5. ONIUM COMPOUNDS
203
As to the biochemical or physiological significance of stachydrine or
related compounds in this group nothing is known.
B. HETEROAROMATIC COMPOUNDS
1. Thiazolium
Compounds
Thiamine is the most important member of this group and the one
about which most is known. A discussion of its chemistry and biochemistry falls entirely outside the scope of this chapter but its cleavage
by thiaminase will be discussed later as it has an important bearing on
discussions of the energetics of the onium bond.
2. Pyridinium Compounds
In addition to DPN and TPN, which will not be discussed here,
there are a number of naturally occurring alkyl-pyridinium compounds,
such as N'-methylinicotinamide, and its oxidation product N'-methyl-3carboxylamide-6-pyridone (96), N'-methylnicotinic acid, and N'-methylpicolinic acid.
Very little is known about the last compound: N'-methylpicolinic
acid is found in high concentrations in mussels such as Area noae (97)
and Mytilus californianus Conrad, or the dinoflagellate Gonyaulax catenella on which it feeds (98), and in the nerves and other tissues of a
variety of marine invertebrates (99). The distribution of the compound
in the nerve muscles of these species is characteristic, and it has been
suggested that by virtue of the ability of N'-methylpicolinic acid to
form a mobile ion (H
+ ) when cleaved it might function in nerve conduction mechanisms (100).
N'-methylnicotinamide is formed from nicotinamide by a transmethylation reaction (101, 102) which represents the major pathway
for the metabolism of nicotinamide in certain mammalian species. The
details of the enzymatic aspects of the transfer reactions will be discussed in a later section. In man and rat, N'-methylnicotinamide is
metabolized further to N'-methyl-3-carboxamide-6-pyridone and present
evidence suggests that this pathway accounts for 80% or more of the
catabolism of N'-methylnicotinamide (103). In other mammals, however, nicotinamide is excreted directly or converted to a number of
different metabolites.
N'-methylnicotinamide is rapidly excreted in the urine owing to a
renal excretory mechanism involving tubular secretion (104).
The methylation of nicotinamide does not seem to be confined to
animal species as it occurs also in corn where this metabolic characteristic is under genetic control. In man N'-methylnicotinamide and the
203
As to the biochemical or physiological significance of stachydrine or
related compounds in this group nothing is known.
B. HETEROAROMATIC COMPOUNDS
1. Thiazolium
Compounds
Thiamine is the most important member of this group and the one
about which most is known. A discussion of its chemistry and biochemistry falls entirely outside the scope of this chapter but its cleavage
by thiaminase will be discussed later as it has an important bearing on
discussions of the energetics of the onium bond.
2. Pyridinium Compounds
In addition to DPN and TPN, which will not be discussed here,
there are a number of naturally occurring alkyl-pyridinium compounds,
such as N'-methylinicotinamide, and its oxidation product N'-methyl-3carboxylamide-6-pyridone (96), N'-methylnicotinic acid, and N'-methylpicolinic acid.
Very little is known about the last compound: N'-methylpicolinic
acid is found in high concentrations in mussels such as Area noae (97)
and Mytilus californianus Conrad, or the dinoflagellate Gonyaulax catenella on which it feeds (98), and in the nerves and other tissues of a
variety of marine invertebrates (99). The distribution of the compound
in the nerve muscles of these species is characteristic, and it has been
suggested that by virtue of the ability of N'-methylpicolinic acid to
form a mobile ion (H
+ ) when cleaved it might function in nerve conduction mechanisms (100).
N'-methylnicotinamide is formed from nicotinamide by a transmethylation reaction (101, 102) which represents the major pathway
for the metabolism of nicotinamide in certain mammalian species. The
details of the enzymatic aspects of the transfer reactions will be discussed in a later section. In man and rat, N'-methylnicotinamide is
metabolized further to N'-methyl-3-carboxamide-6-pyridone and present
evidence suggests that this pathway accounts for 80% or more of the
catabolism of N'-methylnicotinamide (103). In other mammals, however, nicotinamide is excreted directly or converted to a number of
different metabolites.
N'-methylnicotinamide is rapidly excreted in the urine owing to a
renal excretory mechanism involving tubular secretion (104).
The methylation of nicotinamide does not seem to be confined to
animal species as it occurs also in corn where this metabolic characteristic is under genetic control. In man N'-methylnicotinamide and the