Journal of Alzheimer’s Disease 19 (2010) 1331–1336
Uric Acid as a CNS Antioxidant
Gene L. Bowmana,b,∗, Jackilen Shannonb, Balz Freic, Jeffrey A. Kayeaand Joseph F. Quinna
aDepartment of Neurology, Layton Center for Aging and Alzheimer’s Disease Research, Oregon Health & Science
University, Portland, OR, USA
bDepartment of Public Health and Preventive Medicine, Oregon Health & Science University, Portland, OR, USA
cLinus Pauling Institute, Oregon State University, Corvallis, OR, USA
Accepted 8 November 2009
Abstract. Oxidative damage is a consistent finding in a number of central nervous system (CNS) disorders. Uric acid (UA) is a
potent hydrophilic antioxidant that is modified by diet and drug. Several lines of evidence suggest that plasma UA may modulate
outcomes inneurologic disease, but littleattention has been paid toCNSlevels ofUA.Our objective was to testthe hypothesis that
cerebrospinal fluid (CSF) UA is determined by plasma UA, modified by blood-brain barrier (BBB) integrity and associated with
rate of cognitive decline in Alzheimer’s disease (AD). Also, since UA and ascorbic acid may act as antioxidants for one another,
we also explored a potential interaction between them in the brain. Thirty-two patients with mild to moderate AD (Mini-Mental
Status Exam 19 ± 5) participated in a longitudinal biomarker study for one year involving standardized clinical assessments.
CSF and blood were collected at baseline for UA, ascorbic acid, and albumin. Cognitive measures were collected at baseline and
again one year later. CSF UA was independent of age, gender, and AD severity. CSF and plasma UA were positively correlated
(r = 0.669, p = 0.001) and BBB impairment was associated with higher CSF levels of UA (p = 0.028). Neither plasma nor CSF
UA reached significant association with rates of cognitive decline over 1 year. CSF UA and CSF ascorbic acid were positively
correlated (r = 0.388, p = 0.001). The hypothesis that CSF UA is determined by plasma UA and BBB integrity is supported, as
is the hypothesis that UA and ascorbic acid are associated in CSF but not plasma. Adequately powered prospective studies would
help assess any role for UA in primary and secondary prevention of AD.
Keywords: Alzheimer’s disease, ascorbic acid, blood-brain barrier, cerebrospinal fluid, uric acid
Oxidative stress has been implicated in a number of
central nervous system (CNS) disorders and is an early
featureofneuronsin Alzheimer’sdisease (AD)and the
degeneration of dopamine producing cells in Parkin-
son’s disease (PD) [1,2]. In multiple sclerosis (MS),
peroxynitrite and other free radicals can contribute to
the inflammation and demyelination of axons . Pre-
the prognosis of these common CNS disorders.
Uric acid (UA) is an endogenously produced water-
soluble antioxidant that is modified by both drug and
∗Correspondence to: Gene L. Bowman, N.D., Oregon Health
& Science University, 3181 SW Sam Jackson Park Road, CR-131,
Portland, Oregon 97239 USA. Tel.: +1 503 494 6976; Fax: +1 503
494 7499; E-mail: email@example.com.
diet [4–6]. UA accounts for over half of the free radi-
cal scavenging activity in human blood  by quench-
ing superoxide and singlet oxygen, and protecting ox-
idation of vitamin C (ascorbic acid; AA) through the
chelation of iron [8–10]. Others have shown the abil-
ity of AA to repair oxidized urate, which highlights a
potential synergy for maintaining antioxidant capacity
by these two antioxidants . These qualities make
UA an attractive CNS antioxidant because neurons are
remarkably susceptible to oxidative stress, and AA is
concentrated as high as 10,000 µM in neurons .
