Pigments pterine

Several action spectra have been constructed for phototaxis (and photophobic responses) in Eu lena. They are distinctly different from those measured, e.g., in Chlamydomonas and have been suggested to represent the involvement of flavins and pterins. Fluorometric analysis has indicated that pterins, absorbing in the UV-A range of the spectrum with a maximum near 360 nm, function as antenna pigments. Pterins emit at about 450 nm, which corresponds with one of the maxima for flavin absorption. The fluorescence emission of the flavins can be detected at 520 nm (82). Therefore the final photoreceptor is thought to be a flavin. 

Pterins — These are pigments derived from pteridine skeletons. All natural pterins are 2-amino-4-hydroxypteridines bearing various substituents at Cg and C7 and having different oxidation states of N5 and Ng. 

Purines absorb only ultraviolet light and they contribute to structural colors (white and silver) in animals. Pterines are generally yellow, orange, or red pigments. Because they are amphoteric molecules, the absorption spectra depend on the pH and present three or two absorption maxima, usually one in the visible region. Sepiapterin has an absorption maximum at 340 nm in O.IM NaOH and at 410 nm in O.IM HCl." Leucopterin has three maxima 240, 285, and 340 nm. Xanthopterin has two 255 and 391 nm. Because they are conjugated with proteins, pterins show bathochromic shifts in vivo. They also present fluorescence when excited with UV light. 

Rutowski, R.L. et al.. Pterin pigments amplify iridescent ultraviolet signal in males of the orange sulphur butterfly, CoUa eurytheme, Proc. R. Soc. B, 272, 2329, 2005. Oliphant, L.W., Pteridines and purines as major pigments of the avian iris. Pigment Cell Res., 1, 129, 1987. 

First isolated from human urine, biopterin (Fig. 15-17) is present in liver and other tissues where it functions in a reduced form as a hydroxylation coenzyme (see Chapter 18).338 It is also present in nitric oxide synthase (Chapter 18).341/342 Other functions in oxidative reactions, in regulation of electron transport, and in photosynthesis have been proposed.343 Neopterin, found in honeybee larvae, resembles biopterin but has a D-erythro configuration in the side chain. The red eye pigments of Drosophila, called drosopterins, are complex dimeric pterins containing fused 7-membered rings (Fig. 15-17).344 345... 

A combination of decarboxylation and hydroxyla-tion of the ring of tyrosine produces derivatives of o-dihydroxybenzene (catechol), which play important roles as neurotransmitters and are also precursors to melanin, the black pigment of skin and hair. Catecholamines may be formed by decarboxylation of tyrosine into tyramine (step e, Fig. 25-5) and subsequent oxidation. However, the quantitatively more important route is hydroxylation by the reduced pterin-dependent tyrosine hydroxylase (Chapter 18) to 3,4-dihydroxyphenylalanine, better known as dopa. The latter is decarboxylated to dopamine.1313 Hydroxylation of dopamine by an ascorbic acid and... 

Pteridines are widely distributed in nature and function as pigments, biological markers, and cofactors of enzymatic reactions. The oxidized heteroaromatic forms show a characteristic fluorescence which allows easy detection even in low concentrations. However, the more active 5,6,7,8-tetrahydro derivatives are nonfluorescing and oxidizable and create experimental problems in handling this type of compound. So far, all naturally occurring pteridines have turned out to be derivatives of pterin (2) and lumazine (3) which are modified by different substituents and functional groups in the 6- and/or 7-position. 

The pterins include the redox cofactors biopterin and molybdopterin, as well as various insect pigments. Folic acid is a conjugated pterin, in which the pteridine ring is linked to p-aminobenzoyl-poly-y-glutamate it is this linkage that renders folate a dietary essential, because it is the ability to condense p-aminobenzoate to a pteridine, rather than to synthesize the pteridine nucleus itself, which has been lost by higher animals. Biopterin (Section 10.4) and molybdopterin (Section 10.5) are coenzymes in mixed-function oxidases they are not vitamins, but can be synthesized in the body. Rare genetic defects of biopterin synthesis render it a dietary essential for affected individuals. 

Another aspect, through which the degradation of tryptophan becomes a part of the chapter of the genes, must be remembered for its great biological importance. In 1952 Butenandt (B29) succeeded in clarifying the structure of ommochromes. These pigments of the eye of insects, which in the past were considered as pterin derivatives, are today known to be formed directly from kynurenine and 3-hydroxykynurenine. 

The identification of hydroxykynurenine as the cn+-substance which is the precursor of insect eye pigments (ommochromes) has already been described. Ommochromes had previously been considered to be pterin derivatives (140, 534), but there is no obvious route for the conversion of hydroxykynurenine to the pterin type of structure. However, pterins also occur in insect eyes (275). More recent work has shown that the eye pig-... 

Z3. Ziegler, I., Pterin Pigmente, cofaktoren und signalverbindungen bei der zellularen interaktion. Naturwissenschaften 74, 563-572 (1987). 

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