Fish, guanine from

The purine catabolic pathway appears in Figure 8,31, The end-product of purine cataboiism in primates, and in some other vertebrates, is uric acid, Purine catabolism differs in other species. Urate oxidase catalyzes the breakdown of uric acid to allantoin. Allantoin can be further broken down to produce urea and glyoxyJate, Allantoin is the purine excretory pixiduct in some mammals and reptiles. Urea is the purine excretory product in fish. Guanine is the purine excretory product in pigs and spiders. Uric acid is used for the packaging and excretion of waste N from amino acids in birds and some reptiles. 

Guanine (Pearl Essence). Guanine (Cl Natural White 1, Cl No. 75170), is the crystalline material obtained from fish scales and consists principally of the two putines, guanine [73-40-5] (42) and hypoxanthine [68-94-0] (43). The guanine content of the colorant varies from 75% to 97%, whereas the hypoxanthine content ranges from 3% to 25%, depending on the particular fish and tissue from which the crystals ate derived. 

Guanine is obtained fiom various fish including menhaden, herring, and alewives. To prepare the colorant, scales are scraped from the fish, levigated, and washed with water, and then made into one or more commercial forms, depending on the intended use. Typically, guanine is suppHed as a paste or suspension in water, castor oil, or nitrocellulose. Guanine is not a colorant in the strict sense but instead is used to produce iridescence in a product. 

There is strong evidence that DNA adduction by these bulky reactive metabolites of PAHs is far from random, and that there are certain hot spots that are preferentially attacked. Differential steric hindrance and the differential operation of DNA repair mechanisms ensure that particular sites on DNA are subject to stable adduct formation (Purchase 1994). DNA repair mechanisms clearly remove many PAH/ guanine adducts very quickly, but studies with P postlabeling have shown that certain adducts can be very persistent—certainly over many weeks. Evidence for this has been produced in studies on fish and Xenopus (an amphibian Reichert et al. 1991 Waters et al. 1994). 

Guanine is a crystalline amorphous substance that is found in guano, fish scales, and the liver of certain mammals. Guanine is responsible for the silvery iridescence of certain fish scales. Before its discovery, guanine was scraped from fish scales and used to coat beads to produce imitation pearls. Thus it was called pearl essence or pearly white in the 1700s. Guanine obtained from fish scales is used in cosmetics, especially for eye cosmetics and nail polishes. 

Natural pearl essence (Essence d Oriente, Fish Silver) is a pigment suspension derived from fish scales, skin, or bladder. It is the oldest commercial pearl luster pigment, consisting mainly of a mixture of the purines guanine [73-40-5] (75-97%) and hypoxanthine [68-94-0] (25-3%). 

Natural pearl essence is isolated as a silky lustrous suspension from fish scales. The organic pigment particles in the suspension are platelet-shaped with a very high aspect ratio (0.05 pm x 1-10 pm x 20-50 pm), and consist of 75-97% guanine and 3-25% hypoxanthine (natural fish silver) [5.122, 5.123, 5.128, 5.130]. 

The use of bismuth as a beauty care product dates back to antiquity and is still used in common cosmetic products. Bismuth oxychloride is used to impart a pearlescent effect to lipstick, nail varnish, eye shadow, and makeup powder. The resulting brilliance and luster surpasses that of a natural product, guanine, which is extracted from fish scales and is more costly. 

Fig. 18.2. Mutations in DNA caused by nitrosamines. Nitrosamines are consumed in many natural products and are produced in the stomach from nitrites used as preservatives and secondary amines found in foods such as fish. They are believed to be responsible for the high incidence of gastric cancer found in Japan and Iceland, where salt-preserved fish was a major dietary item. Nitrosamine metabolites methylate guanine (the transferred methyl group is shown in blue).

Classification Natural purine Definition Crystalline material obtained from fish scales, consisting of 75-97% guanine and 3-25% hypoxanthine (CAS 68-94-0)... 

Pigments derived from purine (9-49), for example guanine (9-50), xanthine (9-51) and uric acid (9-52), are very important pigments of the animal kingdom. The actual compounds are colourless, but in the form of granules or microcrystals are the bases of white, cream and silvery semi-transparent pigments that are found, for example, in fish scales. 

The simultaneous reflection of light from many parallel layers produces the characteristic pearly luster, the hril-liance of which depends on the uniformity and parallelism of the crystals. Natural pearlescent pigments are composed primarily of guanine crystals derived from fish scales. They are expensive but nontoxic. The synthetic pearlescents are based on crystallized lead or bismuth compounds or platelets of mica coated with a dye or pigment. (Bailey s industrial oil and fat products. Shahidi F, Bailey AE (eds). Wiley, New York, 2005)... 

The first pearl pigment was derived from fish scales as platy crystals of guanine and hypoxanthine. This remained imrivaled in luster for approximately 300 years, until the refinement of crystallization processes for... 

Nucleoproteins are almost insoluble in water, but dissolve in alkalies, and by this means can be extracted from tissues rich in nuclei, such as thymus and pancreas. They were obtained originally from pus, which is an emulsion of white corpuscles. Another rich source is the sperm of fishes. On acid or enzyme hydrolysis, nucleoproteins are resolved into protein and nucleic acid, which further hydrolysis converts into four units, or nucleotides. Each nucleotide is the phosphoric ester of a nucleoside, or pentose derivative of an amino purine adenine or guanine), or an amino pyrimidine cytosine or methyl cytosine). 

The resultant purine bases, guanine and hypo-xanthine (from AMP and dAMP), may be either salvaged (Section 16.6) or converted to xanthine. The oxidation of xanthine by xanthine oxidase yields urate (uric acid) which is excreted in the urine of man and primates. Other organisms are capable of the synthesis of various enzymes which permit the continuation of the pathway to various end products, e.g. allantoin in mammals, urea and glyoxylate in most fishes (not teleost fishes) and amphibians, and ammonia and COj in crustaceans. 

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