Effect of vitamin B-6 deficiency on fasting plasma ...

Effect of vitamin homocysteine Joshua Frank

W Miller, D Morrow,

of vitamin

centrations concentrations ±

Judy D Ribaya-Mercado, Elizabeth F Cochary,

synthesis

through

pyridoxal-5’-phosphate,

deficiency

on plasma

thus

homocysteine

cys-

requires

pyridoxal-5’-phosphate

the

netically

induced

con-

evaluated. Total fasting plasma homocysteine were measured in 1 1 elderly subjects aged 64.4

1.7 y (1 ± SE)

who

consumed

a vitamin

B-6-deficient

diet

study,

measured

fasting

in 3- and

plasma

homocysteine

23-mo-old

rats

concentrations

fed

vitamin

were

B-6-deficient

and were compared with those ofvitamin B-6-replete, pairfed controls. There was no difference in homocysteine concentrations between deficient and pair-fed animals after 6 wk of the diets

dietary

vation

regimen for either age group; was observed in the 3-mo-old

was observed for the that fasting plasma homocysteine elevated in vitamin B-6 deficiency homocysteine concentrations are B-6 status. Am J C/in Mar difference

KEY

WORDS B-6 deficiency,

after 9 wk a modest deficient rats whereas

dcno

23-mo-old rats. It is concluded concentrations are not initially and therefore fasting plasma not a good indicator of vitamin 1992;55:

Homocysteine, humans, rats

1 134-60.

homocysteinemia,

vitamin

known

since

homocysteine

of homocysteine of thrombotic (1, 2). Recent

homocysteine vascular

disease

the early

concentrations

l960s

that

caused

large

increases

by inborn

errors

metabolism are associated with high incidences events, vascular lesions, and mental deficiencies studies

indicate

concentrations

that

(5-7), and premature These associations

even

small

increases

are an independent

(3, 4). Furthermore,

moderate homocysteinemia with early onset cerebral

zymes

it has been

as a cofactor. deficiencies

deficiency

required

vitamin

several 1 2 and

(1, 8-12)

cofactor

demonstrated

ship

between

vitamin

one

2-3

wk did not

the

disulfide

is not

in women

(32),

result

form

of the cofactor

form

disorders to

is beginning folate status and

clear.

showed

that

studies

not disclosed.

To our

en-

of vitamin

is the potential for vitamin Bof plasma

to be used (28-30). The

con-

one

in men

(31)

B-6 depletion

for

vitamin elevation in the

of homocystine, urine.

Whether

were affected

knowledge

as an inrelation-

homocysteine

studies,

concentrations

(E.C.

of the

homocysteinemia;

plasma

Two

of homocysteine,

homocysteine

cause

a relationship the measurement

in a significant

not plasma was

of any

such Moreover,

B-6 status

however,

and

deficiencies

ofgenetic

homocysteine concentrations dicator of vitamin B-12 and

ge-

the conversion

to homocysteinemia. reductase

deficiencies

( 1 8-27).

Alternatively, affect

lead

and

for the synthesis

studies folate

centrations,

that

to methionine also methylenetetrahydrofolate

B-12 (13-17). Implicit in this description for

Se/hub

no studies

or

in these assessing

the effect of vitamin B-6 concentrations in humans

deficiency on plasma have been published.

in rats (33-35) deficiency was concentrations.

(36) demonstrated that vitamin B-6 with elevated plasma homocysteine Smolin and Benevenga (37) subse-

and pigs associated However,

reported that B-6-deficient

after

eating

temporarily increased. The present study vitamin B-6 deficiency

plasma homocysteine rats were not elevated

were

plasma

homocysteine

homocysteine Early studies

concentrations after a 24-h

in fast.

concentrations

was undertaken to determine on fasting plasma homocysteine

the

effect of concen-

in plasma

risk factor recognized

for that

is prevalent, particularly in patients and peripheral occlusive arterial diseases

coronary disease (3). between plasma homocysteine

and vascular and neurological conditions lend importance to an understanding ofthe pathogenesis ofhomocysteinemia in man. The classic inborn error of metabolism that leads to homocysteinemia is a homozygous deficiency of cystathionine /3-synthase (E.C. 4.2. 1 .22) ( I ). This enzyme is responsible for the catabolism of homocysteine through the synthesis ofcystathionine (Fig 1) and 1 154

1 . 1 . 1 .68)

Only

It has been in plasma

of homocysteine These include

quently vitamin

Introduction

enzyme

Jacob

A,n J C/in Nuir

l992;55:h154-60.

