1998 Nobel Prize Winners for Physiology or Medicine

A. F. Vanin

Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia

The Royal Swedish Academy of Sciences has awarded the 1998 Nobel Prize for Physiology or Medicine to American scientists Robert Furchgott (New York State University), Farid Murad (University of Texas at Houston), and Louis Ignarro (University of California at Los Angeles) for the discovery of the functional activity of nitric oxide (NO) in the cardiovascular system. This discovery has already found important medical applications. It is also of considerable importance to general biology. Only a few years after pioneering works of these researchers, it was clear that NO can be involved in functions performed by many metabolic and physiological systems in humans and other animals. An avalanche of publications on the biology of NO beginning from the late 1980s motivated the editorial board of Science to name NO the Molecule of the Year in 1992. It is now clear that NO is produced in animals from L-arginine by an enzymatic process mediated by the constitutive and inducible isoforms of NO synthase. As a vasodilating agent, NO is involved in regulation of blood vessel tone; it inhibits aggregation of platelets and their adhesion to vascular walls. NO causes relaxation of smooth muscles in vascular walls and the gastrointestinal tract. In blood vessels, NO is generated in the endothelial layer, whereas in the gastrointestinal tract it is derived from endings of nerve fibers in the stomach and intestine. The ability of the autonomic and central nervous systems to generate NO by enzymatic means was reported soon after the discovery of the cardiovascular effects of this molecule. This discovery owes much to works done by Furchgott, Murad, and Ignarro. It was also found that NO is involved in the regulation of respiratory and genitourinary organs by efferent nerves. In addition to regulatory functions, NO has cytotoxic and cytostatic effects. Generation of this agent by immune cells protects the body against bacterial and malignant cells. Studies of this mechanism of cell-mediated immunity were started in the 1970s, and the conclusion on the role of NO as an effector agent of this system was made in the 1980s, independently of works performed by Furchgott, Murad, and Ignarro. Nevertheless, their works stimulated the studies of the role of NO in immunity because the general biological significance of this agent has become clear. These data showed that the cytotoxic effects of NO can be observed in tissues and cells that do not belong to the immune system. The inducible isoform of NO synthase can be produced in vascular smooth muscles, myocardium, brain, secreting tissues, and other tissues stimulated by high levels of cytokines or other biologically active substances, by a process similar to that found in immune cells. When generated in considerable amounts, NO produces negative effects and even kills the cells and tissues. Such an overproduction of NO in blood vessels causes endoseptic shock with an abrupt decrease in blood pressure. There is evidence that the cytotoxic effect of NO is due to its involvement in apoptosis (programmed cell death).

The discovery of the functional activity of NO in the cardiovascular system was a full surprise to the scientific community. Nitric oxide was a known industrial pollutant, like sulfur oxides, whose appearance in the atmosphere results in its pollution and causes acid rains. High concentrations of NO are found in tobacco smoke. Because of this, studies of the biological actions of this agent were focused on its adverse effects on humans and animals. Naturally, this precluded studies of its "positive" metabolic effects. The first breakthrough in NO research, which dispelled its image of a deleterious agent, was made in the middle of the 1970s, when Murad et al. showed that NO can activate guanylate cyclase, an important intracellular enzyme that converts GTP into cGMP, a second messenger. This resulted from their search for agents that activate guanylate cyclase directly. In contrast to adenylate cyclase, a membrane-bound enzyme responsible for the synthesis of cAMP (also a second messenger) that can be activated through an appropriate membrane receptor, guanylate cyclase can be activated in a soluble form, that is, this process may not be receptor-mediated. By that time, it had been shown that various vasodilators containing nitro or nitroso groups are potent activators of guanylate cyclase. Murad and his colleagues were the first to hypothesize that this activation was due to the ability of nitro and nitroso compounds to produce NO in biological preparations. Results of their experiments were fully consistent with this hypothesis: treatment of guanylate cyclase preparations isolated from various tissues with NO caused a 10-30-fold increase in cGMP production by these preparations. This result of Murad's work was awarded the Nobel Prize.

