at sat ‘ Review of the Use of Tracers in the Study of Amino Acids Dr. David Horton, A&C ATOMIC ENERGY PRCJECT, U.C.L.A. . In the old search for understanding of the mysteries of living organisms, the study of amino acids and the proteins which are synthesized from them is of primary importance four this reason: whereas the carbohydrates and fats serve chiefly as sources of encrgy (released in their oxidation by the organism), the proteins are intimteiy involved in the intricate processes responsible for the integrity of the orgeniea as a unit. In support of this view it should be sufficient to point out that the enzymes essential for the complex syntheses and degradations which compromise life are all largely protein in nature; and this is to say nothing of their important role in issanolegical specificity, the ticed-clotting mechanism, and the structural units Tne Zievortanes of mul. tin axuremc comple:dty of theirs fine structure (which is germain to their many corphex Panchiens} has meade the study of them very difficult. Hence it wes with no “eee of exeiterent thet students of the metzibclosm of amino acids and proteins gromtsd vhe rotreor sudden availalility of appreciable cuantitises of isotopic forms of cil the sicmental substences important to livine systems. As will become apparent in the course cf the talk, isotones cre extrenely porerful iools in the study of metubkelic processes. Net only can isotopes help solve problems which are very difficult but it is apparent that many problems may now be solved which were pree- viously imrossible of solution, without elemental substances of unnatural isotopic composition and mans for determining this difference. That biolegists (using the term in its broadest aspects) were awaiting just such a tcol is attested by the exceedingly rupid growth of the number of applications of nuclear physics to the biologicel end medical sciences. Despite its really great veuvh, the field has grown to a size difficult tc encompass in three weeks of courses; end in ene heur I cannot hope to cover completely the many instences already in the published litereture on the metabolism of the amino acids.