EFFECT OF U. V. IRRADIATION ON ENZYME SYSTEMS 59 of different theories (12). Perhaps the most widely accepted theory speculates that sunlight sets up some type of free radical reaction which in turn results in the release of some noxious agent thus setting the stage for the classical symptoms of sunburn. While the nature of these initial re- actions is unknown, it appears that it takes several hours for the body to respond via gross clinical or histologic changes. O. 3-- trol cJ - . . irradiated 0.2-- 0.1-- 0.5 1.0 1.5 2.0 Time (min.) Figure 2.--Effect of U. V. irradiation on metabolism of glucose-6-phosphate by rat epidermis. Rate of TPN reduction (increase in O.D. at 340 mr0 rs. Time (min.). It is only reasonable to expect that during this time lapse, enzymatic changes are occurring which initiate some of the cellular responses seen. While the work reported in this paper does not clarify the mechanism of the initial damaging effects of U. V. irradiation, it does point out some of the early enzymatic changes occurring. The damaging effects of U. V. irradiation on many enzyme systems is well known (13). Usually these effects have been studied with in vitro systems

60 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.01 0.81 0.6 i 0.4 I 0.2 i B I I I 0.5 1.O 1.5 Time (min.) A - Control B - U.V. irradiated Figure &--Effect of U. V. irradiation on epidermal metabolism of glucose with added exo- genous hexokinase. Rate of TPN reduction (increase in O.D. at 340 mu) rs. Time (min.). and are often related to denaturation of the enzyme protein itself or in- activation of some co-factor needed for enzyme activity. Thus, for example, it has been reported that U. V. light can inactivate transaminase systems in vitro (14). Exposure of the under side of mouse skin to U. V. irradiation has been reported to cause a decrease in succinic dehydrogenase activity as estimated histologically (15). Exposu, of skin or skin extracts to U. V. causes an increase in phosphorylase activity (16, 17). In our studies, the most marked effects observed as a result of U. V. irradiation of rat skin in vivo were a decrease in the rate of glucose oxidation and a complete inhibition of two transaminase systems present in the epidermis. Both of these enzymatic effects manifested themselves within one hour after irradiation and continued up to four hours after irradiation. At time intervals of one-half hour after irradiation, this inhibition of glucose oxidation was not noted, and indeed in several experiments a slight