Just Say N.O.
By Sean Henahan, Access Excellence
Jolla, CA (October 24, 1997)- A new understanding of the structure
of nitric oxide synthase, the enzyme that regulates nitric oxide activity
in the body, offers key information useful for developing drugs for everything
from high blood pressure to cancer.
Nitric oxide (NO) performs a variety of tasks in the body. At high concentrations
it acts against infectious organisms and cancer cells. At lower concentrations
it helps regulate the circulatory and central nervous system. NO differs
from other neurotransmitters and hormones in that it is not regulated by
storage, release or targeted degradation, but rather solely by synthesis.
Researchers at the Scripps Research Institute have determined the structure
of nitric oxide synthase oxygenase, the enzyme that turns NO on and off.
Defining the structure of nitric oxide synthase (NOS) will help researchers
understand not only how NO is produced in the body but also how NO production
is controlled. "Having this structure is the difference between working
blind and seeing what you're doing in terms of understanding and drug design,"
notes John A. Tainer, Scripps Research Institute.
"NO appears to be one of the most important messenger molecules in the
body. Excess production appears to cause brain damage from stroke and also
inflammatory conditions. Drugs that block the enzyme could be important
therapeutically; this breakthrough may allow scientists to begin to design
drugs to inhibit it," said Dr. Solomon Snyder, a neuroscientist at Johns
Hopkins University whose research group was the first to clone and sequence
The Scripps group identified the three-dimensional structures of the
catalytic site of NOS, showomg in atomic detail how the enzyme recognizes
the amino acid arginine, its substrate, and oxidizes it to form the biological
signal NO. The team used a technique known as protein crystallography to determine
the NOS structures. This involves diffracting x-rays off of crystals grown
from the highly purified enzyme. The x-ray diffraction experiment provides
all the information necessary to create an atomic image of the protein.
X-ray radiation was needed in this experiment because the diffraction only
occurs when the size of the object is similar to the wavelength size of
The research prvides a clear understanding of where the reactive groups
are located and how the enzyme can control their interaction. The researchers
believe that the unexpected discovery of two adjacent binding sites for
the NOS inhibitor imidazole in the active site promises to aid in the design
of drugs to modulate NOS activity and prevent NO overproduction.
Dual-function inhibitors that simultaneously bind both of these sites
would block both arginine and oxygen binding, creating an expanded dual-site
binding region to increase affinity and prevent the formation of toxic,
reactive oxygen species. Since the characteristics of NOS inhibition vary
among different NOS types, these dual-function inhibitors also may lead
to new drugs that target only one of the various forms of NOS, thereby
limiting potential side effects.
The research could open the door for the development of many new drugs.
Diseases thought to involve too little NO production include hypertension,
impotence, arteriosclerosis, and a susceptibility to infection. Diseases
linked to excessive NO production include immune-type diabetes, neurotoxicity
associated with aneurysm, stroke and reperfusion injury, inflammatory bowel
disease, rheumatoid arthritis, cancer, septic shock, multiple sclerosis
and transplant rejection.
The research appears in the October 17, 1997 issue of Science