ANTIOXDANTS CAROTENES

8/11/2019 ANTIOXDANTS CAROTENES

1/3

FEB S Let te rs 384 (1996) 240-242 FEB S 16934

n t i o x i d a n t a c t i v i t i e s o f c a r o t e n e s a n d x a n t h o p h y l l s

N i c h o l a s J . M i l le r a , J u li a S a m p s o n a , L u i s P . C a n d e i a s b , P e te r M . B r a m l e y c ,

C a t h e r i n e A . R i c e - E v a n s a ,

aFree Radical Research Group, UMDS-Guy s Hospital, St Thomas s Street, London SE1 9RT, UK

bGray Laboratory, Cancer Research Trust, Mount Vernon Hospital, Northwood, Middlesex HA6 2 YR, UK

CDivision of Biochemistry, School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK

Received 8 March 1996

Abstract The purpose o f th i s s tudy was to assess the re la t ive

a n t i o x i d a n t a c t iv i t i es o f a r a n g e o f c a r o t e n es a n d x a n t h o p h y l l s

t h r o u g h t h e e x t e n t o f t h e i r a b i l i t i e s t o s c a v e n g e t h e A B T S +

radica l cat ion . The resu l t s show that the re la t ive abi l i t i e s o f the

carotenoids to scavenge the ABT S radica l cat ion are inf luenced

by the presence o f funct ional groups wi th increas ing po lar i t ie s ,

s u c h a s c a r b o n y l a n d h y d r o x y l g r o u p s , i n t h e t e r m i n a l r i n g s, a s

wel l as by the number of conjugated double bonds .

Key words: L y c o p e n e ; C a r o t e n e ; X a n t h o p h y l l ; A B T S ' + ;

R a d i c a l c a t i o n ; R a d i c a l s c a v e n g e r

I . Introduct ion

M a n y s t u d i e s h a v e i m p l i c a t e d 1 3 - c a r o t e n e a s a n a n t i o x i d a n t

i n v i v o a l t h o u g h t h e r e i s l i t t le d i r e c t e v i d e n c e t o s u p p o r t t h i s

a s y e t. I n c o n t r a s t , t h e r e i s c o n s i d e r a b l e i n v i t r o e v i d e n c e f o r

i n t e r a c t i o n o f 1 3 - c a r o t e n e w i t h f r e e r a d i c a l s [ 1 - 5 ] , f o r i t s p r o p -

e r t i e s a s a c h a i n - b r e a k i n g a n t i o x i d a n t [ 2 ] a n d i n s c a v e n g i n g

a n d q u e n c h i n g s i n g l e t o x y g e n [ 6 - 9 ] . 1 3 - C a r o t e n e h a s a l s o b e e n

r e p o r t e d t o p r o t e c t a g a i n s t p h o t o s e n s i t i v i t y d i s e a s e s i n h u -

m a n s [ 10 ]. H o w e v e r , i t o f f e r s l i t tl e p r o t e c t i o n a g a i n s t m e t a l -

c a t a l y s e d L D L o x i d a t i o n e x v i v o [1 1,1 2] a l t h o u g h t h e c a r o t e -

n o i d s w i t h i n t h e L D L a r e c o n s u m e d o n o x i d a t i o n w i t h C u 2 +,

t h e s e q u e n c e o f a c t i v i t y b e i n g l y c o p e n e > I ~ - c r y p t o x a n t h i n -

> l u t e i n / z e a x a n t h i n > c~ ,[ 3 -c a ro t en e , a s d e d u c e d f r o m r a t e s

o f c o n s u m p t i o n d u r i n g 1 h o x i d a t i o n [ 13 ].

