Comparative studies of the effects of molybdenum and sulphur on copper metabolism in sheep Ovis aries and red deer Cervus elaphus

Abstract

The difference between sheep and deer in the effects of molybdenum (Mo) and sulphur (S) on copper (Cu) metabolism was investigated in two dietary experiments (Chapter 4) using 8 animals of each species. In Exp 1 the animals were offered hay diet containing 3, 6, 12 or 24 mg Mo/kg DM and 2.3 g S/kg DM for 35 days. In Exp 2 the hay, containing 6, 12, 24 or 48 mg Mo/kg DM and 5.3 g S/kg DM, was offered for 16 days. With the low S diet (Exp 1) the total plasma Cu was changed from 0.75 to 0.63, 0.77, 0.81 and 1.03 mg/l, respectively, in sheep and from 0.95 to 0.77, 0.79, 0.67 and 1.01 mg/l in deer and the proportion of TCA-soluble/total Cu was reduced by 0.05, 0.09, 0.07 and 0.21 in sheep and by 0.01, 0.05, 0.08 and 0.11 in deer. Caeruloplasmin (CP) activity was not affected by the diet. With the high S diet (Exp2) total Cu concentration increased from 0.81 to 0.82, 0.97, 1.43 and 1.41 mg/l in sheep and changed from 1.08 to 0.72, 0.96, 0.88 and 1.21 mg/l in deer. The proportion of TCA-soluble/total Cu was reduced by 0.20, 0.50, 0.60 and 0.63 in sheep and by 0.05, 0.10, 0.23 and 0.48 in deer. The mean CP activity was reduced from 1.09 to 0.89 u/ml in sheep and from 0.99 to 0.32 u/ml in deer. Direct-reacting Cu (DRCu) was increased from 0.08 to 0.34 mg/l in sheep and from 0.06 to 0.17 mg/l in deer. It was concluded that sheep and deer differed in the effect of dietary Mo and S on Cu metabolism, deer being more sensitive to dietary S than dietary Mo and in both animal species the increase in dietary S enhanced the effect of dietary Mo on Cu metabolism.

To investigate the effect of the quantity and the species of thiomolybdate (TM) before binding of TM-Cu and the degradation of TM-Cu compounds in plasma in the two animal species two experiments were conducted (chapter 5). In Exp1 each of 3 plasma samples derived from 6 sheep or 3 individual deer was divided into nine 35-ml portions, each of which received one of three quantities - low, medium or high - of either di-, tri- or tetra-thiomolybdate (TM) and incubation at 38ºC for 70 min. The amounts of TM added were, for the low treatment, 1.8, 1.9 and 2.0 mg ammonium TM, respectively, and the medium and high levels were 200 and 400% of the amount used on the low treatment. TCA-solubility was reduced after addition of TM2, TM3 and TM4, respectively, by 0.67, 0.90 and 1.0 mg/l in sheep and by 0.60, 0.75 and 0.85 mg/l in deer. Three levels of TM, low, medium and high, reduced TCA-soluble Cu by an average of 0.63, 0.90 and 1.04 mg/l in sheep and by 0.56, 0.72 and 0.91 mg/l in deer, respectively. DRCu was reduced after addition of TM2, TM3 and TM4 by 0.03, 0.05 and 0.05 mg/l in sheep and by 0.02, 0.04 and 0.05mg/l in deer, respectively. DRCu was reduced due to amount of TM by 0.04, 0.05 and 0.05 mg/l in sheep and by 0.03, 0.04 and 0.05 mg/l in deer, respectively. CP activity was not affected by treatment. It was concluded that plasma Cu susceptibility to TMs depends on the species and on the amount of TM; the lowest effect caused by TM2, or by the lowest amounts, and TM4 appearing of the greatest potency. Deer plasma Cu tended to be less affected by TM than sheep plasma Cu.

In Exp2 6 sheep and 6 deer were injected intravenously with TM2, TM3 and TM4, in a latin square designed experiment, to provide 5.4, 5.7, and 6.0 mg ammonium salt/l of estimated plasma volume, respectively. Blood samples were taken before and at intervals between 0.5-180 min after injection. TM2, TM3 and TM4 all increased total plasma Cu similarly in the two animal species from 0.75 to an average of 0.89, 0.83 and 0.82 mg/l, respectively. TCA-soluble Cu concentration was reduced similarly in sheep and deer within 0.5-1 min by 0.60, 0.72 and 0.83 mg/l after TM2, TM3 and TM4, respectively. TCA-insoluble Cu subsequently decreased with a rate constant of 0.25, 0.06 and 0.06 mg/l/min after TM2, TM3 and TM4, respectively, and with an average rate constant of 0.09 and 0.15 mg/l/min in sheep and deer, respectively. It was concluded that the ability of plasma to clear the effect of TM depends on the species of TM, with TM2 being most rapidly cleared. Ability to clear the effect of TM on plasma Cu appear to be slightly greater in deer than in sheep.

The formation of TMs in the rumen was investigated (Chapter 6) using intraruminal injection of ⁹⁹Mo to 4 sheep and 4 deer offered silage diets containing 5.5 g S/kg DM and 12 or 24 mg Mo/kg DM. The TM fractions in plasma were separated by Sephadex G-25 chromatography. TM2 and TM3 were identified in all sheep and TM4 in two sheep. There was no effect of diet. The largest peak of activity was in the TM3 region from 3 sheep. In deer the largest peak was in TM2 region in 3 animals, with no TM peak identifiable in the fourth animal. A small peak of TM3 was recorded from two deer and a very small peak in the TM4 region of the other in two deer. It was concluded that sheep possibly have a greater tendency to produce or absorb greater proportions of TM3 and TM4 than deer.

A feature of all the studies was the considerable variation exhibited between animals of both species in response to dietary Mo and S. However, it was possible to conclude that sheep and deer are different in the effect of dietary Mo and S on Cu metabolism; deer are more susceptible to dietary S than dietary Mo at levels which may be experienced in practice. TM4 is the most potent species of TM in its effect on plasma Cu. Sheep plasma Cu may have greater binding ability for TM than deer plasma Cu. Deer probably have a greater ability to clear the effect of TMs on plasma Cu than sheep and sheep have a greater tendency to produce TM3 and TM4 the deer.