Identification of Carnosine Synthase
As previously reported, carnosine synthase is also not very
specific with respect to the amino compound serving as the
-alanine or ␥-aminobutyryl acceptor (11, 14). Thus, the
chicken enzyme is able to use N--methyl-L-histidine, L-argi-
nine, and L-lysine in addition to L-histidine, whereas the mouse
and human enzymes use L-ornithine and L-lysine fairly well (cf.
Table 3). The -alanyl-L-lysine or L-ornithine synthesizing
activity can now be concluded to be due to an authentic lack of
specificity of carnosine synthase rather than to the presence of
different isoforms with different specificities. Considering that
the concentration of L-lysine in human skeletal muscle is about
1.5-fold higher than that of L-histidine (0.53 and 0.37 mM,
respectively (31)) -alanyl-L-lysine synthesis is expected to pro-
ceed in vivo at ϳ10% of the rate of carnosine synthesis. The
concentration of -alanyl-L-lysine is not known for human
muscle, but it is 1000-fold lower than that of carnosine in rabbit
muscle (32). This low concentration of the “wrong” peptide is
presumably due to the fact that it is degraded by a dipeptidase
that is particularly active in muscle and specifically cleaves the
-alanyl or ␥-aminobutyryl derivatives of L-lysine, L-arginine,
or L-ornithine, but does not act on carnosine or homocarnosine
and thus is different from carnosinase CN2 (33). The specific
accumulation of carnosine in muscle tissue appears therefore to
be due to the existence of two enzymes: one, carnosine syn-
thase, that preferentially makes carnosine, but also synthesizes
other dipeptides, and a second enzyme, not yet molecularly
identified, that should destroy all the unneeded dipeptides.
Other situations where one enzyme serves to compensate for
the lack of specificity of another enzyme have been described.
For example, L-2-hydroxyglutarate dehydrogenase serves to
degrade L-2-hydroxyglutarate, which is “mistakenly” made by
the Krebs cycle enzyme L-malate dehydrogenase (34). Defi-
ciency in the former enzyme causes L-2-hydroxyglutaric acid-
uria, a neurometabolic disorder. Another example is the ATP-
dependent dehydratase that “repairs” the hydrated form of
NADH made by glyceraldehyde-3-phosphate dehydrogenase
(35). It is likely that other enzymes catalyzing such “metabolite
repair reactions” have still to be found.
Perspectives—The identification of carnosine synthase will
allow progress in the understanding of the physiological func-
tion of carnosine and homocarnosine. The creation of knock-
out models or animals in which carnosine synthase is overex-
pressed could help define the role of carnosine as a buffer, a
radical scavenger in muscle and brain, or as a neurotransmitter
in the olfactory system, and the function of homocarnosine as a
␥-aminobutyrate reservoir. Knock-out models would tell us
whether carnosine and/or homocarnosine deficiency lead to
disease. No case of carnosine deficiency has yet been described
in humans, but we surmise that this is because carnosine con-
tent of muscle and brain is only infrequently measured. We
speculate that carnosine synthase deficiency could lead to
symptoms like muscle cramp, myopathy, anosmia or hypoos-
mia, seizures, and most probably other neurological problems.
As the carnosine synthase gene is in the region of IDDM4 on
chromosome 11q13, which is linked with insulin-dependent
diabetes (37), it is also possible that carnosine deficiency leads
to glucose intolerance. It has indeed been shown that carnosine
administration affects the function of pancreatic islets (7). The
identification of the gene encoding carnosine/homocarnosine
synthase will facilitate the diagnosis of carnosine or homocar-
nosine deficiency.
