Riboflavin in urine (^g/g creatine)
Riboflavin in erythrocytes(^g/g hemoglobin)
Erythrocyte glutathion reductase activation coefficient
Niacin: Chemistry, Metabolism, and Functions
Niacin used to be called vitamin B3 or vitamin PP (pellagra preventive). Today, the term "niacin" is used to denominate nicotinic acid and nicotinamide (A). In their biological activity they are quantitative and qualitative equivalents since the body can convert them into one another (B).
Nicotinic acid (NA), nicotinamide (NE), and the metabolically active coenzyme form nicotinamide adenine dinucleo-tide (NAD), and nicotinamide adenine dinucleotide phosphate (NADP), occur in foods.
In order to be absorbed, the coenzymes have to be split and NE partially converted to NA by bacteria of the small intestine. Mucosa cells absorb NA and some NE actively, at higher concentrations also by passive diffusion (C). At low dosages, nearly all niacin is immediately converted to NAD in the liver (first-pass effect, i.e., quantitative conversion during the initial passage through the liver).
There are three pathways for NAD synthesis. The first one uses NA as a precursor and requires 5-phospho ribosyl-1-diphosphate (PRPP), glutamine, and ATP. If NE is used as a precursor, only PRPP and ATP are needed. The third pathway is independent of niacin. In an alternative pathway for the breakdown of tryptophan, nicotinic acid mononu-cleotide (NMN) is made via the intermediates kynurenine and quinolinic acid. NMN can be converted to NAD with glutamate and ATP as above. This pathway is important only in the liver and kidneys, whereas peripheral organs preferentially use NE for the synthesis. On average, 60 mg L-tryptophan can be converted to the same amount of NAD
as 1 mg NE. For this reason, the term niacin equivalents was introduced. Considering this equivalency, niacin may not necessarily be considered a vitamin. However, it becomes non-essential only when there is a large excess of tryptophan. If tryptophan is the limiting amino acid or, as is common with adequate protein supply, there is just enough of it, it is used exclusively for protein synthesis.
In the liver, beyond the synthesis of NAD from NA and tryptophan, there is also constant breakdown to NE and resyn-thesis of NMN. The liver regulates NAD (and NADP) metabolism by breaking them down to NE. And NE is secreted into the bloodstream and thereby made available to other tissues or inactivated and excreted mainly renally as methyl-NE. Humans can store a niacin supply sufficient for up to 2-6 weeks.
The biochemical functions of niacin are based on its role as a coenzyme of various dehydrogenases. The active form is NAD and/or NADP, into which NAD is converted by phosphorylation, using ATP. Both play a role in hydrogen transfer. For this, one H out of H2 attaches to the niacinamide ring bringing with it its pair of electrons (NADH, NADPH) and leaving behind a proton (H+). NAD-dependent dehydrogenases are found mostly in the mitochondria while NADP is predominantly involved in cytosolic synthesis pathways (e. g., fatty acid synthesis).
- A. Chemistry
OH Nicotinic acid
5 3| NHw Nicotinic acid amide N
B. Nicotinic Acid Amide Synthesis t
Nicotinic acid (NA)
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