Vorkommen und Toxikokinetik von Myosmin in Abhängigkeit von Rauchen und Ernährung

Occurrence and toxicokinetics of myosmine depending on smoking and
nutrition Myosmine, a minor tobacco alkaloid, occurs like the major
tobacco alkaloids nicotine, nor-nicotine, anabasine and anatabine
in small amounts in tobacco plants and tobacco smoke. Because of
its low acute toxicity and the weak affinity to nicotinic
receptors, no attention was given to myosmine for a long time.
However, this changed with the discovery that myosmine occurs
independently of nicotine in a great variety of staple foods,
vegetables, fruits, nuts and dairy products and consequently in
human milk, plasma and saliva. The possible health im¬pact of
myosmine was further demonstrated by its conversion into reactive
intermediates with carcinogenic potential. Due to its imine
structure, myosmine is readily nitrosated and peroxi-dated.
Nitrosation yields the tobacco-specific nitrosamine
N'-nitrosonornicotine (NNN), a human carcinogen, which produces
tumours in oesophagus, oral cavity and the respiratory tract of
rodents. In larger quantities, the same reactive intermediates
which are formed by me-tabolic activation of NNN and another
tobacco-specific nitrosamine are generated by nitro-sation and
peroxidation of myosmine. In biomonitoring studies the resulting
DNA and protein adducts did not show the expected correlation with
smoking status. Therefore, myosmine has been postulated to be an
important additional source of these adducts. In the first chapter,
a detailed literature survey is given covering all aspects of
occurrence, biosynthesis, toxicokinetics and -dynamics of myosmine
in the context of other tobacco alkaloids and tobacco-specific
nitrosamines. In the experimental part, an analytical method was
developed to determine myosmine, coti-nine and nicotine by gas
chromatography/mass spectrometry (GC/MS) in different matrices in
order to study the occurrence of myosmine in livestock as well as
in humans. First, myosmine was determined in plasma of pigs in
dependence of the feeding condition. Second, human sa-liva and
toenails from smokers and nonsmokers were analyzed for short and
long-term expo-sure to myosmine and cotinine. The nails were also
analyzed for nicotine. Finally, myosmine was determined in multiple
samples of saliva of eight test persons to study the kinetics of
myosmine under controlled dietary conditions. The results can be
summarized as follows: • The analytical method has low detection
limits for myosmine, nicotine and cotinine in toenails with 0.01,
0.02 and 0.035 ng/mg, respectively, and for myosmine and cotinine
in plasma/saliva with 0.0012 and 0.05 ng/ml. The recovery is very
high with 91-93% in plasma/saliva and 97–102% in toenails. The
intraday precision is ≤ 8% for all analytes in toenails, whereas
for plasma/saliva it is 18% with myosmine and 4% with cotinine. The
analytical method has a high specificity by the use of deuterated
internal standards. • In the plasma of 12 fasting pigs, myosmine
was traceable with only one exception. The concentration of
myosmine was 0.067 ± 0.049 ng/ml. Within 1 - 2 hours after start of
fee-ding the concentration of myosmine in the plasma of 13 pigs was
0.497 ± 0.166 ng/ml, a statistically significant 7-fold difference
to the fasting pigs (p < 0.0001). The swill con-tained 124 ng
myosmine/g wet weight. • In toenails the concentrations of all
analytes were significantly lower in 11 nonsmokers (0.021 ± 0.014
ng/mg) compared to 15 smokers (0.058 ± 0.052 ng/mg, p < 0.01).
This 2.8-fold difference in myosmine concentrations between smokers
and nonsmokers was clearly less than the 14-fold difference in
nicotine concentrations (0.128 ± 0,008 versus 1.789 ± 0.964 ng/mg,
p < 0.001). Cotinine was detectable only in toenails of smokers,
1.136 ± 0.843 ng/mg. Significant correlations exists between the
concentrations of myosmine and nicotine (Spearman r = 0.67) as well
as myosmine and cotinine (r = 0.63). A clearly better correlation
(r = 0.83) was found between cotinine and nicotine values. • In
saliva samples taken in parallel with toenails, the differences
between nonsmokers and smokers were also smaller with myosmine,
0.73 ± 0.65 versus 2.54 ± 2.68 ng/ml than with cotinine, 1.85 ±
4.50 versus 83.14 ± 54.30 ng/ml. Nonetheless, concentrations of
myos-mine and cotinine in saliva were highly correlated (p <
0.0001). • In eight volunteers, the kinetics of myosmine in saliva
after food intake showed large indi-vidual differences. After four
hours of fasting in the morning all subjects took a lunch
containing about 2.7 µg of myosmine within half an hour. The basal
concentrations of myosmine in saliva at lunch were between 0.05 and
0.28 ng/ml. In one subject only, a rapid rise of myosmine to 3.6
ng/ml was observed within three-quarters of an hour. In two
subjects a plateau of 1.1 and 1.5 ng/ml was reached between 2½ and
4½ hours after lunch. The remaining five subjects showed only a
weak rise of myosmine concentrations to a maximum of 0.42 ± 0.06
ng/ml within 1½ and 2½ hours. Fitting the data to the Bateman
function, an elimination half-life of about 1.6 hours could be
estimated which is roughly equivalent to the plasma half-life of
nicotine in humans.