Rapid spot test for glutathione peroxidase activity: comparison with a spectrophotometric procedure and assessment of the test as a measure of selenium in the blood of grazing ruminants

Abstract

The activity of the seleno-enzyme glutathione peroxidase in ovine and bovine blood was measured by using spot and spectropholometric assays, and was used to predict the selenium content of blood. Sources of error were examined in three experiments. In the first, the time from sampling and temperature at which samples were stored, the quantity of heparin used, the temperature of the assay and exercise of the sheep immediately preceding sampling were varied. In the second, relationships were calculated between blood selenium content and glutathione peroxidase activity assayed by the spot test and with a spectrophotometer. In the final experiment sources of variation in the spot assay were examined as affected by variation between animals, operators and days. Changes in glutathione peroxidase activity with time following sampling were small, and activities of samples stored at 3� and -28�C were similar; freezing samples in liquid nitrogen reduced the activity, which was also sensitive to the temperature of the assay. Exercise and the quantity of heparin used did not affect activity. Allometric relationships were calculated between blood selenium content and glutathione peroxidase activities determined by both spot and spectrophotometric assays. The latter was more precise. Differences between operators and between ewes and wethers in enzyme activity were not significant; variation between animals in blood selenium content increased with mean blood selenium content. It was concluded that the spot test would be satisfactory for identifying animals which could respond to selenium supplementation and would be particularly useful when equipment was not available for automated selenium determinations.