C a r o t e n o i d s a r e k n o w n t o b e re a c t i v e t o w a r d s s i n gl e t o x y -

g e n , l y c o p e n e b e i n g t h e m o s t e f fi c ie n t q u e n c h e r i n v i t r o

[ 14 ,1 5] , b u t v e r y f e w m e c h a n i s t i c s t u d i e s o n t h e i r r a d i c a l

s c a v e n g i n g p r o p e r ti e s h a v e b e e n c a r r i e d o u t . T h e i n t e r a c t i o n

o f 1 3 -c a ro t en e w i t h p e r o x y l r a d i c a l s g e n e r a t e d d u r i n g l i p i d p e r -

o x i d a t i o n h a s b e e n s u g g e s t e d t o l e a d t o f o r m a t i o n o f a r a d i c a l

a d d u c t [ [ 3 -c ar .. .R O 2 ] [ 16 ]. M e c h a n i s m s o f f r e e r a d i c a l s c a v e n g -

i n g h a v e b e e n i n v e s t i g a t e d [ 1 7 ] b y p u l s e r a d i o l y t i c g e n e r a t i o n

o f n i t r o g e n d i o x i d e r a d i c a l s ( N O ~ ) , t h i y l r a d i c a l s ( R S ' ) a n d

s u l p h o n y l r a d i c a l s ( R S O ~ ) , w h i c h a r e r a p i d l y s c a v e n g e d b y

I ~ -c a ro t en e . T h e m e c h a n i s m f o r t h e s c a v e n g i n g o f t h e N O ~

r a d i c a l h a s b e e n d e s c r i b e d a s t h a t o f e l e c t ro n d o n a t i o n w i t h

r a d i c a l c a t i o n f o r m a t i o n [ [ 3- c ar o te n e ] + w h e r e a s t h e s c a v e n g -

i n g o f t h i y l ra d i c a l s i n v o l v e s th e g e n e r a t i o n o f a r a d i c a l a d -

d u c t [ 1 3 - c a r . . R S ] ' . T h e r a d i c a l c a t i o n a n d a d d u c t r a d i c a l s a r e

h i g h l y r e s o n a n c e s t a b il i s ed a n d a p p e a r t o u n d e r g o s l o w b i m o -

l e c u l a r d e c a y t o n o n - r a d i c a l p r o d u c t s . O t h e r s t u d ie s h a v e d e -

scr ibed

t h e i n t e r a c t i o n b e t w e e n t h e t r i c h l o r o m e t h y l p e r o x y l

r a d i c a l a n d t h e c a r o t e n o i d s ] 3 - ca r o t en e , c a n t h a x a n t h i n , a s t a x -

a n t h i n , z e a x a n t h i n a n d l u t e i n , p r o d u c i n g a c o m b i n a t i o n o f

*Corresponding au thor . Fax : (44) (171) 955 4983 .

c a r o t e n o i d r a d i c a l c a t i o n a n d r a d i c a l a d d u c t , w i t h n o d i s t i n c -

t i o n b e t w e e n t h e r e a c t i v i t i e s o f t h e d i f f e r e n t c a r o t e n o i d s , e x -

c e p t f o r a s t a x a n t h i n p a r t i c i p a t i n g e x c l u s iv e l y i n r a d i c a l a d d i -

t ion [18] .

T h e p u r p o s e o f t h is s t u d y w a s t o d e l i n e a te t h e d i f f e r e nt i a l

a n t i o x i d a n t a c t i v it i e s o f a r a n g e o f c a r o t e n e s a n d x a n t h o p h y l l s

o f d i e t a r y o r i g i n t h r o u g h t h e e x t e n t o f t h e ir a b i l it i es t o s c a v -

e n g e t h e A B T S + r a d i c a l c a t io n . T h e r e s u lt s s h o w t h a t t h e

r e l a t iv e a b i l it i es o f t h e c a r o t e n o i d s t o s c a v e n g e t h e A B T S +

r a d i c a l c a t i o n a r e i n f l u e n c ed b y t h e p r e s e n c e o f f u n c t i o n a l

g r o u p s w i t h i n c r e a s in g p o l a r it i e s , s u c h a s c a r b o n y l a n d h y d r o -

x y l g r o u p s , i n t h e t e r m i n a l r i n g s a s w e l l a s b y t h e n u m b e r o f

c o n j u g a t e d d o u b l e b o n d s . O v e r a l l, t h e c a r o t e n e s w i t h 11 c o n -

j u g a t e d d o u b l e b o n d s a r e m o r e a c t i v e A B T S + r a d i c a l q u e n c h -

e r s t h a n t h e x a n t h o p h y l l s , w i t h t h e e x c e p t i o n o f 1 3 - c r y p t o -

x a n t h i n .