Epidemiological studies focused on PD have ob-
served lower incidence and better prognosis with high-
er serum UA [13–16]. A postmortemstudy of substan-
tia nigra from PD patients revealed depletion of UA
content compared to age-matched controls . Some
studies in MS have reported lower serum UA in cas-
es as compared to controls, and higher serum UA has
ISSN 1387-2877/10/$27.50 2010 – IOS Press and the authors. All rights reserved
1332 G.L. Bowman et al. / Uric Acid as a CNS Antioxidant
been associated with delayed onset of the first neuro-
logic episode, stressing some potential for a preventa-
tive role [18–20]. Other studies in MS suggest there
may be therapeuticvalue in raising serum UA [21–23].
It is less clear whether UA is important to the devel-
opmentorprogressionof AD [24–29]. Cross-sectional
studies of serum UA have identified lower concentra-
tions in AD [27,28] and mild cognitive impairment
(MCI)  patients compared to healthy controls, but
another report found no difference . A secondary
analysis of MCI clinical trial data found that among
placebo treated subjects, lower serum UA was associ-
ated with more rapid cognitive decline and higher inci-
dence of AD over 3 years, but this observation was not
seen in the other study arms .
Two small studies of CSF UA in AD are inconsis-
tent, with one reportinghigherandthe otherlower
concentrations  of CSF UA compared to controls.
CSF UA is consistently about ten times lower than
serum levels, but very few have examined the relation-
ship between CSF and plasma UA in AD . We are
ly in AD with simultaneous measures of blood-brain
barrier (BBB) integrity and CSF ascorbic acid. Our
primary goal was to measure brain UA and to identi-
fy the determinants and consequences of CSF UA in
AD. We hypothesized that: 1) CSF UA is determined
by plasma UA and modified by BBB integrity; 2) CSF
UA is associated with rates of cognitivedecline; and 3)
since UA and AA may act as antioxidants for one an-
other, we also exploredwhether any relationship exists
between these two antioxidants.
We analyzedbaselineand12monthCSF andplasma
with mild to moderate AD. Study participants were
followed for cognitive change over 1 year.
er study were patients in the NIA – Layton Center for
Aging and Alzheimer’s Research at Oregon Health &
Science University (OHSU) with a diagnosis of proba-
ble AD. Participants were diagnosedby National Insti-
Stroke – Alzheimer’s Disease and Related Disorders
Association criteria  and Clinical Dementia Rating
of 0.5 or 1.0, establishing mild to moderate AD. All
patients from the biomarker study with available bio-
chemical specimens were included, yielding 32 partic-
ipants with mild to moderate probable AD. Informed
consent was obtained in accord with the Institutional
Review Board for Human Study at Oregon Health &
Thirty-two elderly (10 females, mean age 71 ±
ical evaluation included medical history, physical ex-
am, Mini-Mental Status Exam (MMSE) , Clinical
sessment Scale-cognitive subscale (ADAS-Cog) ,
burden, and Geriatric Depression Scale . Lumbar
Lumbar punctures were performed in the morning
under standardized conditions at L3-L4 or L4-L5 in-
terspace. CSF samples were immediately aliquoted
and snap frozen at −70◦C until assayed, at which
point samples had normal cell count and glucose lev-
els. Blood samples collected at the same visit as lum-
bar puncture for CSF were analyzed for albumin, UA,
and AA. Briefly, UA and AA were done by paired-ion
reversed-phase HPLC coupled with electro-chemical
detection. Fifty microliters of plasma was mixed with
an equal volume of cold 5% (wt/vol) metaphosphoric
acid and centrifuged to remove the precipitated pro-
teins. Fifty microliters of the supernatant was mixed
with 15 µl of cold 2.58 M K2HPO4buffer (pH 9.8)
followed by the addition of 185 µl of HPLC eluant
(see below). Forty microliters of this mixture was im-
mediately chromatographed on an LC18DB column
[25 cm × 4.6 mm (i.d.)]
