As may be imagined, the primary difficulty was to prepare nucleic acids of such purity as admitted of their elementary chemical analysis. The necessary researches were to a large extent confined to two types of nucleic acid, one derived from yeast, and the other from the thymus gland; in other words, to representatives of the only two nucleic acids in nature, one derived from the nuclei of animal cells, the other from the nuclei of vegetable cells.
A feature common to nucleic acids of animal and vegetable origin is that, on hydrolysis with boiling mineral acid, they yield two purin derivatives, guanine and adenine, and a pyrimidin derivative, cytosine. From thence as regards their remaining constituent elements they display distinctions. Thus animal nucleic acids yield thymine, and contain a hexose group in their molecule. On the other hand, vegetable nucleic acids give forth uracil and possess a hexose group.
To sum up, nucleic acid is a chemical complex, made up of phosphoric acid with purin bases, pyrimidin bases and carbohydrate radicles. Moreover, nucleic acids, whatever their source, show a striking similarity in structure, containing always two amino-purins (adenine and guanine), two pyrimidines (either cytosine and uracil, or cytosine and thymine), and a carbohydrate. Now, while purin bases are always present, yet, in respect of their carbohydrate group, nucleic acids display variations; this, according as they are of animal or vegetable origin. If the former, the carbohydrate group is a hexose (contains six carbon atoms) with thymine. If the latter, it contains pentose (five carbon atoms) with uracil.
The constancy in the content of the various nucleic acids is such that Levene and Jacobs have felt justified in putting forward the following provisional formula as to the constitution of a nucleic acid of animal origin.
Structural Formula of Nucleic Acid
Distribution of the Enzymes
The enzymes responsible for the disruption of the nucleic acid complex are not to be found in all the body tissues. Moreover, the distribution of the enzymes in the various organs and tissues varies in different species of animals. Of the various organs the liver, spleen, thymus, and pancreas more particularly contain enzymes in abundance. As to their varied location in different animals, it may be noted that the enzyme responsible for the oxidation of xanthine into uric acid, viz., xanthine-oxidase, is found in man only in the liver. In other animals, also, it is of localised distribution, being as a rule only found in the liver or in the liver and kidney. The dog, however, appears to be an exception, xanthine-oxidase being found in a variety of its tissues.
Adenase, the deaminising enzyme, is not to be found in any organs in man. Neither does it exist in any of the tissues of the rat. Consequently, if adenine be injected subcutaneously in rats, it undergoes oxidation, without abstraction of its amino group.