The inactivation of an enzyme (carboxypeptidase) by X- and α -radiation

Abstract

The inactivation of aqueous solutions of pure crystalline carboxypeptidase by X-radiation and α -radiation has been studied. It is concluded that in each case the enzyme molecules are inactivated by collision with a labile product resulting from the ionization of the water. The X-ray ionic yield is approximately constant and equal to about 0·18 enzyme molecules inactivated per ion pair of water at all enzyme concentrations between about 2 x 10 ^-4 and 2 x 10 ^-1 g. enzyme/ml. At lower concentrations a smaller X-ray ionic yield was observed. At an enzyme concentration of 3 x 10 ^-5 g./ml. the results are not affected by the absence of dissolved oxygen. The α -ray ionic yield is only about one-twentieth of the X-ray ionic yield, that is, ~ 0·01 molecules of enzyme inactivated per ion pair of water and appears to increase about twofold over the concentration range 5 x 10 ^-6 to 5 x 10 ^-3 g. enzyme/ml. An attempt is made to correlate the X-ray and α -ray ionic yields with what is known concerning the spatial distribution of the ions. It is concluded that the extremely high concentration of positive ions which forms the core of the α -ray track probably plays a very significant role in the general radiochemistry of α -radiation. The small inactivation of carboxypeptidase brought about by exposure to α -rays could be ascribed to the secondary electrons ( δ -rays) which travel clear of the α -ray track and it is not certain that any inactivation of enzyme is brought about by the primary α -ray ionization.