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3.1
Chapter 3: The synthesis, reactivity and control of nitric oxide
Introduction
The previous chapter detailed the two areas where nitrogen compounds were being
increasingly recognised for their importance; air pollution and biological systems. In air
pollution the nitrogen oxides were associated with excess mortality, in particular respiratory
and cardiovascular morbidity; and in the biological systems, NO was found to be a
widespread fundamental messenger. In this chapter, I will detail how NO is produced and
cover its highly reactive and interactive properties. I will discuss the nitric oxide synthase
(NOS) enzymes and how they are controlled in the physiological roles and the host defence
roles of NO. The properties of NO needed to be understood when preparing to design
experiments to measure it in vivo.
3.2 Properties of nitric oxide
Figure 3.1: The molecular structure of NO
ao OO
oN=O..
NO is a clear, colourless gas. It is a free radical with an unpaired electron (. N=O, abbreviated
NO, see Figure 3.1) and therefore it is highly reactive with a half life of seconds and readily
combines with other free radicals (Beckman and Crow 1993). However it does not self react,
possibly because its bond length is intermediate between double and triple bond lengths
(Braker and Mossman 1975). Its small size and the fact that it is a relatively non-polar
molecule means that it moves readily through hydrophobic lipid membranes (Shaw and
Vosper 1977 Malinski, Taha et al. 1993; Liu, Miller et al. 1998). Its reactivity means that it
binds quickly with transition metals such as iron, copper, cobalt, or manganese that are central
ions to many cytochromes and oxidases, and in the case of iron, haemoglobulin. NO can be an
oxidant or a reducing agent depending on the 'redox' environment. NO is soluble in water up
to 2 millimoles per litre at 200C in one atmosphere, but has a high partition coefficient so it
usually exists as a gas. NO reacts rapidly with oxygen in air producing nitrogen dioxide (NOz)
(Vallance and Collier 1994). NO is very soluble in lipid and water and is therefore fully
diffusible in the environment of the cell (Archer 1993; Henry, Lepoivre et al. 1993).
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