Creatine and Creatinine Metabolism - Physiological Reviews

1162 MARKUS WYSS AND RIMA KADDURAH-DAOUK Volume 80
temperatures of 37–250°C, either dry, in aqueous solution,
or in diethylene glycol-water mixtures which display increased
boiling points of 128–150°C. With the individual
components varied, almost all AIA food mutagens could
be generated in model systems, namely, IQ, MeIQ, IQx,
8-MeIQx, 4,8-DiMeIQx, 7,8-DiMeIQx, 7,9-DiMeIgQx, PhIP,
and TMIP (for reviews and references, see Refs. 119, 236,
417, 424, 434, 492, 544, 710, 902).
Similar to the situation in cooked foods, a variety of
factors influence mutagen yield in model systems, e.g.,
temperature, incubation time, concentration of antioxidants
as well as the nature, concentration, and proportion
of the precursors (for reviews and references, see Refs.
417, 424, 434, 465, 902). Maximal mutagen yield was
achieved by mixing Cr or Crn with an amino acid and a
sugar in a molar ratio of 1:1:0.5. Remarkably, almost the
same ratio between these components is found in bovine
muscle (534). In most instances, omission of any one
component from the ternary mixture greatly reduced mutagenicity.
Crn proved to be a more potent precursor than
Cr. Because, in addition, conversion of Cr to Crn is favored
at elevated temperatures, Crn rather than Cr is
likely to be the actual precursor of AIA mutagens.
The amino acid in the ternary mixture determines not
only the yield, but also the nature of the AIA mutagens
produced. In mixtures with Cr(n) and glucose, Cys and
Thr yield highest mutagenicity, followed by Lys, Ala, Ser,
and Gly. In most of these ternary mixtures, 8-MeIQx and
4,8-DiMeIQx were identified as products. On the other
hand, PhIP was only found in ternary mixtures containing
Phe, Leu, Ile, or Tyr.
The influence of sugars on mutagen production is
somewhat more complex. In most but not all mixtures of
Cr(n) and amino acids, addition of a variety of sugars
(glucose, fructose, sucrose, lactose) increased mutagen
formation. Fructose proved to be more potent than glucose,
and sucrose was more potent than lactose. As mentioned
above, maximal mutagenicity was reached at a
molar ratio of Cr(n), amino acid, and sugar of 1:1:0.5.
When the sugar concentration was further increased, progressive
inhibition of mutagen formation occurred. This
inhibition was suggested to be due either to Maillard
reactions that are expected to become more prominent at
elevated sugar concentrations, or to inhibition of the conversion
of Cr to Crn by sugars.
Model systems also produced new mutagens that
have not been detected so far in cooked foods (for references,
see Refs. 215, 905). From a heated mixture of Cr,
Glu, and glucose, the AIA mutagen 2,6-diamino-3,4-dimethyl-7-oxo-pyrano[4,3-g]benzimidazole
was isolated
(Fig. 15, structure 9). The imidazo-quinoxaline 4,7,8-Tri-
MeIQx was identified in a heated mixture of Ala, Thr, Crn,
and glucose. Finally, pyrolysis of Cr monohydrate at 250–
400°C gave rise to the mutagenic compound Cre-P-1 (Fig.
15, structure 10).
On the basis of the experiments with model systems,
two alternative reaction pathways were proposed for the
formation of imidazo-quinoline and imidazo-quinoxaline
mutagens (for a detailed discussion, see Ref. 424). According
to the first hypothesis, a pyridine or pyrazine and
an aldehyde, both postulated to be formed from amino
acids and sugars through Maillard reactions and Strecker
degradations, react with Crn to yield an imidazo-quino(xa)line
compound. In support of this hypothesis, IQ has
recently been isolated, although in low yield, from a
heated mixture of 2-methylpyridine, Crn, and acetylformaldehyde
(544). According to the second hypothesis, Crn
first undergoes an aldol condensation with an aldehyde to
yield an intermediary creatinine-aldehyde which, subsequently,
combines with a pyridine or pyrazine to give an
imidazo-quino(xa)line compound. This hypothesis is supported
by the identification of the postulated creatininealdehyde
intermediates AEMI and AMPI (Fig. 15, structures
11 and 12) in heated mixtures of Crn and Thr, and
by the generation of AEMI through direct reaction of Crn
with acetaldehyde.
Common to both hypotheses are the postulates that
pyridines or pyrazines are obligatory intermediates and
that these pyridines or pyrazines are formed from sugars
and amino acids through Maillard reactions (see also Ref.
465). The validity of these assumptions is, however, questioned
by several lines of evidence: 1) imidazo-quino(xa)lines
were formed in binary systems lacking sugars,
namely, in mixtures of Cr(n) with Pro, Phe, Ser, Ala, or
Tyr. 2) In mixtures of Cr with Pro or Ser, addition of
glucose did not stimulate mutagen formation. 3) Upon
addition of a variety of pyridines or pyrazines to model
systems, the amount of mutagenicity at most doubled or
did not increase at all. 4) Finally, in cooked meat, a poor
correlation was observed between mutagenicity and the
level of Maillard reaction products (see Ref. 534). On the
other hand, in model systems containing Crn, Thr, and
radioactively labeled glucose, label was in fact incorporated
into 8-MeIQx and 4,8-DiMeIQx (see Ref. 902). Taken
together, these results imply that different reaction pathways
have to be considered and that sugars may be
involved in but are not essential for the formation of
imidazo-quino(xa)lines.
PhIP was identified in binary mixtures of Cr plus Phe,
Crn plus Phe, and Cr plus Leu (see Refs. 424, 902, 989).
When mixtures of Crn and Phe were dry-heated at 200°C,
addition of glucose in a half-molar amount decreased
rather than increased PhIP yield (989). Experiments with
isotopically labeled Phe revealed that the whole phenyl
ring, the 3-carbon atom, and the amino nitrogen of Phe are
incorporated into PhIP (236). In the light of these findings,
it comes as a surprise that in aqueous solution at pH 7.4
and at a temperature of 60°C, PhIP was only detected
when sugars or aldehydes were added to mixtures of Crn
plus Phe (591). It remains to be determined whether,