Creatine and Creatinine Metabolism - Physiological Reviews
Creatine and Creatinine Metabolism - Physiological Reviews
Creatine and Creatinine Metabolism - Physiological Reviews
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July 2000 CREATINE AND CREATININE METABOLISM 1165<br />
first reaction is N-hydroxylation of the exocyclic amino<br />
group, yielding the respective N-hydroxy-AIA. For most<br />
AIA, this reaction is catalyzed predominantly by the cytochrome<br />
P-4501A2 isoenzyme (296, 330, 872, 920). H 2O 2 in<br />
the physiological/pathological concentration range considerably<br />
stimulates metabolic activation of IQ by human<br />
<strong>and</strong> rat cytochrome P-4501A2 (19). PhIP, on the other<br />
h<strong>and</strong>, is hydroxylated at similar rates by cytochromes<br />
P-4501A1 <strong>and</strong> 1A2 (212, 330). In addition, a series of other<br />
cytochrome P-450 isoenzymes (see Refs. 158, 235, 345,<br />
590, 872) as well as prostagl<strong>and</strong>in H synthase (670) were<br />
suggested to participate in N-hydroxylation of AIA, but<br />
their actual contribution to the metabolism of the respective<br />
AIA is only vaguely known.<br />
N-OH-AIA can be activated further by O-acetylation,<br />
O-sulfonylation, O-prolylation, or O-phosphorylation (see<br />
Refs. 119, 159, 235, 743, 872, 920). In human liver, Oacetylation<br />
is mainly catalyzed by the polymorphic Nacetyltransferase<br />
(NAT2) isoenzyme <strong>and</strong> yields the respective<br />
N-acetoxy-AIA (345, 1121). In human mammary<br />
gl<strong>and</strong>, on the other h<strong>and</strong>, O-acetylation is catalyzed predominantly<br />
if not exclusively by the NAT1 isoenzyme<br />
(828). O-Sulfonylation of N-OH-PhIP is catalyzed in human<br />
liver by the thermostable phenol sulfotransferase<br />
isoenzyme (TS-PST), whereas the contributions of thermolabile<br />
phenol sulfotransferase (TL-PST) <strong>and</strong> dehydroepi<strong>and</strong>rosterone<br />
sulfotransferase (DHEA-ST) seem to be<br />
negligible (126). Moreover, no O-sulfonylation was observed<br />
for N-OH-IQ <strong>and</strong> N-OH-8-MeIQx in human liver.<br />
While O-prolylation of N-OH-AIA is catalyzed by L-prolyltRNA<br />
synthetase, the enzyme responsible for ATP-dependent<br />
O-phosphorylation has not yet been identified. All<br />
four activated AIA metabolites are likely to decompose to<br />
the highly reactive nitrenium ion (872; see also Ref. 348)<br />
which then reacts with protein, low-M r thiols like glutathione<br />
or cysteine, or DNA.<br />
Each AIA forms several different adducts with DNA<br />
(e.g., Ref. 110), but most of them involve guanine bases.<br />
So far, IQ, MeIQ, 8-MeIQx, 4,8-DiMeIQx, <strong>and</strong> PhIP were<br />
reported to form adducts with 2�-deoxyguanosine, by<br />
binding of the exocyclic nitrogen of the AIA to the C-8 of<br />
guanine (see Ref.s 235, 617, 872, 886, 922). To a lesser<br />
extent, IQ <strong>and</strong> 8-MeIQx also formed adducts with the N 2<br />
of guanine. The preference of activated AIA for guanine<br />
bases is in perfect agreement with the fact that most<br />
mutations induced by IQ, MeIQ, 8-MeIQx, <strong>and</strong> PhIP in the<br />
p53, Ha-ras, Ki-ras, APC, supF, lacZ, lacI, <strong>and</strong> hprt genes<br />
involve G-C base pairs (see Refs. 296, 682, 872, 1013).<br />
In addition to these activation reactions, several detoxification<br />
pathways have to be considered, in particular<br />
N-glucuronidation, N 2 -sulfamation, ring hydroxylation<br />
followed by sulfation or glucuronidation, N-demethylation,<br />
<strong>and</strong> glutathione- plus GST-mediated suppression of<br />
AIA-DNA adduct formation (see Refs. 215, 235, 448, 521).<br />
