UVB irradiation alters the activities and kinetic properties of the enzymes of energy metabolism in rat lens during aging

Ultraviolet (UV) radiation is one of the major risk factors of cataract (loss of eye-lens transparency). The influence of UVB radiation (300 nm; 100 μW cm−2) on the activity and apparent kinetic constants (Km and Vmax) of rat lens hexokinase (HK; EC 2.7.1.1), phosphofructokinase (PFK; EC 2.7.1.11), isocitrate dehydrogenase (ICDH; EC 1.1.1.41) and malate dehydrogenase (MDH; EC 1.1.1.37) of energy metabolism has been investigated by irradiating the lens homogenate of three- and 12-month-old rats. In the three-month-old group specific activities of HK and PFK are reduced by 56 and 43%, respectively, and there is no change in ICDH and MDH activities after a 24 h exposure. On the other hand, in the 12-month-old group the decreases are 72, 71, 24 and 16% for HK, PFK, ICDH and MDH, respectively. UVB irradiation increases the apparent Km of HK and PFK (in both age groups), whereas the Km of ICDH and MDH is not altered. While the decrease in Vmax of these enzymes due to UVB exposure is only marginal in three-month-old rats, it is more pronounced (significant) in 12-month-old rats. A similar decrease in enzyme activities of HK and PFK is also observed upon UVB exposure of the intact rat lens. The photoinduced changes in energy metabolism may in turn have a bearing on lens transparency, particularly at an older age.