STABLE ISOTOPES AS INDICATORS IN BIOLOGY

Summary 1. The application of stable isotopes as indicators does not differ in principle from the application of radioactive isotopes, though the determination of the latter is generally more convenient. 2. Since deuterium has several physical properties which are markedly different from those of the more abundant isotope of hydrogen it has definite biological effects in high concentrations. In low concentrations there are no appreciable biological effects, and it is not expected therefore that the application of deuterium will be limited in this respect. 3. Deuterium is used as an indicator in two ways: (a)it is used as a label directly attached to the carbon atoms of metabolites, ( b ) reactions are carried out in a heavy‐water medium and the uptake of deuterium in the end‐products is determined. 4. An account is given of the methods for the determination of deuterium and of some of the methods for preparing deutero‐organic compounds. The deuterium content of an organic compound is generally calculated from the density of its combustion water. 5. The stability of the carbon‐hydrogen bond is discussed; although it is often possible to predict the stability of this bond in vitro , it is concluded that the possible wandering of the label under biological conditions would limit its useful application to a few types of compounds, such as the relatively inert fatty acids and steroids. 6. The retention time of a molecule of water in the human organism has been found to be 14 days by experiments which involved the ingestion of heavy water and the determination of the deuterium content of the urine. The permeability of frog skin to water was found to be 400 days at o° C. 7. The metabolism of trideuteroacetic acid by yeast was studied. From the deuterium content of succinic acid isolated it was concluded that the acetic acid was converted into succinic acid, but the deuterium values gave no clue as to the mechanism of this conversion. The steroids isolated from the yeast had a higher deuterium content than the fatty acids, and it was concluded therefore that they were not derived from the fatty acids, but probably from the acetic acid. 8. In feeding experiments with deutero‐fatty acids it was found that they were deposited before being burned, even when the fat content of the diet was abnormally low. Caproic and butyric acids were disposed of by mice within 8 hr., and this disposal did not consist of a conversion into higher fatty acids. From the deuterium content of lipoids and glycerides isolated after the feeding of deutero‐fat it was concluded that ingested fatty acids play an active part in the metabolism of these compounds. The source of increased liver fat which results from starvation, exposure to carbon tetrachloride vapours, and injection of an extract of the anterior pituitary gland, was traced to the depot fat, and it was also concluded that this is not the source of increased liver fat resulting from a high carbohydrate diet. Desatura‐tion and saturation of fatty acids were proved to take place by following the deuterium content of the unsaturated fatty acids of mice which had been fed saturated deutero‐fatty acids, and vice versa. It was shown that stearic acid is converted by the organism into palmitic acid. 9. It was shown that cholestenone and coprostanone are converted by the dog and by man into coprosterol. These two substances can be intermediates in the conversion of cholesterol into coprosterol in oitro . 10. When yeast was grown in a heavy‐water medium on various nutrient hexoses it was found that the least deuterium was incorporated in the glycogen subsequently isolated when the nutrient hexose was fructose. It was accordingly concluded that the most direct conversion of a hexose to glycogen was from fructose, and not from glucose. The deuterium values did not give a satisfactory clue to the mechanism of this conversion. 11. From the deuterium content of the muscle of a mouse injected with heavy water the myosin content was calculated to be 55 %. In experiments of long duration a considerable amount of deuterium was built into the protein of mice, but no conclusions were drawn from the results. A series of deuteroamino acids were isolated from mice which had been maintained with an artificially raised level of heavy water in their body fluids. No deuterium was found in the indispensable amino acid, lysine. 12. Attempts were made to determine the rates of biological synthesis and breakdown of fatty acids and cholesterol from the deuterium content of these compounds isolated from mice which had been kept for various periods with heavy water. The results thus obtained were not conclusive. 13. Heavy nitrogen has also been used as an indicator. It was found that the nitrogen of ingested ammonium salts and of ingested dl‐tyrosine was incorporated in a large number of amino acids, with the exception of lysine. In some cases this incorporation was shown to take place by a process of deamination and reamination. It was shown that the glycine of urinary hippuric acid is not all derived from ingested glycine, even when a superfluous amount is fed. 14. The heavy isotope of oxygen has been used in a study of the source of the oxygen of respiratory carbon dioxide. 15. The possibilities of the application of the heavy isotope of carbon are mentioned.