The use of caeruloplasmin activities and plasma copper concentrations as indicators of copper status in cattle

The aetiology of copper deficiency in cattle involves a complicated interaction where molybdenum-sulphur (thiomolybdates) and iron-sulphur compounds are formed in the rumen and interact with copper rendering it unavailable. If the thiomolybdate compound does not bind with copper in the rumen it is absorbed through the rumen wall and into the blood and subsequently the liver where it can then react with copper, reducing the activity of the copper requiring enzymes. The diagnosis of copper deficiency has been based on blood or liver copper concentrations but these can give a false indication of adequacy when substantial concentrations of thiomolybdate are absorbed by the animal. In such cases a significant proportion of the metabolic copper pool can be biologically unavailable as it is in the copper-thiomolybdate complex. The measurement of either the serum caeruloplasmin or whole blood superoxide dismutase activity have been reported as being indicative of copper status in cattle. We have developed a method of identifying cattle that will respond to copper therapy by estimating the copper that is biologically available and not bound to thiomolybdate. This study has used blood from over 1,500 cattle in which the plasma copper concentrations and caeruloplasmin activities have been determined. Bovine caeruloplasmin is a 132,000 dalton protein, which contains 6 copper atoms per molecule and has a turnover of two to three days. Copper present in caeruloplasmin accounts for about 88% of plasma copper. Regression analysis shows that caeruloplasmin activity, as expressed as milli-grams of caeruloplasmin per decalitre (mg/dl), is correlated with plasma copper concentration (mu mole per litre plasma (r = 0.55; p<0.01). We propose that a method for determining whether cattle will respond to copper therapy can be based on the relationship between caeruloplasmin (CP) activity and plasma copper (Pl-Cu) concentration. The model predicts a CP/Pl-Cu ratio of 2.0:1 for the normal animal. If the ratio is less than 2.0 then there is free thiomolybdate being absorbed into the blood which then renders copper unavailable and reduces the activities of the copper enzymes. This CP/Pl-Cu ratio can therefore be used as a method of identifying cattle which, despite an apparently good copper status, will respond to copper therapy as the clinical problem is one of thiomolybdate toxicity rather than the lack of copper for metabolic function.