EFFECT OF U. V. IRRADIATION ON ENZYME SYSTEMS 57 was evaluated via oxygen consumption and compared to homogenates pre- pared from nonexposed animals. The results indicated that at the one- half hour interval, the rate of O2consumption was often comparable or slightly elevated from that of unexposed skin. However, at all other time intervals, the O2 consumption of the homogenates prepared from the ex- posed animals was markedly depressed. In some cases this depression in oxygen consumption was as great as 75 per cent. Since previous work in our laboratories and others (9-11) indicated the presence of the hexose monophosphate shunt in glucose metabolism in skin, it was decided to investigate further the effect of U. V. irradiation on glucose oxidation by following the rate of TPN reduction in epidermal extracts prepared from exposed and nonexposed animals. Animals were irradiated and sacrificed at intervals of one-half hour and one hour after exposure. Epidermal extracts were prepared from their skins as well as from the skin of a nonexposed animal and the early stages of glucose oxidation followed spectrophotometrically via TPN reduction. The results are presented in Table 1. TABLE 1--THE EFFECT OF U. V. IRRADIATION ON GLUCOSE METABOLISM* Control .•----Irradiated Sacrificed .... Time, min. Nonirradiated 1/= hr. later 1 hr. later 1 0. 258 0. 295 0. 090 2 0.300 0.350 O. 150 * Glucose metabolism measured as rate of TPN reduction (increase in O.D. at 340 m/•). TABLE 2--GOT AND GPT TKANSAMINASE ACTIVITY OF IRRADIATED rS. NONIRRADIATED RAT SKIN GOT Activity GPT Activity Nonirradiated 15 X l0 s units* 3 X l0 s units* Irradiated (sacrificed one hr. 0 0 after irradiation) * One unit equals a decrease in optical density of 0.001 units per minute per gram wet weight of tissue. These results suggest an increase in glucose metabolism at the one-half hour interval and a depression at the one hour interval. Since a marked depression in glucose oxidation was consistently noted at time intervals from one hour to four hours after U. V. irradiation, further attempts were made to find the cause for this inhibition. Rats were irradiated in the usual manner and sacrificed one hour after exposure. Epidermal extracts were prepared and glucose oxidation studied via TPN reduction. As usual, a marked reduction in glucose oxidation by irradiated skin was noted (see Fig. 1). When, however, glucose-6-phosphate was used as a substrate in place of glucose, no dif-

58 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ference was observed between the rate of glucose metabolism in exposed rs. nonexposed animals (Fig. 2). These results suggested that the cause for the decreased rate of glucose metabolism in the irradiated animals' skin was related to some inactivation or inhibition of the glucose phosphorylating mechanism. The supposition was further confirmed by showing that when exogenous hexokinase (the enzyme which converts glucose to glucose-6-phosphate) was added to the epidermal homogenate system, no difference in the rate of glucose metabolism could be observed between irradiated and nonirradiated skin (Fig. 3). The effect of U. V. irradiation on one other enzyme system was investi- gated. In earlier work in this laboratory it was shown that the rat epider- mis contains at least two transaminase systems. These are glutamic- oxalacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT). The activities of these two transaminase systems were compared in epidermal extracts of normal rats rs. epidermal extracts of U. V. irradiated animals. The results (Table 2) indicated a complete inhibition of trstns- aminase activity in the extract from the irradiated animals. D•seuss•o• The time delay between exposure to U. V. irradiation, and the onset of clinical or histological evidence of damage has been explained by a number 0.3• i i i i A - Coz•trol B - U.V. irradiated 0.5 1.O 1.5 2.0 Time (min.) Figure 1.--Effect of U. V. irradiation on glucose metabolism by rat epidermis. Rate of TPN reduction (increase in O.D. at 340 m/z) rs. Time (min.).