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Biological Role of Nitric Oxide: History, Modern State, and Perspectives for Research

A. F. Vanin

Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 117977 Russia; fax: (095) 938-2156; E-mail: mikoyan@center.chph.ras.ru

Received April 20, 1998
This paper is an introduction to this issue of review papers on the biological role of nitric oxide. The history, modern state, and promising directions for research in this field are briefly considered.
KEY WORDS: nitric oxide, biological systems

During the last decade a particularly important discovery has been made in biology. One of the simplest compounds, nitric oxide, was found to be continuously produced in man and animals, acting as a universal regulator of metabolism. Since the late 1980s an avalanche-like growth of publications on NO biology began, and in 1992 Science announced NO as "molecule of the year" [1]. It is now clearly recognized that NO participates in the control of vascular tone as an antagonist of the adrenergic regulatory system. It inhibits aggregation of platelets and their adhesion on a vascular wall. NO causes smooth muscle relaxation not only at the vascular wall, but also at the gastrointestinal tract wall. NO functions both in the central and peripheral nervous systems. It regulates the activity of respiratory system organs and also of the digestive tract and genitourinary systems via efferent nerves. NO also influences the functioning of secretory tissues and cells. In all these systems constitutive isoforms of NO synthetases (NOS), neuronal and endothelial NOS, synthesize NO from L-arginine. The steady-state level of NO maintained by these NOS does not exceed few micromoles. Besides regulatory functions, NO exhibits cytotoxic/cytostatic activity operating as one of major effectors of the cellular immunity system. This action of NO is based on the functioning of inducible NOS which is synthesized in immune-competent cells when stimulated by cytokines, endotoxins, and other biologically active compounds. The steady-state NO level in these cells can reach hundreds of micromoles. Inducible NOS can also be synthesized in other human and animal tissues and cells, but this causes adverse effects. Good evidence exists that these effects are caused by formation of a potent oxidizing agent, peroxynitrite. The latter emerges from the reaction between NO and superoxide anion. Thus, peroxynitrite operates as an junction combining two systems of simple biologically active compounds formed in cells and tissues, NO and active oxygen species.

We should acknowledge an important contribution of various and at first sight unrelated studies to the formation of the NO problem in biology. They have provided huge progress of this field. The most important results are the following.

1. Discovery of the generation of nitrites and nitrates in man and animals from endogenous sources and augmentation of this process during inflammation [2]. It was concluded that nitrites and nitrates are formed via the oxidation of reduced nitrogen forms with possible formation of NO as an intermediate.

2. Discovery of NO generation by activated macrophages [3-5]. The efficacy of this generation correlated with cytostatic and cytotoxic effects of macrophages. NO was shown to mimic activated macrophage effects on target cells. Experiments with macrophages revealed for the first time NO formation from L-arginine during oxidation of amino groups of the guanidine moiety. A few L-arginine derivatives with substituted amino groups in the guanidine moiety effectively inhibited NO synthesis in macrophages.

3. The EPR method detected the formation of a paramagnetic nitrosyl complexes of heme and non-heme iron in animal tissues [6-12]. Exogenous compounds (e.g., nitrite) could be a source of NO formation in these complexes [6-9]. Subsequently, a contribution from endogenous compounds to the formation of these complexes was also demonstrated [10-12].

4. Discovery of soluble guanylate cyclase activation by NO [13-15]. This enzyme is responsible for synthesis of the second messenger, cGMP. Activation resulted from NO binding to heme of guanylate cyclase. This was a key to understanding of hypotensive, spasmolytic, and antithrobmotic effects of various nitroso- and nitro-compounds, particularly, nitroglycerin. The mechanism of these effects is due to the formation of stipulated by NO from such compounds in animals and humans.

5. Discovery and study of the nature of endothelium-derived relaxing factor (EDRF) [16-19]. It is released from vascular endothelial cells in response to acetylcholine, bradikynin, and some other compounds and causes relaxation of vascular smooth muscles. Comparison of the physiological and physicochemical properties of EDRF and NO led Furchgott to suggest that EDRF was identical with NO [17]; this was experimentally confirmed later [18, 19]. However, subsequent studies questioned this identification. Apparently, EDRF represents some nitroso-compound and NO is its active vasodilator component [20, 21]. Discovery of EDRF sharply raised interest in the biological role of NO and stimulated numerous studies in this field.

6. Discovery of EDRF-like activity of brain slices sensitive to inhibition by the above-mentioned L-arginine derivatives [22, 23]. These studies raised a question of the involvement of endogenously produced NO in the central (and later peripheral) nervous system.

7. Discovery of so-called "inhibitory factor" in the animal penis retractor [24, 25]. This factor is activated during acidification of the medium and then at neutral pH it can relax vascular and non-vascular smooth musculature. NO was found to be its active element.

In the early 1990s it became clear that all the mentioned directions of study can be brought together into one problem, investigation of the biological role of NO. This association emphasizes the universal importance of NO for biological systems. It became a basis for the development of a new area in biology, the biology of NO. The development of this area will promote solution of numerous fundamental problems of biology and have practical importance especially for medicine. One of the most interesting aspects of NO biology is the ability of both NO and its derivatives to stimulate expression of important proteins and enzymes (stress-proteins, ferritin, antioxidant defense proteins, transferrin protein receptors, nuclear protein p53 responsible for blockade of malignant growth, etc.) at levels of transcription and translation, to activate or inhibit activity of many proteins and enzymes (guanylate cyclase, ribonucleotide reductase, components of respiratory chain and glycolysis, transcriptional factor NFkB, cytochrome P-450-like proteins, ion channel proteins, etc.). It is important to establish which redox form of NO (neutral or ionized) is responsible for these effects? Solution of this problem is important for the development of exogenous compounds which could mimic biological actions of NO.

These problems are partially reflected in the present collection of reviews on NO biology submitted by Russian and foreign experts in this field. It was of course impossible to cover all aspects of this very interesting area. For those readers who wish to obtain more information on this subject, we can recommend previously published reviews on NO biology [26-30].


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