guard column [2 cm × 4.6 mm (i.d.)] containing the
same material. The eluant, delivered at a flow rate
of 1.0 ml/min, consisted of 40 mM sodium acetate,
0.54 mM Na2EDTA, 1.5 mM dodecyltriethylammoni-
um phosphate, and 7.5% (vol/vol) methanol, taken to
pH 4.75 with glacial acetic acid. The eluate was an-
alyzed with an LC 4B amperometric electrochemical
detector equipped with a glassy-carbon working elec-
trode and an Ag/AgCl reference electrode (Bioanalyt-
(Supelco) preceded by a
G.L. Bowman et al. / Uric Acid as a CNS Antioxidant1333
Study population characteristics (n = 32)∗
CSF uric acid (µM)
Plasma uric acid (µM)
CSF ascorbic acid (µM)
Plasma ascorbic acid (µM)
CSF albumin (µM)
Serum albumin (µM)
CSF Albumin Index
Mini-Mental State Exam
Clinical Dementia Rating – sum of box
Alzheimer’s Disease Assessment Scale – cognitive subscale
Annual change in Mini Mental State Exam
Annual change in AD Assessment Scale – cognitive subscale
Annual change in Clinical Dementia Rating – sum of boxes
∗Mean (SD) unless otherwise denoted.
ical Systems, West Lafayette, IN). The applied poten-
tial was +0.5 V, with a sensitivity setting of 500 nÅ
for urate and 50 nÅ for ascorbate. Urate and ascor-
bate eluted as single peaks with retention times of 5.8
and 11.5 min, respectively . CSF and serum albu-
min were quantified to calculate CSF Albumin Index
as a validated marker of BBB integrity [37,38]. All
biochemical analyses were performed and the results
recorded by staff blinded to subject’s clinical informa-
Our primary hypotheses were that CSF UA was de-
termined by plasma UA, BBB integrity (CSF Albumin
Index), and CSF AA and associated with rates of cog-
nitive decline in AD. Means and SD were generated
for baseline study characteristics. Pearson correlation
coefficientswere used to confirmUA and AA indepen-
dence from age, gender, and AD severity. Regression
analysis was used to examineany relationshipbetween
the predictors (plasma UA, CSF Albumin Index, and
CSF AA) and response (CSF UA) and CSF UA with
rates of disease progression over one year (change in
t-test compared the mean difference in CSF UA by a
priori categoryofBBB integrity(intact,CSF AI <9.0;
impaired, CSF AI ? 9.0). All statistical significance
was two-tailed with α set at 0.05. SPSS version 17
and Macintosh were the software and hardware for the
analysis (Chicago, IL).
The mean age of the sample was 71.0 ± 7.0 years.
levels. Conversely,AAwas higherin CSF (∼3:1). The
mean CSF Albumin Index (BBB integrity marker) was
7.5 ± 3.8 and eight of the thirty-two subjects had BBB
impairment (CSF AI ? 9.0). Baseline MMSE was 19
of follow-up (Table 1).
Relationship between CSF UA and established risk
factors for AD progression
There were no significant correlations between CSF
UA and age, gender, or disease severity (Table 2).
Consequences of UA in AD
We did observe cognitive and functional decline in
our population (Table 1), but CSF or plasma UA did
not explain the annual decline (correlation analysis,
Table 2; regression coefficient data not shown).
Determinants of CSF UA in AD: Plasma UA, BBB
integrity, and CSF AA
Each µmol/L increase in plasma UA was associ-
ated with about a 5% increase in CSF UA (β =
0.092 µmol/L, p < 0.001, 95% CI = 0.054–0.130,
Fig.1A).BBB impairmentwas definedapriori as CSF
Albumin Index of 9.0 or greater and was identified in
1334 G.L. Bowman et al. / Uric Acid as a CNS Antioxidant
Correlation coefficients generated between CSFand plasma antioxidants with age, gender, baseline disease severity and rates ofcognitive
change in Alzheimer’s disease
Baseline cognitive status
Annual change in cognition (∆)
MMSE ∆ ADAS ∆
CSF uric acid (µM)
Plasma uric acid (µM)
CSF ascorbic acid (µM)
Plasma ascorbic acid (µM)
CSF, cerebrospinal fluid; MMSE, Mini Mental State Exam; ADAS, Alzheimer’s Disease Assessment Scale-cognitive subscale; CDR,
Clinical Dementia Rating sum of box score.
