A comparison of the effects of ultraviolet and ionizing radiations on trypsin activity and on its constituent amino acids

Abstract

Photons of 254nm. u.v. light, 60Co γ-rays and 1Mev electrons produce different patterns of destruction of individual amino acids in dried films of trypsin and in the corresponding amino acid mixture. For example, in the amino acid mixture u.v. light destroys tyrosine, tryptophan and cystine, whereas in trypsin only cystine is disrupted but with 10 times the initial yield. Further, in the amino acid mixture loss of half-cystine is a simple exponential function of dose, but in trypsin there appear to be two exponential components of the loss with yields that differ by a factor of 35. Both the γ-rays and electrons destroy half-cystine, tryptophan, histidine and methionine in the amino acid mixture with remarkably high yields, whereas in trypsin doses that destroy almost all of the enzymic activity produce no detectable destruction of amino acid residues. These marked differences between the two preparations show that the radiation-sensitivity of a given amino acid alone and in a protein is different, and suggests that in trypsin there is fairly extensive migration of energy, charge or both with localization of damage at specific sites determined by this enzyme's internal organization. All three types of radiation produce appreciable amounts of ‘damaged’ (not completely inactivated) molecules which are prevented from reassuming an active configuration by the addition of 5·5m-urea; thiol reagents have a similar effect after bombardment with u.v. light or electrons. The patterns of destruction produced by γ-rays and by electrons in both the amino acid mixture and in trypsin are different (some of the yields vary by a factor of 30). This result appears to be inconsistent with the popular belief that most of the energy absorbed from γ-rays is associated with very-high-energy electrons.