Nitric Oxide, friend or foe?
Dr Reynald Altema
Nitric Oxide, friend or foe?
Nitric Oxide (NO) is considered an antioxidant par excellence, because of its known vasodilating property when it interacts with the endothelium. Its discovery was considered such a seminal achievement that three researchers were awarded the Nobel Prize in Physiology/Medicine in 1998 (Furchgott, Ignarro, Murad). The common acceptance of the positive role of an antioxidant is that it is a scavenger of free radicals, ready to cause harm to the human cell. Paradoxically enough, NO is a free radical; how can this be? How can a free radical be an antioxidant? Therein lies the confusion that can be found.
Basic Science.
Some precision is in order. A basic principle is the phenomenon called redox, i.e. there is a type of chemical reaction that forms a bond that requires two things to happen -or else no reaction takes place- one element or group has to give up one or more electron(s) and another has to receive one or more electron(s).The one giving up is being oxidized while the one receiving is being reduced. A given element can undergo either oxidation or reduction, depending on the circumstances. A free radical by definition is an intermediate entity with a short half life and is highly reactive, i.e. very susceptible to either accept or give up one or more electron(s). Therefore it is understood that a compound that gets reduced causes another to be oxidized is called an antioxidant and in the opposite reaction it is considered a prooxidant. In practical term, it means that NO can be an antioxidant as well as a prooxidant. In fact it belongs to the class of highly Reactive Nitrogen Species (RNS). We know that prostaglandins as a class have diametrically opposite properties among its different species. The concept of a compound playing a dual role is analogous.
More specifically, the vaunted reaction for which such acclaim was garnered involves L-Arginine, which in the presence of NO Synthase (NOS) and Oxygen yields NO and Citrulline. However and this is very important to remember, L-Arginine at any given time can react with any number of enzymes such as:
〉 Arginase to produce Urea or with NOS to make NO to end up as Nitrate or Nitrite, requiring an amazing number of six cofactors; Arginase may react with L-Arginine at a rate 1000 times that of NOS, meaning that L-Arginine preferentially makes Urea over NO. In a marvelous cycle of biologic ingenuity, as is common in the body, Citrulline can be recycled and reconstitute L-Arginine and at the level of the endothelium, this is considered an important step in the quick production of NO.
〉 Arginine: Glycine Aminotransferase to make Creatine. There are other enzymes with which it can interact.
NOS exists in different settings, as so-called isomers:
〉 Endothelium induced NOS, aka eNOS, (eNOS, eNOS3). This activity markedly decreases with aging, even in people without any systemic illnesses or cardiovascular conditions. Its decrease is significantly accelerated in the presence of cardiovascular pathologies.
〉 Neuron induced NOS, aka nNOS, (nNOS, nNOS2). It works primarily as a neurotransmitter.
〉 Cytokine-inducible NOS, aka iNOS, (iNOS, iNOS1). Found primarily in macrophages. It is involved in the immune response.
1-NOS + L-Arginine + O2 + NADPH− (and other co-factors) → .NO + Citrulline + NADP+
This is the classic pathway for .NO production; its subsequent vasodilation occurs via an interaction with cGMP. Phosphodiesterase inhibitors enhance this because they inhibit the degradation of cGMP. The absence of co-factor(s) causes a reduction in production of NO.
The end products of NO metabolism are either Nitrite or Nitrate. Either product can regenerate NO and this has therapeutic implications.
2NO + 2O2 → 2NO3-(Nitrate)
2NO + O2 → 2NO2- (Nitrite)
REACTIVE SPECIES.
Both oxygen and Nitrogen can be considered as belonging in this category for their ready tendency to form highly reactive chemicals, some of which are free radicals.
Nitrogen.
• Free radicals: .NO (Nitric Oxide), .NO2 (Nitrogen dioxyde).
• Non-Radicals: OONO- (Peroxynitrite), HNO2 (Nitrous Oxide).
Oxygen.
• Free radicals: .O-2 (Superoxide), .OH (Hydroxyl).
• Non-Radicals: H2O2 (Hydrogen Peroxide).
2-NOS(Fe(II)heme)+O2 → NOS(Fe(III)heme)+O2•−(Superoxide)
In a pathologic condition, NOS can bind to Heme and generate superoxide which can be detrimental to cells.
NO + O2•− →NOO generates Peroxynitrite which is a prooxidant.
SOURCES OF NO:
NO can also be generated from other substrates than L-Arginine, such as Nitrite, Nitroglycerine (NTG), in the presence of Oxygen (Fig 1). In fact NTG exerts its therapeutic effect via the production of NO and its subsequent vasodilatory property.
