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.

The purine bases, other than the three mentioned in the preceding
paragraph, are undoubtedly intermediate products in pro