Creatine and Creatinine Metabolism - Physiological Reviews

1176 MARKUS WYSS AND RIMA KADDURAH-DAOUK Volume 80
catabolism, anemia, a decrease in the circulating platelet
count, central and peripheral neuropathy, pruritus, arrhythmias
and myocardial degeneration, congestion, and,
in later stages of intoxication, anorexia, vomiting, increased
salivation, and diarrhea (see Ref. 53). In vitro
studies have shown that MG has a variety of potentially
toxic effects by inhibiting, for example, mitochondrial
oxidative phosphorylation, growth of cultured cells, or
Na � -K � -ATPase in the brain (for references, see Refs. 168,
735, 1147) or by affecting membrane fluidity (1138). Furthermore,
MG, which is also present in various foods (for
references, see Refs. 223, 294, 685), may be nitrosated
intragastrically and may thereby become a potent mutagen
(223) (see also sect. IXF).
Recently, MG was identified as a nonselective reversible
inhibitor of NOS isoenzymes (see Refs. 341, 581, 930,
931, 1004, 1114). The three major isoenzymes of NOS,
neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible
NOS (iNOS), were inhibited with IC 50 values of
90–370 �M (931, 1114). By inhibiting eNOS, intravenously
administered MG increased mean arterial blood pressure
in rats, thus raising the hypothesis that MG may contribute
to the hypertension seen in patients with CRF (929,
930). Similarly, by inhibiting NOS in kidney and brain, MG
may have an impact on the regulation of glomerular capillary
pressure and on neurological functions, respectively.
Guanidino compounds including Cr, Crn, guanidine,
MG, GAA, GPA, GBA, and GSA were also shown to have
a series of other, potentially harmful, side effects (see also
sect. VIIIB). Insulin resistance is a common finding in CRF.
Accordingly, insulin binding to erythrocyte receptors was
shown to be decreased in uremic patients, but increased
rapidly upon hemodialysis (814). Binding of insulin to
erythrocytes is also depressed by 1 mM concentrations of
Crn, Cr, and guanidinoacetate. Plasma of uremic patients
as well as �- and �-GPA inhibit the hexose monophosphate
shunt in red blood cells (453). GPA, GBA, GAA, and
GSA inhibit the phytohemagglutinin-induced stimulation
of proliferation of normal human lymphocytes (878). This
inhibitory action may possibly explain the depression in
total lymphocyte count and function as well as the greatly
depressed cell-mediated response to protein antigens in
uremia. Finally, guanidine, MG, Crn, Cr, GSA, and GBA
have convulsive effects in animals and may thus contribute
to the neurological symptomatology in uremia (see
sect. IXG).
In conclusion, Crn degradation seems to be virtually
irrelevant under normal conditions when Crn might even
have a beneficial effect by acting as a hydroxyl radical
scavenger. At greatly reduced GFR, on the other hand,
when the serum concentration of Crn as well as oxidative
stress are considerably increased, the formation of toxic
Crn degradation products is favored and may contribute
significantly to further disease progression. Because Crn
degradation is stimulated by ROS and in particular by the
hydroxyl radical, the serum concentrations of creatol and
MG as well as the creatol/Crn and MG/Crn ratios may not
only serve as diagnostic indexes for the degree of CRF,
but may also be used as measures of oxidative stress in
uremic patients (25, 683, 687, 747, 1146).
XI. ANALYTICAL METHODS AND THEIR
IMPLICATIONS FOR CLINICAL DIAGNOSIS
Ever since the suggestion of Popper and Mandel in
1937 (779) that the clearance of endogenous Crn approximates
the GFR, investigation of serum and urinary [Crn]
has been popular in clinical medicine. Even though the
parallelism between Crn clearance and GFR turned out to
be less strict than previously presumed (see sect. IXH),
there is still a broad interest in improving analytical methods
for measuring [Crn] and [Cr] in biological samples.
For many decades, chemical methods prevailed (551,
758, 932, 1071). Crn has been, and still is, measured
mostly by the Jaffé reaction (422) in which Crn and picric
acid under alkaline conditions form an orange-red-colored
complex. Despite considerable effort, the detailed
mechanism of color formation could not yet be resolved
(see Refs. 551, 932, 958). The major disadvantage of the
Jaffé reaction is its lack of specificity due to interference
by a variety of metabolites, e.g., ketones and ketoacids,
protein, bilirubin, and cephalosporins (e.g., Refs. 71, 443,
635, 926, 932, 958, 1071, 1110). Although a large number of
modifications and improvements of the Jaffé reaction
were proposed, none has eliminated all interferences.
This disadvantage was outscored in the past by the usefulness
of the Jaffé reaction in the clinical environment,
by the lack of valid alternatives, and, in particular, by the
low price of the required chemicals. Cr was frequently
determined chemically by the �-naphthol-diacetyl reaction
(1117), but this method is rather cumbersome and
nonspecific as well.
Already in 1937, using crude bacterial extracts, Miller
and Dubos managed to estimate enzymatically the Crn
content of plasma and urine (see Ref. 55). But it is only in
recent years that significant progress has been made toward
developing and improving enzymatic Cr and Crn
determination methods (see Refs. 758, 862, 926, 932). In
keeping with the diversity of Crn degradation pathways
(see Fig. 7), a series of alternative reaction sequences was
proposed to be specific and accurate. 1) A first method is
based on creatininase, creatinase, sarcosine oxidase, and
a peroxidase. The hydrogen peroxide liberated in the
sarcosine oxidase reaction is used by the peroxidase to
produce a colored substance that can be measured spectrophotometrically
or fluorimetrically (for references, see
Refs. 432, 443, 926, 958, 1110). 2) In the reaction sequence
of creatininase, CK, pyruvate kinase, and lactate dehydro-