Isoquinoline is a heterocyclic aromatic organic compound. It is a structural isomer of quinoline. Isoquinoline and quinoline are benzopyridines, which are composed of a benzene ring fused to a pyridine ring. In a broader sense, the term isoquinoline is used to make reference to isoquinoline...

is a chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

, and the structural backbone of many alkaloid
Alkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. This group also includes some related compounds with neutral and even weakly acidic properties. Also some synthetic compounds of similar structure are attributed to alkaloids...

s with a wide variety of structures, including papaverine
Papaverine is an opium alkaloid antispasmodic drug, used primarily in the treatment of visceral spasm, vasospasm , and occasionally in the treatment of erectile dysfunction...

, noscapine
Noscapine is a benzylisoquinoline alkaloid from plants of the Papaveraceae family, without significant painkilling properties. This agent is primarily used for its antitussive effects. It has also been shown to have anticancer activity...

, codeine
Codeine or 3-methylmorphine is an opiate used for its analgesic, antitussive, and antidiarrheal properties...

, morphine
Morphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...

, apomorphine
Apomorphine is a non-selective dopamine agonist which activates both D1-like and D2-like receptors, with some preference for the latter subtypes. It is historically a morphine decomposition product by boiling with concentrated acid, hence the -morphine suffix...

, berberine
Berberine is a quaternary ammonium salt from the protoberberine group of isoquinoline alkaloids. It is found in such plants as Berberis Berberine is a quaternary ammonium salt from the protoberberine group of isoquinoline alkaloids. It is found in such plants as Berberis Berberine is a quaternary...

, protopine
Protopine is a benzylisoquinoline alkaloid occurring in opium poppy, corydalis tubers and other plants of the family papaveraceae. It has been found to inhibit histamine H1 receptors and platelet aggregation, and acts as an opioid analgesic....

 and tubocurarine
Tubocurarine is a neuromuscular-blocking drug or skeletal muscle relaxant in the category of non-depolarizing neuromuscular-blocking drugs, used adjunctively in anesthesia to provide skeletal muscle relaxation during surgery or mechanical ventilation...



Plants producing benzylisoquinoline alkaloids have a common biosynthetic pathway, making use of two units of L-tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

. One tyrosine molecule is metabolised to dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

 which constitutes the isoquinoline part, while the benzylic part is mostly formed from tyramine
Tyramine is a naturally occurring monoamine compound and trace amine derived from the amino acid tyrosine. Tyramine acts as a catecholamine releasing agent...

, itself the decarboxylation product of tyrosine.

Many benzylisoquinolines have a methylated nitrogen atom as well as functional groups containing oxygen (-OH
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

, -OCH3
In chemistry , methoxy refers to the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula O–CH3.The word is used in organic nomenclature usually to describe an ether...

, -OCH2O-) in positions 6, 7, 3' and 4'. The latter come from the precursors mentioned above, namely tyrosine, dopamine and their derivatives.
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