Studies shed new light on how nitric oxide is produced
By Leigh MacMillan
January 19, 2012
Studies that started as part of an exploratory collaboration for Judy Aschner, M.D., took an unexpected turn and have led to a whole new understanding of how nitric oxide — an important signaling molecule in the body — is produced.
“Our findings surprised everyone,” said Aschner, the Julia Carell Stadler Professor of Pediatrics and director of the Division of Neonatology at Vanderbilt University.
In the December issue of Nature Medicine, Aschner and colleagues at Baylor College of Medicine report that an enzyme called ASL (argininosuccinate lyase) — part of the urea cycle that removes ammonia from the body — plays a previously unrecognized and pivotal role in nitric oxide production.
Among its many biological activities, nitric oxide relaxes blood vessels and increases blood flow (this is the basis for nitroglycerin’s use in treating the heart condition angina).
Aschner’s interest in the molecule stems from her clinical experiences in the neonatal intensive care unit, where babies often have lung disease accompanied by pulmonary hypertension (high blood pressure in the arteries of the lungs). Nitric oxide, a gas, is one of few options for treating pulmonary hypertension in newborns, but it’s expensive and difficult to administer, Aschner said.
“We’ve focused for many years on the nitric oxide pathway to find new therapies that might improve outcomes for babies with pulmonary hypertension,” Aschner said.
In the body, the enzyme nitric oxide synthase (NOS) produces nitric oxide from the amino acid arginine. Aschner and colleagues had discovered that NOS is present in a multi-protein complex, which is important for the activation of NOS and production of nitric oxide.
The enzyme ASL is also present in the complex, and the current studies establish that ASL is required for the formation of the complex. The investigators demonstrate that mice with reduced expression of ASL do not assemble the multi-protein complex and do not produce enough nitric oxide.
The findings also provide new insights to a scientific mystery – the so-called “arginine paradox” — that supplementing cells with extra arginine increases nitric oxide production even though there is more than enough arginine in the cell to saturate the NOS enzyme.
Now, Aschner said, it appears that “NOS can’t use just any arginine molecule that’s around; arginine has to be delivered directly to NOS through this multi-protein complex.”
Aschner is particularly excited about the implications of the findings for babies with pulmonary hypertension. Preterm babies have “biochemical immaturity” of many enzymes systems, she said, including urea cycle enzymes like ASL, which could increase their risk for chronic lung disease and pulmonary hypertension by reducing nitric oxide production.
As part of a new National Institutes of Health grant to Aschner, her team is enrolling preterm newborns to examine biomarkers in the nitric oxide-urea cycle pathway (including the amino acids arginine and citrulline).
“If it turns out that babies that go on to develop chronic lung disease have deficiencies in arginine or citrulline, then giving these amino acids would represent a dietary therapy that should be safe and has the potential to be a game changer for premature babies.”
She and her colleagues have already demonstrated in two animal models that dietary citrulline supplements reverse neonatal lung disease.
The idea of citrulline as a therapy (it is being tested for the prevention of pulmonary hypertension in children undergoing heart surgery) was born at Vanderbilt in studies spearheaded by Rick Barr, M.D., and Marshall Summar, M.D. The new findings have added a mechanistic understanding to citrulline’s actions as part of the nitric oxide production cycle.
“I think we’re on the cusp of something that could change the face of how we treat infants at risk for chronic lung disease or with chronic forms of pulmonary hypertension,” Aschner said. “I am really excited about that because we currently have no good preventative or treatment strategies for infants with these conditions.”
The research was supported by grants from the National Institutes of Health.