Biotin is an essential coenzyme in carbohydrate, fat and protein metabolism. It is involved in conversion of carbohydrate to protein and vice versa, as well as conversion of protein and carbohydrate to fat. Biotin also plays an important role in maintaining normal blood glucose levels from metabolism of protein and fat when the dietary intake of carbohydrate is low. As a component of several carboxylating enzymes, it has the capacity to transport carboxyl units and to fix carbon dioxide (as bicarbonate) in tissue.
In carbohydrate metabolism, biotin functions in both carbon dioxide fixation and decarboxylation, with the energy-producing tricarboxylic acid (TCA) cycle dependent upon the presence of this vitamin. Data from Hamilton et al. (1983) suggest an effect of biotin on hepatic pyruvate carboxylase activity in young pigs. Specific biotin-dependent reactions in carbohydrate metabolism are as follows:
- Carboxylation of pyruvic acid to oxaloacetic acid.
- Conversion of malic acid to pyruvic acid.
- Interconversion of succinic acid and propionic acid.
- Conversion of oxalosuccinic acid to alpha-ketoglutaric acid.
In protein metabolism, biotin enzymes are important in protein synthesis, amino acid deamination, purine synthesis and nucleic acid metabolism. Biotin is required for transcarboxylation in degradation of various amino acids. Acetyl-coenzyme A (CoA)-carboxylase catalyzes addition of carbon dioxide to acetyl CoA to form malonyl CoA. This is the first reaction in the synthesis of fatty acids. Biotin is required for normal longchain unsaturated fatty acid synthesis and is important for essential fatty acid metabolism. Deficiency in rats and chicks inhibited arachidonic acid (20:4) synthesis from linoleic acid (18:2) while increasing linolenic acid (18:3) and its metabolite (22:6) (Kramer et al., 1984; Watkins and Kratzer, 1987a). Biotin deficiency resulted in reduced arachidonic acid (20:4) in chicks and therefore reduced plasma prostaglandin E2 (PGE2) as a result of less of the prostaglandin precursor (20:4) (Watkins and Kratzer, 1987b; Watkins, 1989).