2 . M a t e r i a l s a n d m e t h o d s

2.1. Antioxidant activity

AB TS + rad ica l ca t ion was prepared b y pass ing a 5 mM aqueous

s tock so lu t ion of ABTS (2 ,2 ' -az inobis (3-e thy lbenzoth iazo l ine-6-su l -

p h o n i c a c id d i a m m o n iu m s al t) ( A ld r ic h C h e m ic a l C o . ) t h r o u g h m a n -

ganese d iox ide (S igma-Aldr ich) on a Whatman no . 5 f i l te r paper .

Excess manganese d iox ide was removed f rom the f i l t ra te by pass ing

i t th roug h a 0 .2 gM W hatm an PV DF syr inge f il te r . Th is so lu t ion was

then d i lu ted in 5 mM phosphate buffe red sa l ine (PBS) pH 7 .4 to an

absorbance of 0 .70 (+ 0 .02) a t 734 nm and pre- incub ated a t 30C

pr ior to use . F resh A BTS + rad ica l ca t ion so lu t ion w as prepared

each day .

Tro lox (Hoffman-La Roche) (6 -hydroxy-2 ,5 ,7 ,8- te t ramethylchro-

man-2-carboxyl ic ac id) (Aldr ich Chemica l Co . ) was used as an an t i -

ox idan t s tandard as descr ibed prev ious ly for the fe r ry l myoglobin /

ABTS assay [19 ,20] . 2 .5 mM Tro lox was prepared in PBS for use as

a s tock s tandard . A t th is pH and concen tra t ion , the upper l imi t o f the

so lub i l i ty o f Tro lox in PBS is approached and gen t le u l t rason ica t ion i s

required to d isso lve the c rys ta ls . F resh work ing s tandards were pre-

pared da i ly by d i lu t ing 2 .5 mM Tro lox wi th PBS.

A l l c a r o te n o id s w e r e o b t a in e d f r o m H o f f m a n - L a R o c h e e x c e p t f o r

as taxanth in which was a g i f t f rom P .B . Zaga lsky . The caro tenoids

were shown by HPLC to be a t leas t 97% pure except fo r as taxanth in

a t 100%. Caro teno ids were d isso lved in ace tone and then d i lu ted to a

concentra t ion of ca . 100 I .tM in 90% hexane /10% ace tone . Th e exac t

c o n c e n t r a t io n o f e a c h w o r k in g s o lu t i o n w a s d e r i v e d f ro m th e m o la r

absorp t iv i ty a t the spec i f ic absorbance maximum of each caro tenoid

be tween 444 nm and 480 nm. Af te r add i t ion o f 1 .0 ml of ABT S +

so lu t ion to a l iquots o f Tro lox or the caro tenoids (20-300 g l , depend-

ing

on the ac t iv i ty o f the par t icu la r compound) the so lu t ions were

vor tex mixed for exac t ly 30 s ; the absorbance a t 734 nm was taken

exac t ly 1 min a f te r in i t ia t ion of mix ing in a Beckm an spec t ropho-

tometer w i th Pe l t ie r e lec t ron ics se t a t 30C. PBS b lanks and hexane /

ace tone b lanks were run in each assay . The dose-response curve for

Tro lo x ( final concentra t ion 0-25 gM ) cons is ted of p lo t t ing the absorb-

ance a t 734 nm as a percen tage o f the absorba nce of the un inh ib i ted

rad ica l ca t ion (b lank) and was based on t r ip l ica te de te rmina t ions . The

ac t iv i ties o f the caro teno ids were assessed a t a min im um o f th ree

d i f fe ren t concentra t ions which had been de te rmined to be wi th in the

S0014-5793196l$12.00 19 96 Fed erat ion o f Europ ean Bioch emic al Societies. All rights reserved.

