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1. Protein-bound, trichloracetic acid- (TCA) insoluble 99Mo appeared in plasma a few hours after the introduction of 99Mo-labelled molybdate (30 mg Mo) into the rumen of sheep maintained on a basic diet supplemented with elemental sulphur (3 g S/d). 2. Most of the 99Mo could be displaced from its protein carrier in vitro and the labelled compounds displaced were identified by sephadex G-25 chromatography as di- and trithiomolybdate. Tetrathiomolybdate was not detected. 3. In control experiments protein-bound, TCA-insoluble 99Mo predominated in plasma after the direct administration of [99Mo]tetrathiomolybdate, either into the rumen or intravenously. The 99Mo could be displaced in vitro and [99Mo]tetrathiomolybdate identified, although [99Mo]trithiomolybdate was also present. The study provides direct evidence of thiomolybdate synthesis and absorption in ruminants in vivo.

1. The effect of acute duodenal infusion of 99Mo-labelled sodium tetrathiomolybdate on caeruloplasmin (ferroxidase; EC 1.16.3.1) was examined in sheep. The diamine oxidase activity of this enzyme with respect to two substrates, p-phenylenediamine and o-dianisidine (both at their apparent Km concentrations) was inhibited. 2. The 99Mo appeared rapidly in plasma and was at first present predominantly in a trichloroacetic acid insoluble form; inhibition of oxidase activity was related to the levels of TCA-insoluble Mo. The behaviour of the copper prosthetic groups of caeruloplasmin was altered since some plasma Cu precipitated with the protein fraction after TCA treatment. The appearance of TCA insoluble Cu was related to the level of TCA-insoluble 99Mo and corresponded to the inhibition of diamine oxidase activity.

1. Rams, fauna-free from birth and initially of 48-65 kg live weight, were allocated to two groups of ten each and given a diet containing 14 micrograms copper/g dry matter; five additional rams were killed and their livers were analysed for Cu. 2. One group (faunated) was inoculated with a mixed population of ciliate protozoa, and contained between 60 x 10(5) and 195 x 10(5) protozoa/ml rumen fluid throughout the 184 d experiment. The other group remained fauna-free. Following blood sampling, three rams in each group were killed on day 63, two on day 125 and four on day 184. One sheep in each group died during the experiment. 3. Faunated rams showed higher weight gains and feed consumption than fauna-free rams. 4. Plasma Cu concentration (microgram/ml) increased from an initial 0.82 to a final 1.00 in faunated and 1.36 in fauna-free rams. Liver Cu concentration (micrograms/g dry matter) increased from an initial 745 to a final 962 and 1684 in faunated and in fauna-free sheep respectively, representing a 4.3-fold greater increase in the fauna-free than in the faunated group. The absorption and retention of Cu was 38-50% higher in the fauna-free than in the faunated rams. 5. It was suggested that rumen ciliate protozoa increased rumen production of sulphide (through increased breakdown of soluble proteins) which complexed part of the Cu, making it unavailable for absorption and utilization. Therefore, ciliate protozoa could determine susceptibility to chronic Cu toxicity in sheep.