A new bioassay for assessment of copper availability and its application in a study of the effect of molybdenum on the distribution of available Cu in ruminant digesta.

Investigations were carried out on the feasibility of using an oral repletion technique in the rat to assess the bioavailability of copper in experimental sources providing no more than 250 micrograms Cu from any one source. Preliminary studies on the response in plasma Cu of partially Cu-depleted rats given repletion doses of 20-50 micrograms Cu as CuSO4/d on four consecutive days indicated that this index of Cu status was insufficiently sensitive to Cu dose. In contrast, the activity of cytochrome c oxidase (EC I.9.3.I) in the duodenal mucosa of partially Cu-depleted rats showed a measurable and uniform response to 10 micrograms Cu as CuSO4/d given on three consecutive days. Furthermore, when the rats were given 0, 2.5, 5.0 or 10.0 micrograms Cu/d, the increase in cytochrome c oxidase activity above that of the unsupplemented control group was linearly related to Cu dose. The mean response in cytochrome c oxidase activity in groups of eight rats was therefore used to assess the availability of Cu from experimental sources relative to that of Cu as CuSO4, only 240 micrograms Cu being required from each experimental material. The assay was used to study the effect of the Cu-antagonist molybdenum on the distribution of available Cu in digesta from sheep given dried grass either untreated (1.6 mg Mo/kg dry matter (DM)) or treated with ammonium molybdate (11.6 mg Mo/kg DM). The relative availability of Cu in untreated dried grass (75%) was substantially higher than in rumen (12%), duodenal (43%) or ileal (28%) digesta. In all cases, addition of Mo to the diet resulted in a substantial reduction in Cu availability. The effects of Mo on availability of Cu are discussed with special reference to the possible involvement of thiomolybdates in the Cu-Mo antagonism.