The major function of niacin is in the coenzyme forms of nicotinamide, NAD and NADP. Enzymes containing NAD and NADP are important links in a series of reactions associated with carbohydrate, protein and lipid metabolism. They are especially important in the metabolic reactions that furnish energy to the animal. The coenzymes act as an intermediate in most of the H+ transfers in metabolism, including more than 200 reactions in the metabolism of carbohydrates, fatty acids and amino acids. These reactions are of paramount importance for normal tissue integrity, particularly for the skin, the gastrointestinal tract and the nervous system.
Like the riboflavin coenzymes, the NAD- and NADP-containing enzyme systems play an important role in biological oxidation-reduction systems due to their capacity to serve as hydrogen-transfer agents. Hydrogen is effectively transferred from the oxidizable substrate to oxygen through a series of graded enzymatic hydrogen transfers. Nicotinamide-containing enzyme systems constitute one such group of hydrogen transfer agents.
Important metabolic reactions catalyzed by NAD and NADP are summarized as follows:
- Carbohydrate metabolism:
- Glycolysis (anaerobic and aerobic oxidation of glucose)
- TCA (Krebs) cycle
- Lipid metabolism:
- Glycerol synthesis and breakdown
- Fatty acid oxidation and synthesis
- Steroid synthesis
- Protein metabolism:
- Degradation and synthesis of amino acids
- Oxidation of carbon chains via the TCA cycle
- Rhodopsin synthesis
Niacin-dependent poly(ADP-ribose) is involved in the post-translational modification of nuclear proteins. The poly ADP-ribosylated proteins seem to function in DNA repair, DNA replication and cell differentiation (Carson et al., 1987). Poly(ADP-ribose) is synthesized in response to DNA strand breaks. Rat data have shown that even a mild niacin deficiency decreases liver poly(ADP-ribose) concentrations and that poly(ADP-ribose) levels are also altered by food restriction (Rawling et al., 1994). Zhang et al. (1993) suggest that a severe niacin deficiency may increase the susceptibility of DNA to oxidative damage, likely due to the lower availability of NAD. Turnover rates of protein in Japanese quail have been related to niacin deficiency; a high turnover rate due to the deficiency was primarily attributed to enhanced degradation rate of proteins rather than enhanced synthesis rate of proteins (Park et al., 1991).
For ruminants, niacin is particularly required for unique features involving protein and energy metabolism, including liver detoxification of portal blood NH3 to urea and liver metabolism of ketones in ketosis. It is apparent that niacin can increase microbial protein synthesis (Girard, 1998). It may result in an increased molar proportion of propionate in rumen volatile fatty acids and may cause an increased rate of flow of material through the rumen. The primary function of interest for lactating dairy cows deals with the role of niacin in fatty acid oxidation and glucose synthesis, particularly as a preventive and possible treatment for clinical and subclinical ketosis.