The Study of Intermediary Metabolism of Animals With the AId of Isotopes

The study of intermediary metabolism is concerned with synthesis, interconversion and degradation of organic molecules. In adult animals in energy equilibrium the total amount, the distribution and the chemi-cal constitution of body constituents are kept constant within narrow limits. The constituents of normal diets (proteins, fats, carbohydrates, etc.) are in principle the same as those which compose the animal body. The experimenter loses track of them as soon as they pass the intestinal wall and mix with the same substances in blood and organs. Very few experiments, therefore, on the intermediary metabolism of physiological compounds have been carried out with adult animals on normal diets, as in most instances the tracing of the food constituents was impossible. Our present knowledge of intermediary metabolism is based mainly, therefore, on experiments performed under such abnormal conditions as permit accumulation of interinediates or end-products: Use of WI-natural compounds, or of diets unduly enriched or poor in test-sub-st,ances, experiments on poisoned, or sick organisms or on isolated organs, tissues or extracts. The isotopes of the elements C, H, N, S, 0, which constitute organic matter provide tools to follow organic molecules directly in the intact and normal animal. There are two types of isotopes available for biological application: 1, the natural stable isotopes of hydrogen (1)) oxygen (2), nitrogen (3, 4), carbon (5), and sulphur (4), all of which have been made available by Urey and collaborators through fractiona-tion of the natural elements; 2, a great number of artificially produced radioactive isotopes. In the present review, which is concerned with the metabolism of organic compounds, only those radioactive isotopes will be discussed which have been used as markers in organic molecules; namely, 1sP31 and 1GS35, etc. No mention will be made of the extensive work in which radioactive isotopes have been employed in the study of the metabolism of ions.