New Results on the Photochemistry of Biopterin and Neopterin in Aqueous Solution

Abstract

New photochemical studies of the reactivity of biopterin (BPT) and neopterin (NPT) in acidic (pH = 5.5) and alkaline (pH = 10.5) aqueous solutions at 350 nm and room temperature were performed. The photochemical properties of BPT are of particular interest because the photolysis of this compound takes place in the white skin patches of patients affected by vitiligo. The photochemical reactions were followed by UV ⁄ VIS spectrophotometry, HPLC, electrochemical measurement of dissolved O 2 and enzymatic methods for hydrogen peroxide (H 2 O 2 ) and superoxide anion (O 2•) ) determinations. When BPT or NPT are exposed to UVA radiation, a red intermediate, very likely 6-formyl-5,8-dihydropterin, is generated in an O 2 -independent process. That product is rapidly oxidized on admission of O 2 to yield 6-formylpterin and H 2 O 2 . When the photolysis takes place in aerobic conditions, no additional pathways exist. On the other hand, in the absence of O 2 , the intermediate generated is not stable and leads to the formation of many products. O 2•) is also generated during photo-oxidation of BPT and NPT. The quantum yields of reactant consumption depends on the O 2 concentration: the higher the O 2 concentration, the lower the quantum yields. This behavior is discussed in connection with the excited state of the pterins.

Solar radiation exposure of dihydrobiopterin and biopterin in aqueous solution

“…To find out if the solar radiation is enough to photolyze pterin derivatives of biological relevance, the values of E S UV (Fig. 2), obtained in our experiments under outdoor conditions, were compared to the intensity of a source used in previous studies (Vignoni et al, 2009(Vignoni et al, , 2010. In this way, a Rayonet RPR lamp was employed as an artificial source and its corresponding q 0;V n;p values was measured by means of an actinometer (detailed in Section 2.6) (Terazima et al, 1992), for a sample placed in front of the lamp, as described in literature (Vignoni et al, 2009(Vignoni et al, , 2010.…”

“…Thereby, integrations of the peaks at different wavelengths were performed such as reported in the literature Vignoni et al, 2009. Assuming that the peak considered is only due to the reactant and one known product, the concentration of both compounds can be calculated by resolving system of linear equations:…”

“…Biopterin (Bip) and other oxidized pterins, which are not present in mammalians under physiological conditions, accumulate in the skin of patients suffering from vitiligo (Schallreuter et al, 2001). Among them, 6-carboxypterin (Cap), a product of Bip photolysis (Vignoni et al, 2009;Lyudnikova et al, 2009), has been isolated from the affected tissues (Rokos et al, 2002), suggesting that photooxidation of Bip takes place in vivo. Oxidized pterins are photochemically reactive upon UVA radiation exposure in aqueous solution and undergo photooxidation to produce several photoproducts and reactive oxygen species such as singlet molecular oxygen ( 1 O 2 ) and hydrogen peroxide (H 2 O 2 ) (Lorente and Thomas, 2006;Neverov et al, 1996).…”

“…1) (Vignoni et al, 2010;Vignoni et al, 2009;Lyudnikova et al, 2009). Briefly, UVA excitation of H 2 Bip in the presence of O 2 leads to two photochemical pathways: the formation of isomeric dimers with molecular masses equal to exactly twice the molecular mass of the reactant and the oxidation to its aromatic analogue, Bip.…”

Photosensitization of peptides and proteins by pterin derivatives

“…Therefore, under irradiation of skin affected by vitiligo, Fop and Cap are formed, very likely, due to the photolysis of both PteGlu and H 4 Bip [57][58][59]. In fact, Cap has been isolated from the skin of patients suffering from vitiligo [21].…”