Pyridoxine and pyridoxamine are two forms of the water soluble vitamin-B6. Both compounds are composed of a pyridine base from which their names are derived. These compounds are used by both multi-cellular and unicellular life as secondary and primary nutritional source (Burg et al., 1960). Pyridoxal 5-phosphate (PLP) is the biologically active form used as an enzymatic cofactor. Toxicity in humans is rare and has only been observed at megadoses (14,000mg/kg), resulting in sensory neuropathy (Ohnishi et al., 1985).
There are two distinct yet related bacterial pathways for the degradation of pyridoxine. The bacteria that carry out these reactions can use pyridoxine as their sole carbon source. The first step in each pathway is a dehydrogenation reaction; in pathway I it is at the 4 position, and in pathway II it is at the 5 position. After this step, both pathways proceed through a series of pyridine compounds before ring cleavage, but none of the pyridine intermediates are common to both pathways (Nelson et al., 1986).
The following is a text-format bacterial pyridoxine pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions. This map is also available in graphic format.
Graphical Map (17k) | Graphical Map (9k)
Pyridoxamine Pyridoxine Pyridoxine
Pseudomonas sp. MA-1 Pseudomonas sp. MA-1 Pseudomonas sp. IA
| Microbacterium luteolum YK-1 Arthrobacter sp. CR-7
| Mesorhizobium loti |
| | |
| | pyridoxine |
| | 4-dehydrogenase |
| pyridoxamine- | or | pyridoxine
| pyruvate | pyridoxine | 5-dehydrogenase
| transaminase | 4-oxidase |
| | |
| v v
+----------------------> Pyridoxal --------------------+ Isopyridoxal
| | |
| pyridoxal | | isopyridoxal
| 4-dehydrogenase | | dehydrogenase
| | |
v | pyridoxal v
4-Pyridoxolactone | oxidase 5-Pyridoxolactone
| | |
| 4-pyridoxolactone | |
| lactonohydrolase | | 5-pyridoxolactonase
| | |
v | v
4-Pyridoxate <------------------+ 5-Pyridoxate
| |
| 4-pyridoxate | 5-pyridoxate
| dehydrogenase | oxygenase
| |
v v
2-Methyl-3-hydroxy-5- 2-(Hydroxymethyl)-3-
formylpyridine-4-carboxylate (acetamidomethylene)succinate
| |
| | 2-(hydroxymethyl)-3-
| 2-Methyl-3-hydroxy- | (acetamidomethylene)
| 5-formylpyridine- | succinate hydrolase
| 4-carboxylate |
| dehydrogenase |
| V
v 2-Hydroxymethyl-
3-Hydroxy-2-methylpyridine- succinate semialdehyde
4,5-dicarboxylate
|
| 3-hydroxy-2-methylpyridine-
| 4,5-dicarboxylate 4-carboxylyase
|
v
3-Hydroxy-2-methylpyridine-
5-carboxylate
|
| 3-hydroxy-2-methyl-
| pyridinecarboxylic
| acid oxygenase
|
v
2-(Acetamidomethylene)succinate
|
| 2-(acetamidomethylene)
| succinate hydrolase
|
v
Succinic semialdehyde
|
|
|
v
to the
Tyrosine
Pathway
Page Author(s): Erik Reynolds, Nicholas Rossi, Luke Kane
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