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1.
Figure 5

Figure 5. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Depending on the substrate, nitrite may be reduced to free NO at the Molybdenum site of the XO enzyme. Process I is progressively inhibited by oxygen. Process II continues to operate even under normoxia.

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
2.
Figure 4

Figure 4. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Depending on ambient oxygen, myoglobin acts as a dioxygenase or as a nitrite-reductase. Under normoxia, oxymyoglobin acts as an NO-scavenger, protecting the mitochondria from inhibition by NO) (left). Under hypoxia, myoglobin changes its function from a dioxygenase to a nitrite-reductase. Now it converts nitrite to free NO (right), regulating mitochondrial respiration and myocardial function.

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
3.
Figure 7

Figure 7. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Possible sites of nitrite reduction in mitochondria at complexes III and IV of the respiratory chain. Abbreviations: R - Rotenone, M - Myxothiazol, A - Antimycin A, T - Thenoyltrifluoroacetone, CN – Cyanide, UQ – Ubiquinone, UQH2 – ubiquinol, Q•−- semiquinone radical anion, Cyt – Cytochrome.

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
4.
Figure 6

Figure 6. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Kinetics of NO formation in a single flask of cultured BEND3 cells under anoxia and normoxia. The MNIC yields are given as function of incubation time of the trapping experiment. The anoxic values (solid squares) were taken from (19) with permission. The accuracy of the MNIC yield is ca 10 %.

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
5.
Figure 2

Figure 2. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Oxygen-dependence of nitrite reductase activity of hemoglobin. As oxygen tension increases the amount of deoxyHb (blue trace) decreases while the amount of R-state Hb (and free hemes in R-state Hb tetramers, red trace) increases. The nitrire reductase activity (gray) depends on both of these factors and is maximal at the P50. It should be noted that this figure is merely illustrative and the precise oxygen tension dependencies are more complex yet give a similar result. From (7) with permission.

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
6.
Figure 9

Figure 9. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Schematic representation of the ranges of oxygenation where the various pathways for reduction of nitrite are active. For cytochrome P450 no definite data exist yet, but this pathway is likely to function only in near complete absence of oxygen (cf section 7. D). The two different reaction processes for XO were explained in fig 5. The vertical lines indicate the boundaries between anoxia, hypoxia, normoxia and hyperoxia as defined in table III.

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
7.
Figure 3

Figure 3. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

The N2O3 forming reaction of nitrite and hemoglobin may regulate the export of NO from the erythrocyte. Hemoglobin deoxygenation (purple) occurs preferentially at the submembrane of the red blood cell as it traverses the arteriole. Nitrite reacts with deoxyHb to metHb and free NO. Much of this NO binds to hemes of deoxyHb or reacts with oxyHb to form nitrate and metHb. MetHb binds nitrite to form an adduct with some Fe2+-NO2 character (Hb- ). This species reacts quickly with NO to N2O3 which can diffuse out of the red cell, later forming NO or extracellular S-nitrosothiols. Low molecular weight nitrosothiols may contribute to exportable vasodilatory activity. (from (123) with permission).

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
8.
Figure 1

Figure 1. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Kinetics of the reaction between human deoxyHb (50 μM) and nitrite (10 mM) at pH 7.4 and 37° C. (A) UV/VIS absorption spectra were deconvoluted to determine the percentage of each species as a function of time. DeoxyHb is observed to form equal amounts of metHb and iron-nitrosyl-Hb. Deviation from first order behavior is evident in the curve for decay of deoxyHb, having a sigmoidal shape. (B) The instantaneous rate of the reaction shown in panel A where the negative of the slope of the decay curve for deoxyHb is plotted as a function of time. (C) Nitrite (10 mM) was reacted with Hb (50 μM) at various oxygen tensions. The initial rate of the reaction is plotted. (A, B from (114), C from (111) with permission).

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.
9.
Figure 8

Figure 8. From: Nitrite as regulator of hypoxic signaling in mammalian physiology.

Many vegetables including spinach, lettuce and beetroot are extremely rich in inorganic nitrate. Ingested nitrate from dietary sources is rapidly absorbed in the small intestine and in the circulation it mixes with endogenous nitrate from the NO pathway. While much of the circulating nitrate is eventually excreted in urine, up to 25% is actively extracted by the salivary glands and concentrated in saliva. In the mouth commensal anaerobic bacteria effectively reduce nitrate to nitrite by the action of nitrate reductase enzymes. Until recently this entero-salivary circulation of nitrate and bacterial reduction to nitrite has received attention only because of nitrites ability to form potentially carcinogenic nitrosamines. However, the link between dietary nitrate and gastric cancer is still very uncertain despite more than 40 years of active research. More recently it was shown that in the acidic stomach nitrite is spontaneously decomposed to form nitric oxide and other bioactive nitrogen oxides which serve to regulate important physiological functions. Gastric NO helps to kill ingested pathogens and also protects the gastric mucosa against luminal aggressors via stimulation of mucosal blood flow and mucus generation. Moreover, much nitrite is also absorbed intact into the ciculation and can convert back to bioactive NO in blood and tissues. This enterosalivary nitrate circulation and serial reduction to nitrite and NO explains the recently described reduction in blood pressure seen after dietary supplementation with nitrate. It has been suggested (216) that the well-known cardioprotective effects of a diet rich in vegetables is related to their high nitrate content.

Ernst E. van Faassen, et al. Med Res Rev. ;29(5):683-741.

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