510 KAUIATION BIOLOGY 



MumlHT of factons. The t'rytlu'mal .spectrum is ohviou.sly domiiiated by 

 the attenuating? properties of the corneum which determine the spectral 

 minimum at t).28 n and must influence the position of the maximum at 

 0.2U7 M- Some of the attempts to interpret the erythemal spectrum do 

 not take this into account. The most adecjuate attempt to examine the 

 erythemal spectrum in terms of the radiation actually absorbed in the 

 epidermis was that of Mitchell (1938) who used the data of Lucas (1931) 

 to correct for epidermal t ransmission. Mitchell arrived at the conclusion 

 that the light absorber is protein, but sub.se(iuent evidence throws doubt 

 on this. In the first place, neither the inhibition of erythema by longer 

 wave lengths nor the elaboration of dilator substance in the corneum was 

 recognized at the time of Mitchell's analysis. These factors should affect 

 the shape of the erythemal spectrum to an unpredictable extent. It is 

 implicit in Mitchell's analysis that the (juantum efficiency [factor y in 

 Eq. (13-6)] is independent of w^ave length. But, if the inactivation of 

 enzymes by ultraviolet radiation may be taken as characteristic of the 

 action of this agent on proteins in general, this cannot be as.sumed since 

 the inactivation is, in some cases at least, wave-length dependent (e.g., 

 McLaren, 1947). The recent demonstration that dilator substance is 

 formed in the corneum frustrates any attempt at analysis in terms of a 

 single substance. 



It seems altogether reasonable to draw an analogy between the effects 

 of ultraviolet radiation on the cells of the human malpighian layer and the 

 action of this agent on cells in general, which has been discussed elsewhere 

 in this volume. Evidence seems at present to favor the idea that the light 

 absorber in such effects is, as a rule, nucleic acid, and recent findings of 

 Blum et al. (1950) indicate that, in the case of retardation of cell division 

 in sea urchins' eggs, the locus of action is the nucleoprotein. But in the.se 

 experiments, as is true in general, the ultraviolet radiation may po.ssibly 

 produce changes that are not reflected in the particular criterion studied 

 (e.g.. see Blum, Cook, and Loos, 1954). It must therefore be kept in mind 

 that, although luicleoproteins in the malpighian cell are without doubt 

 affected by the ultraviolet radiation that reaches these cells, this should 

 also be true of the simple proteins in the cells to a greater or less extent. 

 Thus, while it seems reasonable that the various phj\siological responses 

 studied in sunburn are initiated by the action of ultraviolet radiation on 

 nucleoproteins, it is possible that some of them involve other photo- 

 chemical reactions. Other light absorbers have been suggested in con- 

 nection with various aspects of sunburn (e.g., Ellinger, 1930; Rothman 

 and Rubin, 1942; Frankenburger, 1933), but these have various objec- 

 tions which will not be di.scussed at length here (see Blum, 1941a, 1945). 

 Even in systems optically much better suited to the comparison of action 

 and absorption spectra, the distinction between nucleic acid and simple 

 protein may be difficult, and there are other cell constituents absorbing in 