I From the Vitamin Bioavailabihity Laboratory, USDA Human Nutrition Research Center on Aging at Tufts University, Boston. 2 The contents ofthis publication do not necessarily reflect the views

or policies of the US Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government. 3 Supported by the US Department of Agriculture under contract no 53-3K06-510. 4 Address reprint requests to I Selhub, Bioavailabihity Laboratory, USDA Human Nutrition Research Center on Aging, Tufts University, 71 1 Washington St. Boston, MA, 021 1 1. Received September 9, 1991. Accepted for publication December 12, 1991. Printed

in USA.

© 1992 American

Society

for Clinical

Nutrition

Downloaded from ajcn.nutrition.org by guest on May 31, 2013

for 20 d. Only 1 ofthe 1 1 subjects was found to have elevated homocysteine concentrations even though all subjects exhibited high urinary xanthurenic acid concentrations after a tryptophan load, a measure indicative of vitamin B-6 deficiency. In a supporting

plasma

Robert M Russell, Douglas C Shepard, James A Sadowski, Stanley N Gershoff and

ofhomocysteine

requires B-6

was

on fasting 4

The catabolism

ABSTRACT tathionine effect

B-6 deficiency

B-6

DEFICIENCY

AND

HOMOCYSTEINE

1155

Acceptor )

S-AdenosylMethionine (SAM)

-

CH3-Acceptor

Al?

S-AdenosylMethiOninC

Homocysteine(SAH)

Sere PLP

o1_ Adenosinc

Homoc

ysteine

MethyleneTH? CH3zy

,-“

a-Kctobutyrate

Cysteine

\‘cccg

FIG I . Homocysteine metabolism. 1 , methylenetetrahydrofolate tathionase. THF, tetrahydrofolate; PLP, pyridoxal-5’-phosphate;

trations

in humans

fasting

plasma

and

animals,

homocysteine

and

to determine

concentrations

the

utility

dicator of vitamin B-6 status. This study was carried out extension of several studies conducted in our laboratories

signed to assess the vitamin and to investigate the effects and old rats (38, 39).

B-6 requirement

ofvitamin

of

as a screening

in elderly

B-6 deficiency

inas an de-

subjects

on young

reductase;

approved

Tufts

by the

and

Vitamin Subjects. histories

Details ofthe

and repletion of the method

volunteers

study

for this study

and

are described

medical

in a separate

communication (38). Briefly, 12 healthy volunteers (6 men, 6 women) aged 64.4 ± 1.7 y (1 ± SE, range 61-71 y) were recruited to investigate vitamin B-6 requirements of elderly people. The volunteers chosen to participate in the study were selected after careful review of medical histories and after comprehensive physical

examinations

that

included

assessments

pulmonary functions, hematological leptic tendencies, glucose tolerance,

Exclusion chronic glucose

criteria

included

sleep problems, tolerance, recent

B-6-containing

One subject in the

results.

(male) The

and

did not complete screening

diabetes and recent that

and

and

the study experimental

Review

consent

Committee

was obtained

from

of

all par-

Protocol. The experimental period in which each volunteer by

a vitamin

design began with ate a self-selected

B-6-depletion

was followed

period

by three

vitamin

B-6

was

of

20 d. The

successive

gradually

a 5-d baseline diet, followed

21-d

depletion

repletion

returned

periods

to the

diet.

The

gig/kg; second repletion repletion period (phase

period

(phase

4), 33.75

3), 22.5 pg/kg;

Diets. The diets used are extensively Briefly, the depletion diet was primarily devoid

of vitamin

B-6 but

delphia)

as a source

in all other

The

consisted

of fiber,

nutrients

(Ross

rest of the depletion

menu

of Avicel

and

Foods,

juices,

cheese,

(Ross

(FMC

Corp,

caseinate

Phila-

(Western

Laboratories),

whey,

egg

The repletion

diets selected fruits and margarine, and other foods such as cereal, and biscuits. The repletion diets contained

omelettes,

affect

the experimental design in the form of pyridoxine hydrochloride (Seltzer Chemicals, mc, Carlsbad, CA) mixed in water. Multimineral (Sundown Vitamins, Inc, Hollywood, FL) and multivi-

vitamin

Caucasian,

or nonsmokers.

and is not included procedures

were

0.5

tamin Lakes,

muffins,

Pro-Mod

sodium

white powder, and gelatin as protein sources. consisted of rice casseroles with vegetables, fruit

KY),

detailed elsewhere (38). a liquid formula (Ensure)

and

of epilepsy,

Louisville,

complete

OH).

