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4.1 Introduction
Chapter 4: Methods to measure nitric oxide
The previous chapter reviewed the production of NO, its reactions and interactions in vivo.
This chapter will examine the available methods of measuring NO. There were three issues
that I believed needed to be addressed; firstly, how the production of this evanescent molecule
could be measured as accurately as possible, secondly, which technique was the most
appropriate for measuring the production of NO in the lung and thirdly, how could it be done
non-invasively such that it could be used with children -
so preferably no venepuncture, no
biopsy or bronchoscopy and lavage required. A technique based on the way it was measured
in airway pollution but adapted to exhaled air in a manner analogous to lung function was
sought.
From the previous chapter (see Chapter 3. 1 'Introduction'), it is known that NO is formed
stereo-specifically from the guanidino nitrogen of L-arginine and oxygen. This is
accomplished via five separate steps (see Figure 3.3) and requires cofactors
(tetrahydrobiopterin and flavins FAD, FMN) and cosubstrates (O2, NADPH) as well as
calmodulin and calcium.
Therefore the possible factors that could be measured to assess the activity of this pathway
are:
a
o
o
o
O
o
o
The decrease in L-arginine
The formation of L-citrulline
The activity of cyclic guanosine monophosphate (cGMP)
The formation of NO
The formation of methaemoglobinuria
The formation of nitrite (NO2')
The formation of nitrate (NO3)
These, plus the measurement of co-factors and the other nitrogen compounds have all been
explored. A detailed review of all methods to measure NO in all compounds and in all tissues
is presented in "Methods in Nitric Oxide Research" edited by Martin Feelisch & Jonathan J
Stamler, published by John Wiley & Sons Ltd, Baffins Lane, Chichester, West Sussex,
England, 1996. It is immediately obvious that many of these compounds will be inappropriate
to use for detection in lung exhalate. I will briefly review the options before reviewing in
more depth the technique of NO measurement with the use of a chemiluminscence analyser.
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