Creatine and Creatinine Metabolism - Physiological Reviews

July 2000 CREATINE AND CREATININE METABOLISM 1139
side reactions (biodegradation), and the extent of feedback
repression of AGAT exerted by the Cr analog.
To get a feeling for the potencies of the Cr analogs
that have been used so far, the available knowledge will
shortly be summarized (Table 1). The Cr analogs and
corresponding synthetic phosphagens cover a wide range
of potencies to act as substrates for CK. cCr and PcCr are
rather good substrates, both in vitro and in vivo (810). On
the other hand, GPA and PGPA, with 1,000- to 1,500-fold
decreased V max/K m values, are expected to be poor CK
substrates. Nevertheless, upon changes in workload in
the perfused myocardium of control and GPA-treated
rats, [PCr] and [PGPA], respectively, displayed very similar
changes, both as far as the absolute concentration
changes and the kinetics of the alterations are concerned
(135). This finding points to an “overcapacity” of the CK
system and suggests that under normal in vivo conditions
as well as under most experimental regimens, PGPA is a
satisfactory substitute of PCr. Therefore, conclusions as
to the function of PCr in organs of animals fed these
analogs may be misleading. hcCr and especially PhcCr are
extremely poor substrates of CK. As a matter of fact,
PhcCr is very slowly consumed under in vivo conditions
(810). 3-Guanidinobutyrate is not phosphorylated at all in
vivo (1163). Even though, on first sight, hcCr and 3-guanidinobutyrate
seem to be perfectly suited for investigations
of the physiological functions of the CK/PCr/Cr system,
they both suffer the limitations to be accumulated
rather inefficiently by the tissues and to replace PCr and
Cr only incompletely (810, 1163). These may be the reasons
for GPA being the most widely used Cr analog in
physiological experiments.
When porcine carotid arteries, containing almost no
Mi-CK, were superfused with either GPA, methyl-GPA
(mGPA), guanidinoacetic acid (GAA), or cCr, all of these
Cr analogs entered the tissue and became phosphorylated
(74). When, subsequently, oxidative metabolism was inhibited
by cyanide, PCr, PGPA, PGAA, and PmGPA were
consumed over a similar time course, despite different
kinetic properties of these analogs in vitro. PcCr, on the
other hand, was utilized at a significantly lower rate.
Nevertheless, the results show that these Cr analogs may
all serve as substrates for cytosolic CK. In contrast, only
Cr and cCr stimulated respiration in isolated rat heart
mitochondria, whereas GAA, mGPA, and GPA were ineffective
(74, 130, 1163; see also Ref. 1044). These findings
may indicate that Mi-CK displays narrower substrate
specificity than the cytosolic CK isoenzymes or, more
unlikely, that the outer mitochondrial membrane is impermeable
to GAA, mGPA, and GPA.
Compared with CK, the Cr transporter seems to be
considerably less specific toward Cr analogs (Table 1).
This conclusion has recently been corroborated by experiments
on the cloned Cr transporters from Torpedo, rabbit,
and human (see also sect. VIIC) (318, 319, 691, 840,
927). In cells transiently transfected with Cr transporter
cDNA, nonlabeled Cr and GPA inhibit the uptake of radioactive
Cr very efficiently and to approximately the
same extent. 4-Guanidinobutyrate, guanidinoacetate, and
2-amino-3-guanidinopropionate are somewhat less inhibitory,
while Crn has only little effect. In experiments on the
repression of AGAT activity in embryonic and developing
chicken liver, Cr, cCr, N-ethylguanidinoacetate, and Nacetimidoylsarcosine
proved to be powerful repressors,
whereas N-acetimidoylglycine, 1-carboxymethyl-2iminohexahydropyrimidine,
N-propylguanidinoacetate,
and N-methyl-3-guanidinopropionate caused either no or
just a small reduction in AGAT activity (809, 1078).
To conclude, an ideal Cr analog unfortunately does
not exist. Therefore, experiments with Cr analogs have to
be analyzed and interpreted with caution. Definite conclusions
should only be based on a detailed comparison of
several analogs with different kinetic and thermodynamic
properties and/or on a variety of experimental approaches:
ultrastructural, physiological, and biochemical.
B. Cr Analog Administration as a Means
of Studying CK Function: Facts and
Potential Pitfalls
Cr analogs have been used frequently for studying the
relevance of the CK system for cellular energetics in
various tissues. Most of these studies have been performed
on skeletal muscle where Cr analog (in particular,
GPA) administration caused a series of functional
changes (for a review see Ref. 1125). For example, GPA or
3-guanidinobutyrate (GBA) administration decreases developed
muscle tension as well as the rates of tension
development and relaxation (556, 636, 637, 656, 890, 1044,
1073, 1074), suppresses posttetanic twitch potentiation
and the staircase phenomenon (see Refs. 636, 637), improves
endurance performance (556, 637, 983, 1044, 1073,
1074), and causes ultrastructural abnormalities (535) (see
also below). Even though these results have been taken to
indicate that the CK system is important for proper muscle
function, some authors arrived at opposite conclusions
by finding either no or only minor changes in muscle
mechanics upon GPA feeding (889). Either way, results
and conclusions on CK function based on Cr analogfeeding
experiments must be interpreted with caution,
since these compounds may have a series of confounding
side effects.
GPA is a physiological constituent of mammalian
blood plasma, erythrocytes, brain, liver, kidney, aorta,
and urine, and its concentration is considerably increased
in uremic patients (see, e.g., Refs. 163, 181, 321, 554, 555,
597, 628, 998, 1132). GPA may be formed by AGAT
through transamidination between Arg and �-alanine
(1089) (see also sects. VIIA and IXH). It may thus be