Creatine and Creatinine Metabolism - Physiological Reviews

1118 MARKUS WYSS AND RIMA KADDURAH-DAOUK Volume 80
replenished. The observed or suspected effects of an
elevated serum [Cr], namely, to downregulate the expression
and/or activity of AGAT and possibly also the Cr
transporter, would therefore help to spare precursors of
Cr (Arg, Gly, Met) and to maintain normal, steady levels of
Cr and PCr in CK-containing tissues. As a consequence,
the rate of Cr biosynthesis is highest in young, healthy,
fast-growing vertebrates under anabolic conditions on a
balanced, Cr-free diet (1077).
VI. PHOSPHOCREATINE AND CREATINE AS
PURPORTED ALLOSTERIC EFFECTORS
In some recent articles (e.g., Refs. 92, 641, 1068), the
opinion has still been expressed that Cr and PCr may act
as allosteric regulators of cellular processes. As a matter
of fact, a number of studies, mainly performed in the
1970s, seemed to demonstrate that physiological concentrations
of PCr inhibit glycogen phosphorylase a, phosphofructokinase,
glyceraldehyde-3-phosphate dehydrogenase,
pyruvate kinase, lactate dehydrogenase, AMP
deaminase, and 5�-nucleotidase from a variety of species.
Similarly, PCr was claimed to activate fructose-1,6diphosphatase
from rabbit skeletal muscle, while phosphorylarginine
was suggested to inhibit phosphofructokinase
from oyster adductor muscle (for references see
Refs. 207, 247, 580, 666, 834, 951, 1012, 1099).
Subsequent studies, however, proved that inhibition
of at least phosphofructokinase, pyruvate kinase, lactate
dehydrogenase, AMP deaminase, and 5�-nucleotidase, but
probably also of all other enzymes listed above, is not
afforded by PCr itself; rather, these effects were due to
contaminants present in the commercial preparations of
PCr which, at that time, were no more than 62–75% pure
(1099). The contaminating inhibitors were identified as
inorganic pyrophosphate for AMP deaminase (1099) and
oxalate for lactate dehydrogenase and pyruvate kinase
(1012).
When added to the bathing medium of differentiating
skeletal and heart muscle cells in tissue culture, Cr increased
rather specifically the rate of synthesis as well as
the specific activity of myosin heavy chain (406, 1153). In
slices of the rat neostriatum, Cr inhibited the GABAsynthesizing
enzyme glutamate decarboxylase as well as
the veratridine-induced release of GABA, but significant
effects were only observed at an unphysiologically high
Cr concentration of 25 mM (864). In rat basophilic leukemia
cells, PCr was seen to stimulate phospholipase C
activity (196). Finally, in cell-free extracts of white gastrocnemius,
soleus, heart muscle, and liver of the rat, PCr
affected the extent of phosphorylation of various proteins,
in particular of phosphoglycerate mutase and of a
18-kDa protein (742). Again, these findings are unlikely to
be due to direct allosteric effects of Cr and PCr but are
probably mediated indirectly, e.g., via alterations of the
energy status of particular microcompartments or whole
cells (195, 563, 742, 1153).
In the unicellular alga Gonyaulax polyedra, several
functions show circadian rhythmicity, for example, cell
division, photosynthesis, bioluminescence, motility, and
pattern formation. If cultures of Gonyaulax are first
grown under a 12:12-h light-dark cycle, and if the conditions
are then changed to constant dim light, the circadian
rhythmicity persists for several weeks. This condition is
called free-running circadian rhythmicity, with its period �
depending on the color and intensity of the constant dim
light.
Extracts of several eukaryotic organisms, including
bovine and rat brain and muscle, shorten the period of the
free-running circadian rhythms in Gonyaulax. The substance
responsible for this effect was identified as Cr. In
the micromolar range (2–20 �M), Cr accelerates the circadian
clock by as much as 4 h/day (Fig. 6A) (816). The Cr
effect on � is very pronounced in constant dim blue light,
whereas it is virtually absent in constant dim red light
(Fig. 6B) (817). This finding, together with other lines of
evidence, suggests that Cr interferes with light transduction
pathways and in particular with the pathway(s) coupled
to blue-sensitive photoreceptors. In addition to its
effect on �, Cr also affects light-induced phase changes of
the circadian rhythmicity (817).
A period-shortening substance with properties similar
to Cr is present in extracts of Gonyaulax itself (816) and has
been identified as gonyauline (S-methyl-cis-2-[methylthio]cyclopropanecarboxylic
acid) (815). Its rather close structural
similarity to Cr (Fig. 6C), the complete lack of Cr in extracts
of Gonyaulax (815), as well as the indication that Cr is not
active as such but has to be metabolized to exert its effects
on the circadian rhythmicity (see Ref. 817) all suggest that,
at least in algae, Cr itself is not a physiological component or
modulator of the circadian clock.
To conclude, the data suggesting that Cr and PCr
act as direct (allosteric) regulators of cellular processes
(other than the repression by Cr of AGAT and
possibly also of the Cr transporter) must be treated
with skepticism, at least until new, convincing data are
presented. There may be four notable exceptions that
deserve further attention: 1) PCr stimulates glutamate
uptake into synaptic vesicles (1127; see also sect. IXG).
A series of control experiments showed that the effect
of PCr is not mediated indirectly via CK and ATP.
Remarkably, PCr-stimulated glutamate uptake was
even higher than that stimulated maximally by ATP. 2)
PCr at relatively high concentrations of 10–60 mM was
found to promote efficient endonucleolytic cleavage of
mammalian precursor RNA in vitro (364), which is a
prerequisite for subsequent poly(A) addition. Neither