Practical applications:
NO is the basis of pharmacologic treatment in the following products:
1-Nitroglycerin.
2-Phosphodiesterase inhibitor, such as Cialis.
3-Pulmonary Hypertension in newborns as an inhaled agent (NO).
Alternate sources of and metabolism of Nitric Oxide. (Fig 1).
NO IN HEALTH AND DISEASE.
In healthy endothelium, eNOS produces NO which then leads to vasodilation, at the same time as aging occurs, this ability is reduced. In conditions which cause endothelium dysfunction, this ability is markedly reduced. We will take a look at some conditions. The role that eNOS plays in illness and health can’t be underestimated. Two different groups will be used as examples to drive home this notion:
1-High altitude living.
In a study done among indigenous populations living at high altitudes such as in Tibet and the Bolivian Andes, high level of NO is found in their lungs when compared with a population living in the US at sea level. Blood flow is also found to be greater in such populations to compensate for the chronic exposure to lower level of Oxygen. This is a healthy adaptation to a suboptimal environment.
2- African-Americans.
It is a well known fact that Hypertension has the highest prevalence among African-Americans in the world. Why this is so is subject of a lot of conjectures. Nonetheless an interesting link is found in the presence of G6PD deficiency, an anomaly found in close to 15% of such a population. This enzyme is necessary in the generation of NADPH, a co-factor needed for the generation of NO. The presence of G6PD deficiency adds another pathology contributing to the prevalence and morbidity of both DM and HTN in such patients.
Metabolic Syndrome.
This condition is characterized by Hypertension, Obesity, Dyslipidemia, Insulin Resistance either in total or if three are present. They are also associated with endothelium dysfunction in general but specifically with impaired eNOS activity. The ensuing propensity for clot formation, vasoconstriction, plaque accumulation/rupture, all hallmarks of such a condition can take place via a number of identified mechanisms:
〉 Hypertension. The dysfunction seems to be similar to that seen in normal aging, except it occurs at an accelerated pace.
〉 Diabetes.
◦ There is a little-known pathway that Insulin uses to cause vasodilation. It is called the PI-3K. It activates the eNOS directly and causes the production of NO. This pathway is impaired in Insulin Resistance.
◦ The presence of AGEs (Advanced Glycating End Products) from impaired glucose metabolism is associated with their reaction with NO, diverting its normal vasodilating mode of action.
◦ The presence of ROS (Reactive Oxygen Species) from both smooth muscle and endothelium impair eNOS activity.
Alzheimer’s disease.
Evidence keeps accumulating that the presence of amyloid-b peptides (Ab) may impair the activity of NOS on one hand. The type of NOS generated can be either beneficial (nNOS, eNOS) or detrimental in the case of iNOS. There is a complex interchange. However proper NO activity seems to be grossly impaired.
Therapeutic interventions.
L-Arginine.
It has become a popular therapeutic undertaking to give L-Arginine as an exogenous source of NO. Some physicians even advocate it as an add-on to Viagra to enhance the success. Unfortunately, the evidence of the success is not there; in fact there is plenty of evidence to say that it doesn’t work for reasons that are not so intuitive. For one thing, when ingested, upward of 40% of it is eliminated. In addition, L-Arginine serves as a substrate for many different enzymes with different affinity and even under the best of circumstances, only 3-5 % would go toward NO production. In patients with endothelial dysfunction, the L-Arginine is not being converted to NO, at least not in significant quantity and patients with ED often fall in this category. In fact in one trial, its use in post MI was found to be associated with increased risk of complications. (VINTAGE-MI).
Nitrite/Nitrate.
As shown previously, NO can generate Nitrite and Nitrate while at the same they can be reduced to NO also. The exogenous use of either or both on the other hand is found to be a good source of NO via the eNOS pathway. Beets are a good source of it in the diet.
References:
Gerhard M, Roddy MA, Creager SJ, Creager MA. Aging progressively impairs endothelium-dependent vasodilation in forearm resistance vessels of humans. Hypertension 1996;27:849-53.
Pie JE, Baek SY, Kim HP, et al. Age-related decline of inducible nitric oxide synthase gene expression in primary cultured rat hepatocytes. Molecules and cells 2002;13:399-406.
Berkowitz DE, White R, Li D, et al. Arginase reciprocally regulates nitric oxide synthase activity and contributes to endothelial dysfunction in aging blood vessels. Circulation 2003;108:2000-6.
Beall CM, Laskowski D, Strohl KP, Soria R, Villena M, Vargas E, Alarcon AM, Gonzales C, Erzurum SC. Pulmonary Nitric Oxide in Mountain dwellers. Nature. 2001;414:411-412.