Differences between serum and plasma ceruloplasmin activities and copper concentrations: investigation of possible contributing factors.

Values for ceruloplasmin activities and copper concentrations were found to be lower in serum than in paired samples of plasma in both sheep and cattle. Ceruloplasmin activities in serum were 13-40% lower relative to plasma for nine different groups of animals, and 10-65% lower for individual animals (n = 112). As the values are not directly interchangeable, plasma rather than serum should be used when estimating copper nutrition in these animals. Maximum effects in serum were apparent 3-4 h after collection, the earliest time at which serum could be obtained. Lower ceruloplasmin and copper values in serum could not be attributed to the type of blood collection vessel used, subsequent storage of samples, the methods used for measuring ceruloplasmin activities and copper concentrations, the formation of fibrin in blood, or to the effects of dietary molybdenum. In contrast, the addition of neuraminidase to whole blood before clotting decreased the differences between serum and plasma ceruloplasmin activity and copper concentration in a dose-dependent manner. Of the two major copper-containing enzymes present in blood, effects of clotting were only evident with ceruloplasmin. Cu-Zn-containing superoxide dismutase activity in erythrocytes was unaffected by clot formation. The results indicate that ceruloplasmin and the copper associated with this protein are sequestered into the clot during clot formation by attachment of the enzyme to the blood cellular fraction. The minimizing of this effect by the addition of neuraminidase suggests that this attachment may be through sialic acid residues.