MICRONUTRIENT ROLES IN PHYSIOLOGY AND HEALTH
Location: Grand Forks Human Nutrition Research Center
Title: SERUM SUPEROXIDE DISMUTASE 3 (EXTRACELLULAR SUPEROXIDE DISMUTASE)ACTIVITY IS A SENSITIVE INDICATOR OF CU STATUS IN RATS
Submitted to: Journal of Nutritional Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 14, 2005
Publication Date: August 2, 2005
Citation: Johnson, W.T., Johnson, L.K., Lukaski, H.C. 2005. Serum superoxide dismutase 3 (extracellular superoxide dismutase) activity is a sensitive indicator of Cu status in rats. Journal of Nutritional Biochemistry. 16:682-692.
Interpretive Summary: Examination of usual dietary copper intakes by humans compiled in the National Health and Nutrition Examination Survey II, Continuing Survey of Food Intakes by Individuals II, and Total Diet Studies indicates that up to 50% of men and 75% of women fall below the recommended dietary allowance for copper. While severe copper deficiency in humans is rare, the results of the diet surveys indicate that marginal copper deficiency may be a common occurrence in humans. The detection of marginal copper deficiency in humans requires measuring several indices of copper status in blood or serum in order to obtain an accurate assessment of copper status. Superoxide dismutase (SOD) is a copper- and zinc-dependent enzyme found in red cells and to a much lesser extent, in serum. SOD activity in red cells is often used as an index of copper status. However, the measurement of red cell SOD activity requires several steps, including the isolation of red cells from the blood and removal of hemoglobin before the activity can be measured. Although SOD activity in serum may be a more convenient index of copper status, its usefulness as an index is hindered by its extremely low activity in serum. In the present study, we used a method for assaying serum SOD activity that increased the sensitivity of the assay and reduced the chance of interferences. It was found that serum SOD activity assayed under these conditions was responsive to marginal copper deficiency in rats, paralleled changes in more common biochemical indices of copper status to dietary copper intakes, and was directly related to liver copper concentrations. In addition, it was found that serum SOD activity was more sensitive to marginal copper deficiency than it was to marginal zinc deficiency. Thus, SOD activity in serum can be used as an index of copper status and is particularly useful to detect marginal copper deficiency when used in conjunction with other serum indices of copper status.
Sensitivity of the assay for Cu, Zn-superoxide dismutase 3 (SOD3), the predominant form of SOD in serum, can be increased and interferences caused by low molecular weight substances in the serum can be reduced by conducting the assay at pH 10 with xanthine/xanthine oxidase and acetylated cytochrome c as superoxide generator and detector, respectively. Serum SOD activity was assayed under these conditions in an experiment where weanling, male rats were fed diets for 5 weeks containing 3, 5 and 15 mg Zn/kg with dietary Cu set at 0.3, 1.5, and 5 mg Cu/kg at each level of dietary Zn. Serum SOD responded to changes in dietary Cu but not to changes in dietary Zn. A second experiment compared serum SOD activity to traditional indices of Cu status in weanling, male and female rats after they were fed diets containing, nominally, 0, 1, 1.5, 2, 2.5, 3 and 6 mg Cu/kg for seven weeks. Serum SOD activity was significantly lower (P<0.05)in male rats fed diets containing 0 and 1 mg Cu/kg and female rats fed diet containing 0 mg Cu/kg compared to rats fed diet containing 6 mg Cu/kg. These changes were similar to changes in liver Cu concentrations, liver cytochrome c oxidase activity and plasma ceruloplasmin in males and females. Serum SOD activity was also strongly, positively correlated with liver Cu concentrations over the entire range of dietary Cu concentrations (R2=0.942 in males, R2=0.884 in females, P<0.0001). Plots of serum SOD activity, liver Cu concentration, liver cytochrome c oxidase activity and ceruloplasmin as functions of kidney Cu concentration all had two linear segments that intersected at similar kidney Cu concentrations (18-22 ug/g dry kidney in males, 15-17 ug/g dry kidney in females). These findings indicate that serum SOD activity is a sensitive index of Cu status.