Fig. 1. Association between uric acid, ascorbic acid, and blood-brain barrier integrity in AD.A) Plasma UA versus CSF UA,r = 0.669, p =
0.001; B) Mean difference in CSF uric acid between BBB intact (CSF Albumin Index < 9.0) and BBB impaired (CSF Albumin Index ? 9.0)
was 6.2 µmol/L (p = 0.028); Standard error bars set at 1.0; C) CSF Albumin Index versus CSF-to-plasma UA, r = 0.442, p = 0.011; D) CSF
AA versus CSF UA, r = 0.388, p = 0.001.
G.L. Bowman et al. / Uric Acid as a CNS Antioxidant1335
25% (8/32) of the sample. CSF UA was 6.2 µmol/L
higher in subjects with BBB impairment (p = 0.028,
Fig. 1C). Regression analysis demonstrated a contin-
uous and positive linear association between CSF-to-
plasma UA ratio and CSF Albumin Index (β = 0.004,
p = 0.011, Fig. 1D). Each µmol/L increase in CSF
AA was also associated with about 6% higherCSF UA
content (β = 0.053 µmol/L, p = 0.028, 95% CI =
0.006–0.100, Fig. 1B). Plasma UA and AA had a pos-
itive correlation initially (r = 0.463, p = 0.008), but
after careful examination of the data, it was apparent
that this association was driven by an outlying plasma
AA value greater than 2 SD from the norm (163 µM).
After excluding this outlier, plasma UA and AA were
not correlated (r = 0.293, p = 0.110, n = 31).
Thesedatashowthat CSF UA is determinedbyplas-
ma UA and BBB integrity, and may be modified by
CSF AA. Although CSF UA did not predict the rate of
the rate of neurodegenerationin AD cannot be exclud-
edwith certainty,giventhe relativelysmall sample size
and short duration of follow-up.
Althougholder reportsindicatethat the humanbrain
ies have established the presence of this enzyme in
brain, so that the brain has the capacity to generateuric
acid in situ. However, the positive correlation between
plasma and CSF UA and the 10 fold higher plasma UA
metabolite is manufactured peripherally, and that ac-
cess to the brain is limited by the BBB. Other studies
have proposed CSF-to-plasma UA ratio as a marker of
Our positive correlation between CSF-to-plasma UA
ratio and CSF Albumin Index supports this view.
We were unableto identifyanyrelationshipbetween
CSF. Synergybetweenthese two antioxidantshas been
demonstrated as an apparent mechanism for maintain-
ing antioxidant capacity of serum. One study suggests
UA as an antioxidant for AA and another experiment
by AA [9,11]. The correlation between CSF UA and
AA is a new finding to our knowledge and raises the
possibility that these antioxidants may also interact in
the brain. The possibility that UA might influence AA
levels in the brain is intriguing but requires confirma-
ofCSF UA in AD, but sample size remainsa limitation
to making firm conclusions regarding the effect of UA
upon rate of progression in AD. Other larger studies
have suggested that plasma UA may modify the rate
of progression from MCI to AD , so further study
of this relationship may be in order. However, it ap-
pears from the present study that CSF levels are not
more informative than plasma levels. Pharmacologic
means for modulatingplasma UA are well established,
so this question has clear implications for clinical trial
(GLB), NCCAM P01 AT002034 (BF), VA Advanced
NIA P30 AG08017 (JAK)
Authors’ disclosures available online (http://www.j-
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