S S D I S 0 0 1 4 - 5 7 9 3 ( 9 6 ) 0 0 3 2 3 - 7


2/3

N . J . M i l l e r e t a l . I F E B S L e t t e r s 3 8 4 1 9 9 6 ) 2 4 0 - 2 4 2

Table 1

Mo la r ex t inc t ion coef f ic ien ts and ab sorbance max ima o f the ca ro tenes and xan tho phy l l s [21]

241

Caro te no id Abso rbance max imum (nm) Mo la r ex t inc t ion coeff ic ien t (cm -1 M -1 )

As taxa n th in 482 106.0 x

a-C aro ten e 448 145.02

~-C aroten e 454 134.42

[3-Cryptoxanthin 452 131.03

Can thax anth in 480 118.21

Lu tein 445 145.14

Lyco pene 474 185.02

Zea xan thin 452 133.12

So lven t key : 1 , benzene ; 2 , ace tone ; 3 , e the r ; 4 , e thano l .

range o f the dose - response cu rve . Each ca ro teno id was ana lysed in

t r ip l ica te a t these th ree concen t ra t ions . By re fe rence to the T ro lox

dose- response cu rve , the mean T ro lox equ iva len t an t iox idan t capac i ty

( T E A C ) v a l u e w a s d e r i v e d f o r e a c h c o m p o u n d . E a c h c o m p o u n d w a s

assessed on a min imum of 3 separa te days ( i . e . a t leas t 27 d i f fe ren t

analyses).

For HPLC ana lys is , e thy l -~-apo-ca ro tenoa te was used as the in te r -

na l s tandard (F luka) . Sam ples (50 ~tl a l iquo ts ) were ch roma tograph ed

wi th an i soc ra t ic mobi le phase o f ace ton i t r i le /ace tone /methano l

( 7 5 :2 0 : 5 b y v o l u m e ) ( H P L C g r a d e , R a t h b u r n ) w i t h a f l o w r a t e o f

0 .7 ml /min . A Beckmann 507 au tosample r coup led to a Beckmann

p h o t o - d i o d e a r r a y d e t e c t o r a n d a 1 26 p r o g r a m m a b l e s o l v e nt m o d u l e

was used. H PL C ana lys is was pe r fo rm ed wi th a Nuc leos i l RP 3 Ix OD S

co lumn, 250 3 mm (Phenomen ex) and th e de tec t ion wave leng th was

4 5 0 n m . T h e a m o u n t o f c ar o t e n o i d a p p li e d t o t h e H P L C c o l u m n w a s

ca lcu la ted us ing the mola r ex t inc t ion coef f ic ien ts in Tab le 1 . Recov-

e r ies o f ca ro teno ids f rom the co lumn were ca lcu la ted f rom ca ro teno id

absorbance a t 450 nm in re la t ion to the absorbance a t 450 nm of the

in te rna l s tandard . The iden t i f ica t ion o f each ca ro teno id on H PL C

analysis w as ve r i f ied by comp ar ison o f the re ten t ion t ime wi th tha t

o f the re la t ive s tandard compound , a s we l l a s by ana lys is o f the

spec t ra l p ro f i le and wi th l i te ra tu re va lues [21] . Caro teno id concen t ra -

t ions in the sample were ca lcu la ted f rom ca l ib ra t ion cu rves genera ted

f rom peak a rea ra t ios o f the ca ro teno id to the in te rna l s tandard .

3 R e s u l t s a n d d i s c u s s i o n

T h e h i e r a r c h y o f a n t i o x i d a n t a c t i v i t y f o r t h e c a r o t e n e s a n d

x a n t h o p h y l l s i n s c a v e n g i n g t h e A B T S + r a d i c a l c a t i o n i n c o m -

p a r i s o n w i t h T r o l o x u n d e r t h e d e f in e d c o n d i t i o n s i s s h o w n i n

T a b l e 2 . T h e t h r e e C - 4 0 c a r o t e n e s s t u d i e d s h o w d i f f e r e n t a b i l -

i ti e s t o s c a v e n g e t h e A B T S r a d ic a l c a t i o n w i t h l y c o p e n e

( ~ , w c a r o t e n e ) b e i n g m o r e e f f e c t i v e t h a n 1 3 - ca r o te n e ( 13 ,[ 3- ca r-

o t e n e ) w h i c h i n t u r n w a s m o r e p o t e n t t h a n i t s i s o m e r c ~ - c a r -

o t e n e ( 1 3 , e- c ar o te n e ). A c o m p a r i s o n o f th e i r s t r u c t u r e s s h o w s

t h a t b o t h a c y c l ic l y c o p e n e ( T E A C 2 . 9 m M ) a n d l ~ -c a r ot e n e

