Effects of aging and physical activity on superoxide dismutase in human erythrocytes

This study investigated the relationship of the superoxide scavenging enzyme, superoxide dismutase (SOD), with respect to age, gender and physical activity levels of subjects. Blood was collected from 129 healthy human volunteers (80 males and 49 females) between the ages of 7 and 87. A physical activity index (in kcal/wk) was calculated for each subject. A continuous line assay for Cu,Zn-SOD activity was performed and a function analogous to a Lineweaver-Burke equation in a nonclassical enzyme assay was used to derive a linear reciprocal plot, whose slope was proportional to (kcat Et/Km)'. This kinetic parameter is a reflection of total SOD activity in the RBC. In order to isolate the influences, if any, of age, physical activity and gender on (kcat E m ) ' , subjects were divided into the following groups: (1) Active vs Inactive, (2) Male vs Female, (3) 20 year age brackets, and (4) 20 year age brackets subdivided into Active or Inactive individuals. With advancing age there was seen to be a gradual decline in (kcat EtIKm)'. The drop in this parameter across the age spectrum was estimated to be approximately 50%. Physical activity, however, influences (kcat E W ) ' in the opposite direction, as the active individuals displayed a higher mean (kcat Et/Km)' than their inactive counterparts. The mean value for the active group was 0.64 and for the inactive group it was 0.46 (pc0.01). A multivariate regression analysis indicated that the following linear function described the data: (kcat E m ) ' = -0.00188 * Age (years) + 0.000014 * Physical Activity (kcal/wk) Hence, the decrease in (kcat Et/Km)' due to one additional year of life could be equally compensated for by an overall 134 kcal increase in weekly energy expenditure. The experimental parameter, (kcat Et/Km)', seems to correlate with metabolic rate. This assumption is supported by the following points: (1) SOD activity decreases by approximately 50% over a lifetime; this parallels a similar drop in metabolic rate, (2) This study showed that males had higher (kcat Et/Km)' values than females which agrees with differences in metabolic rates between the two sexes, (3) Active individuals were found to have higher (kcat Et/Km)' values than their inactive counterparts. In this study it was also noted that extract protein concentration was higher in the active (6.57 ug/mL) versus inactive subjects (6.16 ugIrnL) (difference in means was not statistically significant) and in the younger subjects as compared to the older ones. Presumably, oxidatively damaged proteins are more fragile and, therefore, cannot survive the harsh chloroform/ethanol extraction procedure used for partial purification of SOD. Hence, extract protein concentration is an index of cellular oxidative stress in which higher values are diagnostic of better health. Arrhenius plots were used to resolve whether the higher SOD activities in young and active subjects was due to an increase in catalytic efficiency of the enzyme or changes in the amount of the SOD protein. No significant difference was detected between the Arrhenius plots of any of the subgroups studied Thus, the increase in SOD activity was attributed to increased concentrations of the enzyme due to either enhanced gene expression or decreased SOD degradation rates.