Creatine and Creatinine Metabolism - Physiological Reviews
Creatine and Creatinine Metabolism - Physiological Reviews
Creatine and Creatinine Metabolism - Physiological Reviews
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
July 2000 CREATINE AND CREATININE METABOLISM 1177<br />
genase, degradation of Crn (or Cr) is coupled stoichiometrically<br />
to the oxidation of NADH to NAD � , which can<br />
be measured spectrophotometrically (65, 80, 926, 1110).<br />
3) Some Crn deaminase-based methods involve quantitation<br />
of the liberated ammonia either in a bromophenol<br />
blue indicator reaction (for references, see Ref. 758) or<br />
via glutamate dehydrogenase. In the latter case, Crn degradation<br />
is coupled stoichiometrically to NADPH oxidation<br />
(258, 333). These Crn deaminase-based methods have<br />
the advantage of comprising only two steps which may<br />
limit the risk of interferences. On the other h<strong>and</strong>, they<br />
involve quantitation of a volatile substance, ammonia,<br />
which might have a negative impact on the accuracy of<br />
the methods. 4) The last procedure involves five steps<br />
<strong>and</strong>, thus, is the most intricate one: Crn deaminase,<br />
1-methylhydantoin amidohydrolase, N-carbamoylsarcosine<br />
amidohydrolase, sarcosine oxidase, <strong>and</strong> a peroxidase<br />
(715, 893). The second part of this sequence is,<br />
evidently, identical to that of method 1.<br />
Even though enzymatic methods are deemed to be<br />
quite specific, they are still not free from interferences.<br />
For example, peroxidase detection systems suffer from<br />
bilirubin or ascorbate interference, thus necessitating incorporation<br />
of potassium ferrocyanide, bilirubin oxidase,<br />
<strong>and</strong>/or ascorbate oxidase in the assay (e.g., Refs. 70, 432,<br />
715, 862, 926, 932, 1094). On the other h<strong>and</strong>, cytosine<br />
derivatives such as the antibiotic 5-fluorocytosine may act<br />
as substrates of Crn deaminase (see sect. VIIG), a problem<br />
to be circumvented by the use of a more specific Crn<br />
deaminase either obtained from a natural source (298) or<br />
engineered biotechnologically. Further disadvantages of<br />
the enzymatic methods are the potentially lower storage<br />
stability <strong>and</strong> the (still) much higher price relative to the<br />
Jaffé reaction.<br />
It goes without saying that the enzymatic methods<br />
also have distinct advantages. Although it was not possible<br />
to eliminate the interference problem for the Jaffébased<br />
methods despite considerable effort over several<br />
decades, comparative studies have shown that already<br />
now, enzymatic methods produce less outliers <strong>and</strong>, in<br />
general, display a better performance than the Jaffé methods<br />
(see Refs. 70, 71, 258, 333, 443, 958, 1110). Even more<br />
importantly, molecular engineering offers great promise<br />
for further improving the properties of the enzymatic<br />
methods. For instance, mutagenesis approaches gave rise<br />
to more stable creatinase <strong>and</strong> sarcosine oxidase mutants<br />
(498, 709, 869, 871), <strong>and</strong> it is expected that more elaborate<br />
molecular design principles will allow us in the future to<br />
engineer enzymatic Crn <strong>and</strong> Cr determination methods<br />
virtually devoid of interference problems.<br />
HPLC (see, e.g., Refs. 43, 111, 1137, 1158), capillary<br />
electrophoresis (186, 1015), <strong>and</strong> biosensors with immobilized<br />
enzymes (582) represent alternative approaches for<br />
Cr, PCr, <strong>and</strong> Crn determination that may be less prone to<br />
interference problems than the Jaffé methods. Improved<br />
analytical tools for Crn <strong>and</strong> Cr determination are likely to<br />
find wide application not only in the diagnosis <strong>and</strong> monitoring<br />
of kidney function, but also in investigations <strong>and</strong><br />
clinical evaluation of energy metabolism in various bodily<br />
tissues. Moreover, they may be instrumental in monitoring<br />
the efficacy of Cr supplementation in sports physiology<br />
(see sect. XI) <strong>and</strong> in the treatment of diseases responsive<br />
to Cr therapy. Even though, for instance, the<br />
nonspecific Sakaguchi reaction has been suggested to be<br />
useful for the screening of GAMT deficiency (867), more<br />
specific enzymatic tests for the quantitation of guanidinoacetate<br />
<strong>and</strong>/or Cr are likely to facilitate <strong>and</strong> improve the<br />
accuracy of such screenings.<br />
XII. CREATINE SUPPLEMENTATION<br />
IN SPORTS PHYSIOLOGY<br />
With increasing commercialization of the sports business,<br />
improving muscle performance by any means has<br />
become a critical issue. The efficacy of carbohydrate<br />
loading is widely accepted. Carbohydrates can be consumed<br />
in sufficient amounts via natural foods. On the<br />
other h<strong>and</strong>, stimulating muscle growth by, e.g., <strong>and</strong>rogens<br />
is banned, since unphysiologically high concentrations of<br />
these hormones are required. Over the last few years, Cr<br />
supplementation as an ergogenic aid has boomed tremendously<br />
(for reviews, see Refs. 46, 103, 128, 305, 441, 674,<br />
829, 1106). The success of British sprinters <strong>and</strong> hurdlers<br />
at the beginning of the 1990s has been associated with Cr<br />
supplementation (see Ref. 1106). Nowadays, there is<br />
widespread enthusiasm about the performance-boosting<br />
effects of Cr, which is seemingly used by many top athletes<br />
in explosive sports disciplines. Cr supplementation<br />
is popular, for example, among bodybuilders, wrestlers,<br />
tennis players, cyclists, mountain bikers, rowers, skijumpers,<br />
or cross-country skiers as well as among ski,<br />
rugby, h<strong>and</strong>ball, basketball, football, <strong>and</strong> ice hockey<br />
teams. It is not the principal goal of this section to share<br />
the widespread enthusiasm, but to critically discuss the<br />
scientific data published so far on Cr supplementation <strong>and</strong><br />
its effects on muscle performance.<br />
During high-intensity exercise, ATP hydrolysis is initially<br />
buffered by PCr via the CK reaction. Whereas PCr is<br />
available instantaneously for ATP regeneration, glycolysis<br />
is induced with a delay of a few seconds, <strong>and</strong> stimulation<br />
of mitochondrial oxidative phosphorylation is delayed<br />
even further. On the other h<strong>and</strong>, the PCr stores in muscle<br />
are limited so that during high-intensity exercise, PCr is<br />
depleted within �10 s. Therefore, if it were possible to<br />
increase the muscle stores of PCr <strong>and</strong> thereby to delay<br />
PCr depletion, this might favorably affect muscle performance.<br />
The normal muscle concentration of total Cr is �125<br />
mmol � (kg dry mass) �1 (46). As early as in the 1920s, oral