Studies performed by Murad et al. explained the mechanism of vasodilator effects of various nitro and nitroso compounds, including widely used cardiovascular drugs such as nitroglycerin. These drugs generate NO, which activates vascular guanylate cyclase. Cyclic GMP generated by this enzyme activates a complex of biochemical processes that release calcium from vascular smooth muscle cells, relax them, and relieve spasms. The group led by Nobel Prize winner Ignarro was among the first to study the effects of NO on isolated blood vessels. Their results were fully consistent with these hypotheses. Thus, a specific historical circle was closed. Indeed, the founder of the Nobel Prize provided its financial basis by his invention of dynamite, whose active component is nitroglycerin. And now studies of the biological effects of this compound are awarded the Nobel Prize! This peculiar relationship of discoveries aimed at extermination of humanity on one side and saving human lives on the other deserves more profound analysis in the future.

(Interestingly, nitroglycerin, a drug with a newly discovered therapeutic effect, was prescribed to Nobel who suffered from heart disease before his death. The great patron of the science refused to believe in the beneficial effect of an explosive matter and rejected the physician's recommendation (Chief-Editor's note)).

Having discovered the "positive" biological activity of NO, Murad pursued this line of research. He postulated that this agent is produced in the body from endogenous sources when hormones or other biologically active substances act on cells or tissues. This prescient assumption was fully confirmed by Furchgott, who discovered the so-called endothelium-derived relaxing factor (EDRF) in 1980. This discovery resulted from studies of the effects of acetylcholine on the tone of isolated blood vessels. This agent was known to contract vascular smooth muscle cells. The endothelial layer was removed in order to facilitate the contact of acetylcholine with smooth muscle cells because vascular endothelium was regarded as a barrier protecting these cells. In one experiment the endothelium was erroneously left intact, and acetylcholine surprisingly relaxed rather than contracted this preparation. Detailed examination of this phenomenon led Furchgott to conclude that vascular endothelium works not only as a mechanical barrier. Its more important function is to produce, upon a contact with acetylcholine or other physiologically active substances used in Furchgott's experiments, the agent called EDRF, which can relax vascular smooth muscle cells. Data on the dilating effect of NO on blood vessels reported by that time, as well as previous results of Furchgott who found that nitrite anions exert vasodilating effects, helped him to identify the nature of this product. In 1986, Furchgott and Ignarro found in independent studies that EDRF and nitric oxide display very similar physicochemical and functional properties. They suggested that NO was at least the active principle of EDRF, which was confirmed by chemico-physiological studies of S. Moncada in 1987. These results of Furchgott and Ignarro were awarded the 1998 Nobel Prize.

The Nobel Committee thus marked the formation of a large field of biological science--the biology of nitric oxide. This field embraces a broad spectrum of research in virtually all branches of biology. Currently, considerable interest is attracted to the ability of NO and its derivatives to regulate the expression of synthesis of certain proteins at the levels of transcription and translation, as well as its activating and inhibitory effects on many enzymes. An intriguing problem is which redox form (ionized or neutral) of NO causes the biological effects. Studies of the role played by NO in apoptosis are promising. Current research accumulates evidence that determination of mechanisms of biological effects of NO will facilitate resolution of many fundamental problems of biology and result in considerable progress in medicine because of better understanding of the nature of various diseases and designing new medicinal drugs.

Readers who wish to expand their knowledge in this new field of biology are addressed to research and review publications [1-7] and an issue of Biochemistry (Moscow) (Vol. 63, No. 7, 1998) entirely devoted to the biological role of NO.

REFERENCES

1.Furchgott, R. F., and Jothianandan, D. (1991) Blood Vessels, 28, 52-61.
2.Murad, F. (1994) Neurotransmission, 10, 1-4.
3.Ignarro, L., and Murad, F. (eds.) Nitric Oxide: Biochemistry, Molecular Biology, and Therapeutic Implications (1995) Adv. Pharmacol., 34, 1-516.
4.Moncada, S., Palmer, R. M. J., and Higgs, E. A. (1991) Pharmacol. Rev., 43, 109-142.
5.Nathan, C. (1992) FASEB J., 6, 3051-3064.
6.Stamler, J. S. (1994) Cell, 78, 931-936.
7.Drapier, J.-C. (1996) BioEssays, 18, 549-556.

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