Effects of cobalt or hydroxycobalamin supplementation on vitamin B-12 content and (S)-methylmalonyl-CoA mutase activity of tissue from cobalt-depleted sheep.

Lambs (3 months of age) and ewes were fed ad libitum a depletion diet low in cobalt (0.06 ppm) for 7 months. Five sheep were then assigned to each of the following treatments: 330 micrograms hydroxocobalamin (OH-B-12) intramuscularly, 2.7 mg cobalt (Co) orally, and no supplementation (no suppl). Treatments were given on alternate days for 9 weeks. Both forms of resupplementation increased the animal's body weight at slaughter compared to nonsupplementation, while absolute weights and protein concentrations of brain, liver, heart, rumen and kidney were not affected. Supplementation increased concentrations of vitamin B-12 in all tissues; Co and OH-B-12 being equally effective in brain and ruminal mucosa, whereas OH-B-12 had a greater effect in heart, liver and kidney. The greatest concentrations of vitamin B-12 were observed in liver (2630 +/- 160, 1500 +/- 230 and 60 +/- 20 ng/g wet liver for OH-B-12, Co and no suppl groups, respectively) and kidney, although liver contained the greatest absolute amount of vitamin B-12. Activity of (S)-methylmalonyl-CoA mutase, assayed in the presence of added coenzyme B-12, was not increased with resupplementation except in kidney. The activity of mutase without coenzyme added in vitro was correlated with tissue content of vitamin B-12. Through this study we demonstrate that in sheep tissue the activity of (S)-methylmalonyl-CoA mutase is limited by coenzyme rather than enzyme per se. Liver possesses the greatest quantitative activity of mutase and is most responsive to alterations of vitamin B-12 status.