Creatine and Creatinine Metabolism - Physiological Reviews

July 2000 CREATINE AND CREATININE METABOLISM 1121
as catalyzed by the 1-methylhydantoin amidohydrolases
of Pseudomonas, Brevibacterium, Moraxella, Micrococcus,
and Arthrobacter strains, is stoichiometrically coupled
with ATP hydrolysis and is stimulated by Mg 2� and
NH 4 � or K � . In addition, hydantoin is hydrolyzed by these
enzymes at a much lower rate than 1-methylhydantoin. In
contrast, the 1-methylhydantoin amidohydrolases of anaerobic
bacteria (357) are not affected by ATP and Mg 2� ,
and hydantoin is hydrolyzed at a similar rate as 1-methylhydantoin.
3) In various Alcaligenes, Arthrobacter, Flavobacterium,
Micrococcus, Pseudomonas, and Tissierella
strains, still another set of enzymes is induced when they
are grown on Cr or Crn as sole source of nitrogen and/or
carbon. Creatininase (Crn amidohydrolase) converts Crn
to Cr which is then further metabolized by creatinase (Cr
amidinohydrolase) to urea and sarcosine (see Refs. 115,
257, 335, 487, 708, 884). Even though creatinase has also
been detected in human skeletal muscle (655), this finding
awaits confirmation and demonstration of its physiological
relevance.
Sarcosine formed in degradation pathways 2 and 3
may be degraded further to Gly by a sarcosine oxidase or
sarcosine dehydrogenase (487, 708, 884), or possibly to
methylamine by the action of a sarcosine reductase (see
Refs. 334, 335, 439). It also seems worth mentioning that
glycocyamidine and glycocyamine (guanidinoacetate) can
be degraded by microorganisms almost exactly as shown
in pathway 3 for Crn degradation. Glycocyamidinase converts
glycocyamidine to glycocyamine, which is then split
by glycocyaminase (guanidinoacetate amidinohydrolase;
EC 3.5.3.2) into Gly and urea (1150, 1151).
4) Finally, Pseudomonas stutzeri seems to convert
Crn quantitatively to methylguanidine and acetic acid
(1049). Methylguanidine was shown to be split in an Alcaligenes
species by a highly specific methylguanidine
amidinohydrolase into methylamine and urea (685).
The distinction between four alternative degradation
pathways for Crn represents an oversimplification. For
example, two of the degradation pathways may occur in
the same organism, with the relative expression levels of
the individual enzymes depending primarily on the nitrogen
source used (884). When the Pseudomonas sp. 0114 is
grown on Crn as main nitrogen source, Crn is degraded
chiefly via Cr. When the same species is grown on 1-methylhydantoin,
the 1-methylhydantoin amidohydrolase and
N-carbamoylsarcosine amidohydrolase activities are induced
so that in this case, Crn degradation via 1-methylhydantoin
and N-carbamoylsarcosine prevails. The different
Crn degradation pathways may also overlap. In the
Pseudomonas sp. H21 grown on 1-methylhydantoin as
main nitrogen source, creatinase activity is undetectable.
However, Cr can still be degraded, but only indirectly via
Crn, 1-methylhydantoin, and N-carbamoylsarcosine (884).
When the same species is grown on Crn, creatinase is
induced, and Cr can be degraded directly to sarcosine.
Finally, the distinction between pathways 1 and 2 may
seem arbitrary, even more so if it is taken into account
that Clostridium putrefaciens and C. sordellii grown on
basal medium degrade Cr and Crn solely to 1-methylhydantoin,
while the same strains grown on a minced meat
medium further degrade 1-methylhydantoin to sarcosine
(278).
Clearly, microbial Crn degradation is at present only
incompletely understood. To get a deeper insight into this
topic, a wide evolutionary screening and detailed characterization
of all enzymes involved are essential prerequisites.
Relevant questions to be addressed are whether and
how the expression of Cr- and Crn-degrading enzymes is
regulated, and in which microorganisms Crn and cytosine
deamination are catalyzed by a single or by separate
enzymes.
VIII. PROTEINS INVOLVED
IN CREATINE METABOLISM
A. L-Arginine:glycine Amidinotransferase
Depending on the species, the highest activities of
AGAT in vertebrates are found in liver, kidney, pancreas,
or decidua (see sect. III). Although the yolk sac of the
hen’s egg was reported to contain significant amounts of
AGAT by Walker (1077), it was suggested not to do so by
Ramírez et al. (793). Mostly in kidney and pancreas, but
also in rat decidua, the levels of AGAT are influenced by
a variety of dietary and hormonal factors. These factors
and the underlying mechanisms of AGAT regulation are
discussed in detail in section IV. Notably, AGAT expression
was shown to be downregulated in Wilms’ tumor, a
renal malignancy with complex genetic and pathological
features (37).
AGAT is confined to the cortex of human and rat
kidney (623, 634), which is in line with higher concentrations
of GAA in the cortex than in the medulla of rat and
rabbit kidney (555). Both immunolocalization and microdissection
experiments revealed that AGAT activity and
immunoreactivity are restricted to epithelial cells (in a
basilar position) of the proximal convoluted tubule of the
rat nephron (623, 976, 977). AGAT therefore coincides in
location with the site of Arg biosynthesis in the kidney,
which was shown to be highest in the proximal convoluted
tubule, somewhat lower in the pars recta of the
proximal tubule, and almost negligible in all other segments
of the nephron (553). In contrast to AGAT, Cr is
present in all nephron segments tested, although in varying
amounts (974, 977), whereas CK was localized to the
distal nephron of the rat kidney in the thick ascending
limb of Henle’s loop and the distal convoluted tubule
(264).