3eeeAmino Acids Careful consideration mist ulso be given to the lability of the tracer atom in the orgenic chemical structure it is to help in trecing through the orgenism. [iill the tag stzy with the structure whose metabolic fate is of interest, or will it be exchanged with other materials and be lost in the ercat poo: of metabolites? Such exchanges are often ‘interesting in themselves but in other instances muy weil determine the success or failure of the experiment. As experiments dealing with simple, readiiy-avaiiable tugged materials (such as carbon dioxide and the minerals) become exhausted or lead to. more complex questions, the student of the metabolism of tagged materials mist come to re.y more end more on the insenuity of the synthetic organic chemist. He must not only be able to place the tagging atom in a desired and unecuivocaliy-inown position in the comroundc, but he must also errive at this endproduct with the least possible loss of precious isotope concentrate. He mict devise a synthesis with a minimum of operations, with maximum yields at cach step, fant one vnich will involve the tagsec starting materiel as clcsc as pessible to the end ef the synthesis, Irn the study ef certain mctabelic processes cne must face the preblem of the precursor of interest beinz destroyed before rcaching the site of conversion; others, such aS protcins or phesphoryleted compounds, may require hydrolysis, (thus possibly destrcying an integrity cssential to the ex:erinent) by the orranism befcre entering the metabolien such problems can be studied using simpler organisns, where the precursor can be aprlied directly te the cell, but here uncertainties exist in the Legithscy of such results when trenslated to more complex organisms, Extreme ifdjiity of a precursor is sometimes counterected by the "flooding" tecinique, in which unusually large amounts of precursor are fed to the organism; althcugh this technique may give results unobtainable othervise, it is wasteful of tagged muterials and may lead to abnormal results. Ancther atteck is on the compiexity of the organism by employing slices or hotogenates of a specific tissue; such studics mzy give very valuable information, but it is difficult to justify a quantitative translation of the results to the situation in the intact animal; here too, a negative result is in doubt because cf the abnormal conditions