( 1 .9 m M ) h a v e 11 c o n j u g a t e d d o u b l e b o n d s , b u t t h e l a t t e r i s

b i c y cl i c. H o w e v e r , b e c a u s e o f s t e ri c h i n d r a n c e , p a r t i c u l a r l y

b e t w e e n t h e m e t h y l s u b s t i t u e n t a t C - 5 o f t h e r i n g a n d t h e

h y d r o g e n a t o m a t C - 8 o f t h e c h a i n i n 1 3 - ca r o te n e , c o p l a n a r i t y ,

c a n n o t b e a c h i e v e d a n d t h e m o l e c u l e a d o p t s a p r e f e rr e d c o n -

f o r m a t i o n i n w h i c h t h e r i n g s a n d t h e C - 5 , 6 a n d C - 5 ' , 6 ' d o u b l e

b o n d s a r e t w i s t e d o u t o f p l a n e . O r b i t a l o v e r l a p i s t h e r e f o r e

r e d u c e d a n d t h e c o n t r i b u t i o n o f th e r i n g d o u b l e b o n d s t o t h e

c h r o m o p h o r e i s s m a l l. I n t h e c a s e o f a - c a r o t e n e ( T E A C 1 .3

m M ) , t h e d o u b l e b o n d o f t h e s e c o n d r i n g is n o t p a r t o f t h e

c h r o m o p h o r e , l e a v i n g 9 c o n j u g a t e d d o u b l e b o n d s p l u s t h e I~-

r i n g. T h e r e f o r e , i t a p p e a r s t h a t t h e a b i l i ty o f c a r o t e n e s t o

s c a v e n g e t h e A B T S ra d i c a l c a t i o n i n c r ea s e s o n e x t e n s i o n

o f th e c h r o m o p h o r e a n d m a x i m u m o v e r l a p o f th e c a rb o n -

c a r b o n d o u b l e b o n d m o l e c u l a r o r b i t a l s .

T h e p r e s e n c e o f f u n c t i o n a l g r o u p s o n t h e t e r m i n a l t i n g s

m o d u l a t e s t h e r a d i c a l s c a v e n g i n g a c t i v i t y . T h e i n c o r p o r a t i o n

o f c a r b o n y l g r o u p s i n t h e t in g s h a s a p r o f o u n d s u p p r e s s iv e

e f f ec t o n t h e A B T S + r a d i ca l s c a v e n g i n g r e s p o n s e . F o r e x a m -

p l e , e c h i n e n o n e ~ , l ] - c a r o t e n e - 4 - o n e ) h a s l e s s t h a n 5 0 % o f t h e

a n t i o x i d a n t a c t i v i t y ( T E A C 0 . 7 m M ) d e s p i t e a l o n g e r c h r o -

m o p h o r e o f 1 2 c o n j u g a t e d d o u b l e - b o n d s . E x t e n s i o n o f t h e

c h r o m o p h o r e t o 1 3 c o n j u g a t e d d o u b l e b o n d s i n c a n t h a x -

a n t h i n ( 1 3 , 1 3 - c a r o t e n e - 4 , 4 ' - d i o n e ) a n d f u r t h e r h y d r o x y l a t i o n

a s in a s t a x a n t h i n ( 3 , 3 ' - d i h y d r o x y - I ] , 1 3 - c a r o t e n e - 4 , 4 ' -d i o n e ) r e -

d u c e d t h e a b i li t y t o s c a v e n g e A B T S + r a d i c a ls t o T E A C v a -

l u e s o f 0 . 02 a n d 0 .0 3 m M r e s p ec t i ve l y . T h e e l e c t r o n - w i t h d r a w -

i n g c h a r a c t e r o f t h e c a r b o n y l o x y g e n a t o m s i n e c h i n e n o n e ,

c a n t h a x a n t h i n a n d a s t a x a n t h i n r e d u c e s th e u n p a i r e d e l e c t r o n

d e n s i t y i n t h e 1 1 - d o u b l e - b o n d e d c a r b o n s k e l e t o n [2 2] , r e s u l t -

i n g i n a d e c r e a s e i n t h e r e a c t i v i ty o f t h e h y d r o g e n a t o m s / e l e c -

