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The absorption of labelled molybdenum compounds was studied in pairs of sheep exchanging digesta via re-entrant duodenal cannulae. Tri- and tetrathiomolybdate 99Mo were rapidly absorbed from the rumen to circulate in plasma in a protein-bound and in a TCA-insoluble form. The compounds were also absorbed from the small intestine although some breakdown was evident. Initially, molybdate was poorly absorbed from the rumen but after several hours the concentration of protein-bound, TCA-insoluble 99Mo increased in plasma. This provides evidence of rumen thiomolybdate synthesis. The results indicate that thiomolybdates are absorbed directly from the rumen and despite the sensitivity of the compounds to acid, some absorption from the small intestine occurs after passage through the abomasum. Rumen absorption could be a contributory factor to ruminant sensitivity to the effects of Mo compounds.

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.

The effect of molybdate, dithiomolybdate, tetrathiomolybdate and diethyldithiocarbamate on caeruloplasmin amine oxidase activity was examined using o-dianisidine as a substrate. Enzyme activity was strongly inhibited by tetrathiomolybdate (1.5 X 10(6)M), oxidation of another substrate, p-phenylenediamine, was inhibited only slightly.

The synthesis and purification of the sodium salts of di (MoO2S2(2-)), tri (MoOS3(2-)) and tetra (MoOS4(2-)) thiomolybdate is reported. All three compounds were reversible inhibitors of the ovine ceruloplasmin (Cp) catalysed oxidation of O-dianisidine. Na2MoO2S2 inhibited via a non-competitive mechanism whereas Na2MoOS3 showed mixed non competitive and Na2MoS4 competitive mechanisms. All showed upward curving slope replots. Molybdenum trisulfide (MoS3) was synthesized and displayed mixed type inhibition kinetics but with a linear slope replot. Preliminary evidence suggested that both MoOS3(2-) and MoS4(2-) may also be substrates for ovine Cp.