A Proposed Mechanism of Protein Inactivation 289 



amino acid, and as much as fifteen times as efTective as a quantum which spht 

 a peptide bond. 



(h) The electron spin resonance measurements reported by Gordy (11) 

 indicate that irradiation of proteins usually converts some of the disulfides 

 into free radicals. 



(i) The native configuration of the ribonuclease molecule can be greatly 

 disrupted (as indicated by viscosity measurements) by destroying its H-bonds 

 with urea without destroying its function; however, as soon as the disulfides 

 are oxidized activity is lost immediately (12). 



(j) The recent findings of Leone (13) are in excellent agreement with the 

 two main aspects of the hypothesis. First, he found that the antigenic properties 

 of y-irradiated serum albumin were the same whether an average of 9 or 90 eV 

 per molecule had been absorbed indicating that irradiation caused this protein 

 to unfold in a characteristic fashion. Second, the single S— S bond of serum 

 albumin was likely involved since ultracentrifugation patterns of the irradiated 

 material contained only monomers, dimers and small amounts of di- and 

 tripeptides, with no evidence of larger aggregates. Setlow and Doyle (10) 

 also noted smaller fragments produced by ultraviolet irradiation of trypsin, 

 but found that the degradation components produced by a wavelength very 

 favorable to a cystine effect were more prominent and homogeneous than those 

 produced by a less favorable wavelength. They interpreted this as evidence 

 for more than one inactivation mechanism. 



(k) Studies of the inactivation of protein monolayers (2,3,14,15) yielded 

 results which were consistent with the model; however, the data could only 

 disprove but not prove the hypothesis. For example, molecules in compressed 

 monolayers show reduced inactivation from both surface forces (14) and 

 irradiation (15). This was expected, since the scheme proposed here would 

 predict that an external force, such as the monolayer film pressure, should 

 lower the probability of the second S — S bond being ruptured; and even if 

 step 3 occurred, an external pressure should be able to maintain molecular 

 structure sufficiently intact so that restitution would be enhanced. However, 

 it was estimated that the proposed mechanism might account for no more 

 than two-thirds of the inactivation observed. 



Some proteins, such as ovalbumin (16) and serum albumin, contain fewer 

 than two S — S bonds. In such proteins other bonds which (i) have comparatively 

 small rupture energies and (ii) are involved in latching large segments of the 

 molecule together, would assume the functions of the cystine in this scheme. 



The present model provides a specification of the 'target volume' for irradia- 

 tion inactivation. Associated with each atom is a probability that energy 

 will be absorbed and migrate to the weak link in amounts sufficient to rupture 

 that structure completely. The sum of these probabilities over the whole 

 molecule gives the 'effective target volume'. Thus, the actual physical target 

 need not have sharp boundaries (see also discussions by Lea (17), Burton (18) 

 and Setlow and Doyle (10) of target elements having probabilities other than 

 or 1). 



The probabilities, and thus the 'size' of the target volume, will depend 

 upon a number of factors. For instance, the possibility— discussed by Platz- 

 MAN and Franck (7) — of complementary effects between thermal and absorbed 