T a b l e 2

Carotenoid Structure

Lycopene ~ 2.9 1:0.15 [3]

if-C aro ten e ~ 1.9 t: 0.1 [4]

c~-Ca rotene ~ 1.3 1:0.04 [3]

13-Cryptoxanthin ~ 2.0 t: 0.02 [3]

Zea xan thin ~ 1.4 0.04 [3]

~o

oH

L u t e i n ~ 1.5 0.1 [3]

NO

Ech inen one ~ 0.7 1:0.2 [3]

o

Astaxanthin 0.031:0.03 [3]

Ca ntha xan thin ~ 0.02 l:0.02 [3]

~

TE C mM


3/3

242

N.J. Miller et al./FEBS Letters 384 1996) 240-242

t r o n s o n t h e c a r b o n a t o m s i n c a r o t e n o i d s k e l e t o n w i t h t h e

A B T S r a d i c a l c a t i o n .

I n c o n t r a s t , t h e i n s e r t i o n o f a s i n g l e h y d r o x y l g r o u p i n t h e

1 3 -r in g o f t h e a l l t r a n s I ] - c a r o t e n e , f o r m i n g 1 3 - c r y p t o x a n t h i n

( [ 3 , [ 3 - c a r o t e n e - 3 - o l ) , d o e s n o t g r e a t l y m o d u l a t e t h e a b i l i t y t o

s c a v e n g e t h e A B T S + r a d i c a l c a t i o n . H o w e v e r , t h e p r e s e n c e o f

o n e h y d r o x y l g r o u p o n e a c h o f t h e p - ri n g s as i n z e a x a n t h i n

( 1 3 ,1 3 '- c a r o te n e - 3 ,3 '- d i o l) s i g n i f i c a n t l y d e c r e a s e s t h e a n t i o x i d a n t

a c t i v i t y t o 1 .4 m M . A s i m i l a r m o d i f i c a t i o n o f a - c a r o t e n e i . e.

i n l u t e i n ( [ 3 , e - c a ro t e n e - 3 ,3 ' - d io l ) h a d a m i n o r s u p p r e s s i v e e f -

f ec t, T E A C 1 .4 m M a n d 1 .3 m M f o r lu t e i n a n d a - c a r o t e n e ,

r e s p e ct i v el y . A p p a r e n t l y , t h e p r e s e n c e o f o n e h y d r o x y l g r o u p

a t C - 3 ( 1 3 - c r y p to x a n t h in ) d o e s n o t r e d u c e t h e c a r o t e n o i d ' s

a b i l i t y t o s c a v e n g e A B T S + r a d i c a l c a t i o n b u t h y d r o x y l s a t

b o t h C - 3 a n d C - 3 ' ( z e a x a n t h i n ) r e s u l ts i n a s i g n i f ic a n t r e d u c -

t i o n i n a c t i v i t y .

T h e a n t i o x i d a n t p r o p e r t ie s o f c a r o t e n o i d s h a v e b e e n s u g -

g e s t e d t o r e f le c t n o t o n l y t h e r a t e s o f f r e e r a d i c a l s c a v e n g i n g

b u t a l s o t h e r e a c t i v i t y o f t h e r e s u l t a n t c a r o t e n o i d - d e r i v e d r a d -

i c al s . C a r o t e n o i d r a d i c a l - c a t i o n o r a d d u c t r a d i c a l s h a v e b e e n

s h o w n t o b e h i g h l y r e s o n a n c e s t a b i l is e d a n d p r e d i c t e d t o b e

r e la t iv e l y u n r e a c t i v e ; t h e y m a y f u r t h e r u n d e r g o b i m o l e c u l a r

d e c a y t o g e n e r a t e n o n - r a d i c a l p r o d u c t s o r , i n th e c a s e o f c a r -

o t e n e r a d i c a l - a d d u c t s , m a y t e r m i n a t e r a d i c a l r e a c t i o n s b y

b i n d i n g t o t h e a t t a c k i n g f r ee r a d i c a l [ 1 ,1 7] . I n t h e s y s t e m

d e s c r i b e d h e re , t h e a n t i o x i d a n t a c t i v i t y m e a s u r e d i s a m a r k e r

o f t h e i n i t i a l r e a c t i v i t y o f t h e c a r o t e n o i d i n i t s a b i l i ty t o s c a v -

e n g e t h e A B T S + r a d i c a l c a t i o n a n d d o e s n o t t a k e i n t o a c -

c o u n t t h e s u b s e q u e n t r e a c t i v i ty o r s t a b i l i s a t i o n o f t h e c a r o t e -

n o i d - d e r i v e d r a d i c a l s .

