23 Zeoeamino Acids I can only hope to acquaint you with some of the things which have been done and are being done and the techniques used in doing them. Some of the things I shell sey about the techniques of tracer studies in amino acid metabolism have very probably been covered, at lezst to some extent, in previous lectures in this cours2, but considering the host of material which you are given to absorb, some repetition can certainly not be amiss. So the following remarks apply to isotcne metabolism stucies in general: The basic assumption in such studies is thet the organism cannot distinguish appreciably between isotopes; this is essentiglly true for all but the isotopes of hydrosen, whece steric weights are 10C or 200 rer cent different from the common isotope; the cilities eporeciable physiolerical effects of "heavy water" have been much publicized; thece cffiets are very probably due to the inhibition ef cortain enzyme systems, recently shown in gem ccece. Tet living c;siems co not distinguish eppreciably between isotopes differinc in clombe wii lht by only «= few ver cent is indicsted by the fect that isotope concertrétion is not markedly ciffrivent in the stmosphere, certain cf whese constituents “oave beer procesced snd rerprocesse2 oy plant and unimels for a long time, from those same ecastituents from other scurces presuuibly not aubdect to such processes. vith radioactive Lsctores, particularly those emitting particles of high energy, cne my oure ct seme anamclicus results; these can be ininimized by using lew concentration, 3 © anc it is fortunete thet such concerntratior can be dstected and determined with con- siderable accuracy. Tagged materials ingested by the orgenism will naturally be subject to considerable dilution and the isolation of the metabolites of these meterials may not be quantitative by any meane; in setting up the experiment, c.ireful consideration must be given to the dilution fscter, isolation efficiency, half-life and emmission energy of tag (if radioecttive), and isotopic potency of the ingested materiel whose fate in the organism is to be studied. ‘