N-Glucuronidation has been established for N-OH-IQ,<br />
N-OH-8-MeIQx, <strong>and</strong> N-OH-PhIP. In the case of N-OH-PhIP,<br />
two distinct glucuronides, namely, N-OH-PhIP N 2 -glucuronide<br />
<strong>and</strong> N-OH-PhIP N 3 -glucuronide, were identified,<br />
with their ratio differing considerably between rat, dog,<br />
<strong>and</strong> human. Ring hydroxylation, like N-hydroxylation, is<br />
catalyzed by cytochrome P-4501A isoenzymes, although<br />
different isoenzymes may be involved in the two processes.<br />
In the case of PhIP, ring hydroxylation is catalyzed<br />
predominantly by cytochrome P-4501A1 <strong>and</strong> yields 4�-OH-<br />
PhIP, which displays no or only low mutagenicity. In<br />
addition, 5-OH-PhIP may be formed via a cytochrome<br />
P-450-independent pathway.<br />
In in vitro studies, glutathione inhibited the covalent<br />
binding of N-acetoxy-IQ <strong>and</strong> N-acetoxy-8-MeIQx to DNA<br />
by �40%, <strong>and</strong> further addition of human or rat GST isoenzymes<br />
had no extra effect (561). In contrast, covalent<br />
DNA binding of N-acetoxy-PhIP was only slightly inhibited<br />
by glutathione alone, but inhibition was greatly amplified<br />
by further addition of human GST isoenzyme A1–1<br />
or rat GST isoenzyme 1–2. The human GST isoenzymes<br />
A1–2 <strong>and</strong> P1–1 as well as rat GST 12–12 had lesser effects.<br />
Analysis of incubation mixtures containing N-acetoxy-<br />
PhIP, reduced glutathione, <strong>and</strong> GST A1–1 failed to reveal<br />
glutathione conjugates of PhIP. Only oxidized glutathione<br />
<strong>and</strong> PhIP, in a molar ratio of �1:2, were detected as<br />
reaction products.<br />
The contribution of the different activation <strong>and</strong> detoxification<br />
reactions to the total metabolism of a given<br />
AIA differs to a large extent between individual AIA,<br />
between different tissues, <strong>and</strong> between species. It therefore<br />
goes without saying that our current knowledge on<br />
this matter is still incomplete. In humans, after the consumption<br />
of a test meal, 90.8 <strong>and</strong> 69.9% of 8-MeIQx <strong>and</strong><br />
PhIP, respectively, were metabolized via cytochrome<br />
P-4501A2, 2.6 <strong>and</strong> 1.2% were excreted unchanged in the<br />
urine, <strong>and</strong> 6.6 <strong>and</strong> 28.9% were eliminated by other routes<br />
of metabolism (81). Metabolic activation of AIA by Nhydroxylation<br />
<strong>and</strong> O-acetylation occurs predominantly in<br />
the liver of rodents <strong>and</strong> primates. Accordingly, for all AIA<br />
except PhIP, highest DNA adduct levels are found in liver,<br />
followed by kidney, heart, <strong>and</strong> colon (160, 920). For PhIP,<br />
on the other h<strong>and</strong>, highest DNA adduct levels were detected<br />
in rat pancreas, followed by prostate, colon, lung,<br />
<strong>and</strong> heart, whereas liver contained 25- to 50-fold lower<br />
levels of PhIP-DNA adducts than the pancreas (447, 886).<br />
It is surprising that metabolic activation of PhIP nevertheless<br />
occurs predominantly in the liver <strong>and</strong> that Nhydroxy-PhIP<br />
<strong>and</strong> N-acetoxy-PhIP are the major metabolites<br />
that subsequently are transported via the blood<br />
circulation to those extrahepatic tissues that are subject<br />
to the mutagenic effects of PhIP (see Ref. 872). This<br />
apparent discrepancy suggests that an efficient detoxification<br />
mechanism is operative in the liver. Evidence supporting<br />
this notion in fact exists, indicating that protection<br />
may be brought about by glutathione <strong>and</strong> GST (539,