REFERENCES
1. Gulewitch, W., and Admiradzibi, S. (1900) Ber. Dtsch. Chem. Ges. 33,
1902–1903
2. Crush, K. G. (1970) Comp. Biochem. Physiol. 34, 3–30
3. Cameron, J. N. (1989) J. Exp. Biol. 143, 543–548
4. Boldyrev, A. A. (2007) Carnosine and Oxidative Stress in Cells and Tissues,
Nova Scientific Publisher, New York
5. Kish, S. J., Perry, T. L., and Hansen, S. (1979) J. Neurochem. 32,
1629–1636
6. Hipkiss, A. R. (2009) Exp. Gerontol. 44, 237–242
7. Sauerho¨fer, S., Yuan, G., Braun, G. S., Deinzer, M., Neumaier, M., Gretz,
N., Floege, J., Kriz, W., van der Woude, F., and Moeller, M. J. (2007)
Diabetes 56, 2425–2432
8. Bonfanti, L., Peretto, P., De Marchis, S., and Fasolo, A. (1999) Prog. Neu-
robiol. 59, 333–353
9. Brown, C. E., and Antholine, W. E. (1980) Biochem. Biophys. Res. Com-
mun. 92, 470–477
10. Kohen, R., Yamamoto, Y., Cundy, K. C., and Ames, B. N. (1988) Proc. Natl.
Acad. Sci. U.S.A. 85, 3175–3179
Evolution of Carnosine Synthase—An intriguing aspect in the
structure of carnosine synthase is that its sequence has about
twice the length expected for a member of the ATP-grasp fam-
ily. Sequence comparisons indicate that it comprises two ATP-
grasp domains, suggesting that the gene encoding carnosine
synthase resulted from the fusion of two genes encoding two
different ligases. The high conservation of the C-terminal
domain indicates that this part of the protein comprises the
catalytic site responsible for the ligation of -alanine to L-histi-
dine and related amino acids. The N-terminal domain is much
less conserved and presumably has no catalytic activity in ver-
tebrates. Interestingly, the tripeptide -alanyl-L-ornithyl-L-or-
nithine has been described in a bivalve (36). Because carnosine
synthases are rather good at synthesizing -alanyl-L-ornithine,
we speculate that this tripeptide is made by a bifunctional
enzyme similar to carnosine synthase. Whether the N-terminal
domain has now taken another function or is simply a remnant
of evolution in present day vertebrate carnosine synthase is
unknown.
11. Kalyankar, G. D., and Meister, A. (1959) J. Biol. Chem. 234, 3210–3218
12. Stenesh, J. J., and Winnick, T. (1960) Biochem. J. 77, 575–581
13. Skaper, S. D., Das, S., and Marshall, F. D. (1973) J. Neurochem. 21,
1429–1445
14. Horinishi, H., Grillo, M., and Margolis, F. L. (1978) J. Neurochem. 31,
909–919
15. Teufel, M., Saudek, V., Ledig, J. P., Bernhardt, A., Boularand, S., Carreau,
A., Cairns, N. J., Carter, C., Cowley, D. J., Duverger, D., Ganzhorn, A. J.,
Guenet, C., Heintzelmann, B., Laucher, V., Sauvage, C., and Smirnova, T.
(2003) J. Biol. Chem. 278, 6521–6531
16. Murphey, W. H., Lindmark, D. G., Patchen, L. I., Housler, M. E., Harrod,
E. K., and Mosovich, L. (1973) Pediatr. Res. 7, 601–606
17. Willi, S. M., Zhang, Y., Hill, J. B., Phelan, M. C., Michaelis, R. C., and
Holden, K. R. (1997) Pediatr. Res. 41, 210–213
18. Bradford, M. M. (1976) Anal. Biochem. 72, 248–254
19. Lienhard, G. E., and Secemski, I. I. (1973) J. Biol. Chem. 248, 1121–1123
20. Van Den Neste, E., Bontemps, F., Delacauw, A., Cardoen, S., Louviaux, I.,
Scheiff, J. M., Gillis, E., Leveugle, P., Deneys, V., Ferrant, A., and Van den
Berghe, G. (1999) Leukemia 13, 918–925
21. Vertommen, D., Ruiz, N., Leverrier, P., Silhavy, T. J., and Collet, J. F. (2009)
Proteomics 9, 2432–2443
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JOURNAL OF BIOLOGICAL CHEMISTRY 9355