Acute and Subchronic Toxicity?of Teucrium polium Total Extract in Rats
Iranian Journal of Pharmaceutical Research (2006)
4: 229-230
Copyright ? 2005 by School of Pharmacy Shaheed Beheshti University of Medical Sciences and Health Services
Editorial
Does Nature Prefer Heterocycles?
Hossein Komeilizadeh
Heterocycles make up an exceedingly important
class of compounds. In fact more than half of all known organic compounds
are heterocycles. Many natural drugs such as quinine, papaverine, emetine,
theophylline, atropine, procaine, codeine, morphine and reserpine are
heterocycles. Almost all the compounds we know as synthetic drugs such as
diazepam, chlorpromazine, isoniazid, metronidazole, azidothymidine,
barbiturates, antipyrine, captopril and methotrexate are also heterocycles.
Some dyes (e.g. mauveine), luminophores, (e.g. acridine orange), pesticides
(e.g. diazinon) and herbicides (e.g. paraquat) are also heterocyclic in
nature.
All these natural and synthetic heterocyclic
compounds can and do participate in chemical reactions in the human body.
Furthermore, all biological processes are chemical in nature. Such
fundamental manifestations of life as the provision of energy, transmission
of nerve impulses, sight, metabolism and the transfer of hereditary
information are all based on chemical reactions involving the participation
of many heterocyclic compounds, such as vitamins, enzymes, coenzymes, ATP,
DNA, RNA and serotonin. Why does nature utilize heterocycles? The answer to
this question is provided by the fact that heterocyles are able to get
involved in an extraordinarily wide range of reaction types. Depending on
the pH of the medium, they may behave as acids or bases, forming anions or
cations. Some interact readily with electrophilic reagents, others with
nucleophiles, yet others with both. Some are easily oxidized, but resist
reduction, while others can be readily hydrogenated but are stable toward
the action of oxidizing agents. Certain amphoteric heterolcyclic systems
simultaneously demonstrate all of the above-mentioned properties. The
ability of many heterocycles to produce stable complexes with metal ions has
a great biochemical significance. The presence of different heteroatoms
makes tautomerism ubiquitous in the heterocyclic series. Such versatile
reactivity is linked to the electronic distributions in heterocyclic
molecules. Evidently, all the natural products and the synthetic drugs
mentioned above are good examples of nature?s preference for heterocycles
whose biological activity cannot be determined by one or a combination of
two or three of the above-mentioned properties.
The following is a good example of how man
learnt to imitate nature by incorporating heterocycles into drug molecules
to enhance their biological activity: Red sulfanilamide and the less toxic
white sulfanilamide (second generation) were the first sulfa drugs, and
these contained no heterocyclic fragments. However, the intensive research
work that followed their discovery demonstrated that modification of the p-aminobenzenesulfonamide
structure by the introduction of heterocyclic substituents into the amide
markedly enhanced their biolgical activity. Many derivatives of this type,
including the well-known sulfathiazole, sulfadimidine, sulfadimethoxine and
others, were gradually introduced into clinical treatment. Sufla drugs are
highly efficient against many bacterial species and against some protozoa.
Gastrointestinal infections, meningitis, tuberculosis, scarlet fever and
other diseases have been successfully treated by such prepartions. With the
passage of time, however, the increasing evidence of clinical toxicity of
these drugs has led to a diminution in their use, and they have been
replaced by penicillins, cephalosporins and, more recently, quinolone drugs
which are all heterocyclic in structure.
The role played by the hetrocycle imidazole
in the interaction of enzymes with substrates is perhaps the most
illustrative example of the importance of heterocycles in biochemical
systems.
Let?s consider this:
The histidine residue is a constituent of the
active site in many enzymes. The imidazole ring of histidine has a series of
unique properties, enabling it to show catalytic activity. Firstly, the
rather high basicity enables histidine both to form strong hydrogen bonds
and also to abstract a proton from the acidic groups, such as the OH group
of water and alcohols. Since an RO- anion is a much stronger nucleophile
than a neutral ROH molecule, the imidazole ring can catalyze a nucleophilic
addition to a carbonyl group.
In a living organism, this type of catalysis
is represented by the hydrolytic cleavage of protein amide bonds. The
participating enzymes are called proteases. Histidine and serine residues
are constituents of the protease chymotrypsin. The following is a simplified
mechanism for the action of chymotrypsin. Within the enzyme, the imidazole
ring of a histidine and the hydroxy group of a serine are bound by hydrogen
bonding. When a protein molecule approaches, the imidazole nitrogen
abstracts a proton from the OH group, thus activating the serine oxygen atom
towards attack at the carbonyl carbon atom in the polypeptide. The unstable
activated complex is called an ?enzyme-substrate complex?. Further
conversion brings about cleavage of the amide bond and acylation of the
enzyme at its? hydroxy group. By an analogous catalytic mechanism,
subsequent hydrolysis of the ester bond occurs with elimination of acid
(RCO2H) and regeneration of the enzyme.
The imidazole ring in chymotrypsin functions
in an amphoteric manner, i.e., both as an NH acid and a base. Significantly,
the basicity of histidine is such that it exists 50% as the neutral form and
50% as the imidazolium cation at the physiological pH of 7.4.
In conclusion, it can be questioned why it is
specifically appropriate to emphasize the role of heterocycles, since
analogies to the roles of other classes of organic compounds are easily
found. In fact, dyes, luminophores, pesticides, herbicides and drugs do not
necessarily have to be hetorocyclic in structure. In a similar fashion there
are many common features in chemistry and physics between such related
compounds as pyrrole and aniline, or between pyridine and nitrobenzene.
Nevertheless, nature selected the heterocycles pyrrole and pyridine, and not
the homocycles aniline and nitrobenzene, as the basis of most essential
biological systems. We now know the reason for this: the introduction of a
heteroatom into a cyclic compound imparts new properties. Heterocycles are
chemically more flexible and better able to respond to the many demands of
biochemical systems.
Dr. Hossein Komeilizadeh is currently associate professor of chemistry
and head of the Deparment of Pharmaceutical Chemistry, School of Pharmacy,
Shaheed Beheshti University Medical Sciences.
He can be reached at koushiar2004@yahoo.com.