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In a previous paper [Dale, 1940] the effect of X-rays on crystalline carboxypeptidase and polyphenol oxidase was investigated mainly in regard to the radiosensitivity of these enzymes as a function of their concentration. One of the findings of this investigation was that enzymes are affected by reasonably small doses of X-radiation. Since their high chemical activity allows the measurement of radiation effects on them in concentrations as low as those in which they occur in living matter, they are well suited for a more differentiating examination of their radiosensitivity at such concentrations.The work of Theorell [1937] and Warburg & Christian [1938] has established that a number of enzymes consist of conjugated proteins, and' are separable into a chemical compound of a relatively low molecular weight (prosthetic group sometimes called. coenzyme) and a highly specific protein, both of which are necessary for enzymic activity. There is, therefore, an opportunity of measuring the effect of radiation'on these components singly, together, and in combination with the substrate and other compounds.oc-Amino-acid oxidase [Krebs, 1933, 1, 2;1935], as one of those enzymes whose prosthetic group and specific protein can be separated, has been chosen as the subject of this investigation. Also its prosthetic group, alloxazinadeninedinucleotide, is of nucleotide nature and this forms a first step to the study of the radiosensitivity of nuciear substance on which attention has been focused for a long time.

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