This document summarizes information about the vitamins niacin and pyridoxine. It states that niacin, also called vitamin B3, exists in tissues as nicotinamide and acts as a coenzyme in many metabolic reactions. Niacin is absorbed in the stomach and small intestine and is involved in over 200 oxidation-reduction reactions. Pyridoxine, also known as vitamin B6, exists in three forms - pyridoxine, pyridoxal, and pyridoxamine - and all three forms are converted to the active coenzyme pyridoxal phosphate. Pyridoxine is absorbed in the small intestine and transported bound to albumin, then stored in tissues.

2. NIACIN
q It is a water soluble vitamin
 The term niacin(Vitamin B3) is considered a
generic term for nicotinic acid and
nicotinamide
 Nicotinic acid is pyridine 3 carboxylic acid.
 It occurs in tissues as nicotinamide
 Nicotinamide when heated in strong alkaline
or acid solution is converted to nicotinic
acid.
 Niacin is Stable to heat, not destroyed by
autoclaving at 120°C for 20 mins in acid
and alkaline medium

3. That white needle shaped crystals
nicotinamide exists in human and animal tissues as
coenzyme 1 (NAD) and coenzyme 11 (NADP)
It forms 2 phosphorylated compounds which act as
coenzymes to dehydrogenases in metabolic reactions
1) NAD- contain 1 niacinamide+ 2-D ribose + 2
phosphoric acid + 1 adenine molecule
2) NADP- contain 1 niacinamide + 2-D ribose + 3
phosphoric acid + 1 adenine molecule

4. BIOSYNTHESIS
 Precursor – dietary tryptophan
 The coenzyme of niacin (NAD and NADP) can be
synthesized by the essential amino acid tryptophan
 Conversion of Niacin to nicotinamide takes place in
the kidney, brain and liver
 60mg of tryptophan is required to make 1mg
of niacin
 Conversion of tryptophan to niacin in the body takes
place through series of intermediate steps.
 Also synthesized to some extent by intestinal bacteria
 Diet deficient in tryptophan produce deficiency of
niacin
TRYPTOPHAN
KYNURENINE
3 HYDROXY KYNURENINE
3 HYDROXY ANTHRANILIC
ACID
QUINOLINIC ACID
NICOTINIC ACID
MONONUCLEOTIDE(NAM)
NAD
NIACIN

5. ABSORPTION
Niacin is readily absorbed from the stomach
and small intestine
Excess is excreted in the urine in the form of
nicotinamide and methyl carboxamide
pyridoxine
Niacin is retained in tissues by being converted
to NAD and NADPH
q Niacin is essential for the normal functioning
of the skin, intestinal tract and nervous system
q Therapeutically used for lowering plasma
cholesterol

6. FUNCTION
 NAD and NADP participate in 200+ reaction in body
 Coenzyme NAD and NADP are involved in variety of
oxidation – reduction reactions
 NAD and NADP participate in almost all the
metabolism(carbohydrate, lipid, protein)
 Involved in electron transport chain to generate ATP
 Reduced NADP(NADPH) is also involved in enzyme
reaction in fatty acid synthesis, synthesis of cholesterol and
steroid hormones
 NAD-involve in catabolic reaction
 NADP-involve in anabolic reaction

7. PYRIDOXINE
 Pyridoxine is a water soluble vitamin
 3 forms of vitamin B6 occur in nature –
pyridoxine, pyridoxal and pyridoxamine
 Pyridoxine-(alcohol)
 Pyridoxamine -(amine)
 Pyridoxal ( aldehyde)
 It is heat stable, sensitive to light and alkali.
 All 3 forms are equally active as precursors of the
potent pyridoxine coenzyme pyridoxal phosphate

8. METABOLISM
 ABSORPTION
It occurs in proximal jejunum by passive diffusion
In the mucosal cells, all forms of pyridoxine are
converted to pyridoxal
 TRANSPORT
It is transported in the circulation bound to albumin
 STORAGE
It is stored in the tissues as its coenzyme form PLP
Mainly stored in liver, brain, kidney and muscle
 EXCRETION
It is excreted as 4- pyridoxic acid excreted in urine