By Sean Henahan, Access Excellence

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 NOS,

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 radiation.

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