Sometimes the way you turn something off can be just as importantas how you turn something on. In the case of the cellular messengernitric oxide (NO), scientists are now just beginning to discover howthe body controls this vital messenger.

Researcher from Johns Hopkins University School of Medicine, inBaltimore, have discovered a protein that not only turns off theproduction of NO, but appears to be a protein so biologicallyimportant that its sequence has been highly conserved throughout thecourse of evolution. Their results are published in the Nov. 1Science.

"It is pretty likely that this protein will play many roles," saidSolomon Snyder professor at the Johns Hopkins University School ofMedicine.

NO plays a crucial role in a number of delicately regulated biologicalprocesses ranging from the dilation of blood vessels and othersmooth muscles to serving as a neurotransmitter in the brain. For allthat, controlling the amount of NO at any given time relies upon thebody's ability to control the enzymes that produce it. As anevanescent gas, NO cannot be stored for later use like all the otherneurotransmitters, and because it is such a tiny gaseous molecule,there is no way to break it down once it has been made.

"You have to make it when you need it," Snyder said. "And, makesure you don't make it when you don't need it."

As a result, the various nitric oxide synthases (NOS) are the mostregulated enzymes in biology. While scientists have discoveredseveral mechanisms to turn on NO production, no protein inhibitorfor these enzymes had been discovered. Snyder and graduate studentSamie Jaffrey began a search for a protein inhibitor of neuronal nitricoxide synthase (nNOS) by looking at the proteins made in thehippocampus of rats.

They found an 89 amino acid protein that they subsequently calledPIN (protein inhibitor of nitric oxide synthase) which at very lowconcentrations could turn down the production of NO by nNOS.Because nNOS functions as a dimer, PIN stymies the enzyme bypreventing the two identical halves of the nNOS from associating.

One of NO's primary duties in the brain is to cause cells to makecyclic guanosine monophosphate which sets off a cascade ofactivities that cause neurons to fire. The researchers found that PINcould interrupt this normal activity by preventing nNOS fromproducing NO.

PIN, however, isn't specific for nNOS. The researchers found that itbinds to between 10 and 20 other proteins that they have yet toidentify. Surprisingly, when Snyder and Jaffrey searched throughseveral gene data bases, they found that PIN is highly conservedamong species. The PIN protein appears to be 100 percent conservedbetween the human and the mouse. Between humans and the bacteriaSchistosoma mansoni (parasitic flat worms) there is a 63 percenthomology.

"This may very well be the most conserved protein in biology,"Snyder said. "Presumably, PIN must have all kinds of functions. Thechallenge is to find out what it does."

Rockefeller University professor of neuroscience Paul Greenguardagreed that PIN may well have very broad biological functions. "Themain things are the protein's ubiquity and the fact that it is a potentinhibitor of [nNOS]," Greenguard said. "[Jaffrey and Snyder's]findings strongly suggest that [PIN] is going to turn out to be a veryimportant regulatory substance in both smooth muscle and brain." n

-- Lisa Seachrist Washington Editor

(c) 1997 American Health Consultants. All rights reserved.