الكاتب: Multiple References

For their supply of vitamin B,, herbivorous animals are dependent on its production by bacteria which inhabit the alimentary tract and which utilize cobalt from the host’s diet for this purpose. In some species, such as the rabbit, coprophagy is essential for the animal to obtain the vitamin B,, which results from synthesis in the lower gut, whilst ruminants rely on synthesis of the vitamin by rumen bacteria. Many of the early observations on the Co requirements of sheep and on the metabolic consequences of Co deficiency have been reviewed and summarized by Underwood (1977). Suffice it to mention here that the characteristic features of Co deficiency in sheep, namely inappetance and associated weight loss and anaemia, usually become apparent when the serum content of the vitamin falls to approximately 200 pg/ml. Because vitamin B,, is an essential component of at least two enzyme systems in the body, namely, methylmalonylCoA mutase (EC 5.4.9.92) and a methyltransferase involved in methionine biosynthesis, the metabolic consequerices of reduced availability of the vitamin to the sheep include enhanced urinary outpul of methylmalonic acid (MMA) and of formiminoglutamic acid (Gawthorne, 1968). Because ruminants are particularly dependent on gluconeogenesis from propionate, the urinary loss ofits primary metabolite, MMA, could have serious physiological consequences.