The assessment of biotin status in man and animals.

The metabolic role of biotin in animals is as a prosthetic group in enzymes involved in carboxylation reactions. There are three enzymes of principal importance. Pyruvate carboxylase (PC; EC 6.4.1. I ) catalyses the conversion of pyruvate to oxaloacetate within the mitochondrion. Oxaloacetate is an intermediate in the biosynthesis of phosphoenol pyruvate and, ultimately, glucose. Thus, PC plays an essential role in gluconeogenesis via pyruvate. Liver and kidney are the tissues with greatest gluconeogenic capacity and they also contain highest activities of the enzyme. The conversion of pyruvate to oxaloacetate also influences lipogenesis, since acetyl-CoA generated within the mitochondrion must combine with oxaloacetate to form citrate before it can pass into the cytoplasm. Once there, citrate is cleaved to release acetyl-CoA which can then enter the lipogenic pathway. Lipogenesis also requires the presence of another biotin-dependent enzyme, acetyl-CoA carboxylase (ACC; EC 6.4. I .2), for normal function. This enzyme catalyses the conversion of acetyl-CoA to malonyl-CoA, the starting point in the biosynthesis of fatty acids. The third enzyme is involved in propionate metabolism. Propionyl-CoA carboxylase (PCC ; EC 6.4. I .3) converts propionyl-CoA to methylmalonyl-CoA which can ultimately enter the citric acid cycle after conversion to oxaloacetate. This pathway is especially important for ruminants which, because of ruminal degradation of glucose, have to rely on it almost entirely for their supply of glucose. Biotin is thus directly involved in the vitally important metabolic process of glucose and fat synthesis and can also have profound effects on other pathways by its influence on many metabolic intermediates. It is therefore necessary for normal utilization of nutrients, growth of body tissues, reproduction and ultimately, life. This general metabolic role, together with its involvement in certain specific disorders, means that an inadequate supply of biotin can have serious consequences and, in farm animals, can result in considerable economic loss as a result of both decreases in performance and increases in mortality. However, synthetic biotin is a comparatively expensive vitamin and an over-supply to intensively-housed animals can be wasteful. Thus it is of considerable importance to be able to assess the extent to which an animal’s need for biotin is being met.