T h e r e s u l ts s h o w t h a t , o f t h e c a r o t e n e s s t u d i e d , l y c o p e n e

s c a v e n g e s t h e A B T S + ra d i c a l c a t i o n m o r e e x t e n s i v e l y t h a n

[ ~ - c a ro t e n e , i . e . r e d u c e s m o r e r a d i c a l s a t e q u i v a l e n t t i m e

p o i n t s . T h i s i s c o n s i s t e n t w i t h t h e o b s e r v a t i o n s o f o t h e r s c o n -

c e r n i n g r e a c t i v i t ie s w i t h s i n g l e t o x y g e n [ 14 ,1 5] . B o t h o v e r a l l

a r e m o r e r e a c t i v e t h a n t h e x a n t h o p h y l l s ( w i t h t h e e x c e p t i o n o f

[ 3 -c r y pt o x an t h in ) , c a n t h a x a n t h i n a n d t h e a s t a x a n t h i n b e i n g

r e l a t iv e l y u n r e a c t i v e u n d e r t h o s e c o n d i t i o n s . T h e s e q u e n c e

f o r r e l a t i v e r a d i c a l s c a v e n g i n g a b i l i t i e s i s l y c o p e n e > 1 3 -c ry p -

t o x a n t h i n ~ 1 3 -c a ro te n e > l u t e i n ~ z e a x a n t h i n > c t - c ar o t e -

n e > e c h i n e n o n e > c a n t h a x a n t h i n = a s t a x a n t h i n . T h e c a r o te -

n o i d s s t u d i e d i n t h is w o r k h a d a n t i o x i d a n t a c ti v i ti e s v a r y i n g

f r o m t h r e e t i m e s th a t o f v i t a m i n E ( T E A C = 1 .0 m M ) [ 1 9 ,2 3 ]

a s f o r ly c o p e n e ( T E A C = 2 . 9 m M ) t o n e g l i g i b l e a c t i v i ty , a s i n

t h e c as e o f a s t a x a n t h i n a n d c a n t h a x a n t h i n .

T h e p r o p e r t i e s u n d e r l y i n g t h e a c t i v it i e s o f c a r o t e n o i d s t o -

w a r d s f r ee r a d i c a l s a n d t h e i r s c a v e n g i n g e ff e ct s r e la t e s p a r t i c -

u l a r l y to t h e i r a b i l it i e s t o d o n a t e e l e c t r o n s o r h y d r o g e n a t o m s

a n d t o t h e i r r e l a ti v e p r o p e n s i t i e s t o u n d e r g o o x i d a t i o n . T h e y

m a y a l so a c t t h r o u g h f o r m i n g r a d ic a l a d d u c t s w h i c h c a n u n -

d e r g o b i m o l e c u l a r d e c a y . T h e f i n d i n g s h e r e d e m o n s t r a t e t h a t ,

o v e r a l l , th e c a r o t e n e s a r e m o r e e f fi c ie n t q u e n c h e r s o f t h e

A B T S + ra d i c a l c a t i o n t h a n t h e x a n t h o p h y l l s ( w i t h t h e e x c ep -

t i o n o f 1 3 - c ry p t o x a n th i n ) a n d t h a t t h i s m a y b e i n f l u e n c e d b y

t h e i n c r e a s i n g p o l a r i ti e s o f t h e f u n c t i o n a l g r o u p s i n t h e t e r m -

i n a l r i n g s .

Acknowledgements:

Cather ine Rice -Evans and Pe te r Bramley thank

the Biotechnology and Biological Sciences Research Co uncil and the

Min is t ry o f Agr icu l tu re , F i she ries and Foo d fo r f inanc ia l suppor t fo r

their research. Gif ts of carotenoids from Roche, Henkel and P.B.

Zaga lsky are acknowledged with grat i tude.

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