The double bonds, in each case except those between the 4 and 5 carbon atoms, are easily broken apart and readjusted, so that other atoms or groups can be attached to any atom in the nucleus except the 4 and 5 carbon atoms. In all of the statements with reference to the structure of the purine bases, the term "oxy" is used to mean an oxygen atom attached by both its bonds to one of the carbons in the nucleus, instead of its customary use to mean the monovalent OH group replacing a hydrogen, as in the case of all other nomenclature of organic compounds. With this understanding, reference to the numbered nucleus formula above will make plain the structure of all of the purine bases which are included in the following list: Hypoxanthine, C_{5}H_{4}N_{4}O, = 6-monoxypurine. Xanthine, C_{5}H_{4}N_{4}O_{2}, = 2,6-dioxypurine. Uric acid, C_{5}H_{4}N_{4}O_{3}, = 2,6,8-trioxypurine. Adenine, C_{5}H_{3}N_{4}NH_{2}, = 6-aminopurine. Guanine, C_{5}H_{3}N_{4}ONH_{2}, = 2-amino-6-oxypurine. Theobromine, C_{5}H_{2}N_{4}O_{2}(CH_{3})_{2} = 3,7-dimethyl-2,6-dioxypurine, or dimethyl xanthine. Theophylline, C_{5}H_{2}N_{4}O_{2}(CH_{3})_{2} = 1,3-dimethyl-2,6-dioxypurine. Caffeine, C_{5}HN_{4}O_{2}(CH_{3})_{3} = 1,3,7-trimethyl-2,6-dioxypurine, or trimethyl xanthine. In order to make these structural relationships quite clear, the following formulas for uric acid and for caffeine are presented as typical examples: H N--C=O CH_{3}--N--C=O | | | | O=C C--N-H O=C C--N-CH_{3} | ║ \ | ║ \ | ║ C=O | ║ CH | ║ / | ║ // HN--C--N-H CH_{3}-N--C--N Uric acid Caffeine =Uric acid= is found in the excrement of all animals; in the urine of mammals, and in the solid excrement of birds and reptiles. It is not known to occur in plants. =Xanthine= and =hypoxanthine= occur in animal urine, and also in the tissues of both plants and animals. =Adenine= and =guanine= are constituents of all nucleic acids (see below) and, hence, are found in all plant and animal tissues. Guanine is the chief constituent of the excrement of spiders, and is found also in Peruvian guano. It is also a constituent of the scales of fishes. =Caffeine=, =theophylline=, and =theobromine= are not found in animal tissues, but are fairly widely distributed in plants. Caffeine and theobromine are the active constituents of tea leaves and coffee seeds and are found also in cacao beans and kola nuts. The use of these three compounds in the metabolism of the plants which elaborate them is wholly unknown. They are not so directly related to protein metabolism as are the other purine bases.