was the opioid used. This is significant because on a urine immunoassay drug screen, the test typically tests for morphine, which is a metabolite of a number of legal and illegal opiates/opioids such as codeine, morphine sulphate, and heroin. 6-MAM remains in the urine for no more than 24 hours so a urine specimen must be collected soon after the last heroin use, but the presence of 6-MAM guarantees that heroin was in fact used as recently as within the last day. 6-MAM is naturally found in the brain, but in such small quantities that detection of this compound in urine virtually guarantees that heroin has recently been consumed.
Heroin is rapidly metabolized by esterase enzymes in the brain and has an extremely short half life . It has also very weak affinity to μ-opioid receptors because the 3-hydroxy group which is essential for binding to the receptor is masked by the acetyl group. So heroin is just a pro-drug , it is 6-MAM and other minor metabolites that actively bind with μ-opioid receptors.  
6-monoacetylmorphine already has a free 3-hydroxy group and shares the high lipophilicity of heroin, so it penetrates the brain just as quickly and does not need to be deacetylated at the 3-position in order to be bioactivated; this makes 6-monoacetylmorphine somewhat more potent than heroin, but it is rarely encountered as an illicit drug due to the difficulty in selectively acetylating morphine at the 6-position without also acetylating the 3-position. This can however be accomplished by using acetic acid with an appropriate catalyst to carry out the acetylation, rather than acetic anhydride, as acetic acid is not a strong enough acetylating agent to acetylate the phenolic 3-hydroxy group but is able to acetylate the 6-hydroxy group, thus selectively producing 6-MAM rather than heroin.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Monoacetylmorphine". A list of authors is available in Wikipedia.|