ABSTRACT
Nitric oxide is formed by a number of purely chemical reactions [294]:
Fe2+ + NO2 ! + 2 H+ Fe3+ + NO + H2O (1)
2 NO2 ! + 2 I! + 2 H+ 2 NO + I2 + H2O (2)
Reaction (2) may be used for the quantitative iodometric determination of nitrite. Both of these reactions are reductions of NO2
!. However, it is well known that nitrite (NO
NO via the following pathway:
3 HONO HNO 3 + 2 NO + H
These reactions occur readily in the acid environment of the stomach, where nitrites produce nitrosamines, which are highly carcinogenic. The formation of NO occurs at only slightly acidic or even neutral pH to a limited extent. The pH in smooth muscle cells is between 5.9 and 7.0, within the range for these reactions. HONO can also give rise to N
(2 HONO N 2 O
nitrosation of secondary amines:
R 2 NH + N
acid (AH2):
AH2 + HNO2 AH@ + @NO + H2O
This reaction is of physiological significance in the acid environment of the stomach. Ascorbic acid inhibits N-nitrosamine formation in the stomach [681] by converting HNO2 to @NO. Nitric oxide does not nitrosate amines directly, but only after oxidation to N2O3. This reduction by ascorbic acid has been suggested as a means by which neurons regulate their own oxygen supply [682].
