Two divergent theories of the mechanisms involved in U.V. inactivation of enzymes have been developed over a period of years. The one proposes that the random destruction of any amino acid residue causes inactivation. The second emphasizes the importance of the disruption of a cluster of specific cystines and hydrogen bonds responsible for the spatial integrity of the active center. Consistent with the latter postulates, previous studies in this series have shown that the number of cystines disrupted in trypsin is correlated with loss in enzymic activity and in ribonuclease the disruption of cystines is very non‐random. In the present experiments only 1–1.1 titrable (with pCMB) SH groups are formed for each cystine disrupted. The available evidence suggests that about 0.1 of the disrupting events produce two SH groups: 0.2 lead to the formation of one ‐SH and one ‐SOnH: and about 0.7 involve the cleavage of a C‐S bond with the appearance of one ‐SH plus an oxide (perhaps ‐C = O) of the C moiety. The disruption of cystine is such that neither serine, alanine, glycine nor H2S is formed in large amounts. The kinetics of SH production emphasize that the various cystines in RNase and trypsin have unequal radiation sensitivities; some must differ by at least a factor of 10. Tentatively it is concluded that u.v. absorbed directly by cystines (ca 20 per cent) causes random, symmetric breakage of S‐S, whereas quanta absorbed in aromatic residues (ca 80 per cent) lead to efficient energy transfer and localization in chromophores adjacent to critical cystines followed by addition of hydrogen to specific C‐S bonds resulting in their cleavage.

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