Laboratories, Columbus, was semisynthetic and

and third

zg/kg.

or impaired use of vitamin

were

former

Investigation

Informed

epi-

stability.

history

All volunteers

ofalcohol,

profiles,

psychosocial

or medications

metabolism.

or noningesters

ofcardiac

urinary

or family

nephrohithiasis, alcohol abuse,

supplements

B-6 or tryptophan occasional

personal

and

Human

B-l2.

amounts of vitamin B-6 ingested in the various phases of the study were as follows: depletion period (phase 1 ofthe protocol), 3 g-kg body wt’#{149}d; first repletion period (phase 2), 15

in humans

of recruitment

vitamin

ticipants.

period

methods

B-6 depletion

and 3, cys-

$-synthase;

methyhated

University.

in which

Subjects

2, cystathionine

and CH3-B-l2,

mg

vitamin

supplements NJ)

were

B-6

with

devoid provided

the

rest

ofvitamin

throughout

provided

as stipulated

B-6 (Arther,

the study.

mc,

by

Mountain

The

protein


t1EJ

Cystathionine

1156

MILLER

ET

AL

content of all the diets was kept constant at either 0.8 or I .2 g kg body wt1 d’. The diets were isocaloric to maintain body .

.

weight. Analyses. Fasting blood samples and 24-h urinary specimens were collected approximately every fifth day throughout the study; they were processed and stored until analysis as described (38). Total fasting plasma homocysteine concentrations were determined by the fluorimetric method of Araki and Sako (40). This method also was used to determine a reference range for total

fasting

plasma

volunteers

(ages

reference

range

sistent

with

others

(19,

homocysteine

60-70

y) recruited

28,

after

Mountain

Lakes,

imol/L

determined 41-43).

for

The

method

of Hoes

Vitamin

B-6 depletion

load

was

study.

adults

urinary

a 5 g L-tryptophan NJ)

‘C U C 0 .C

C 0

This

in studies

xanthurenic

(Tryptacin,

determined

acid

Arther,

by the

0 C

by

mc,

10

study

(39).

treatment

I

animals used in this study and during the experiment All

results

ofdeficient

experimental

and their were de-

protocols

were

ap-

ofa

lack

for plasma and

four

ofsufficient

plasma

homocysteine

pairs

ofpair-fed

rats

sam-

are based

on

for both

age

presented

separately,

depletion

phase,

tion

for these

from

39.52

subsequent concentration

vendor

every

Body

mined

a 14-h

Results variate

were

analyzed

repeated-measures

1.0; and was

total

were

was fasting

period

choline

last repletion

analysis

a com-

weekly

and food

9 wk ofthe

as previously

described

by

a univariate

of variance

phases.

in-

dietary

increased

(highest

measures

20-d

56-fold

± 15.23

amount

(range

phase,

xanthurenic baseline

nor did it change time

the acid by the

B-6 supplemenincluding

plasma

urinary 4-pyriaminotransferase

38), also changed signifiand normalized during vi-

total fasting significantly

at no

16-fold)

h. During

urinary reaching

B-6 status,

B-6-

concentra35-1

mol/24

ofvitamin

of vitamin

The mean not change

vitamin acid

(PLP) concentrations, and erythrocyte aspartate

Furthermore,

24.06

jzmol/L)

plasma homocysteine during the vitamin

during

did

the

the three mean

repletion

homocysteine

one

and

centration decreased

multi-

(48).

subject

subject

study

2 illustrates the effect of vitamin B-6 depletion and on fasting plasma homocysteine concentrations as reflected by urinary xanthurenic acid after a tryptophan load. Values for 10 of the 1 1 subjects are included. (The 1 lth subject is

not

the

included

a reference

of vitamin

in the

posttryptophan

results

load

population

The

total

one

subject,

on the

above.

xanthurenic

For acid

the depletion in concordance

this con-

phase and with the

for the other 10 subjects. This increase was of increases observed in these other subjects.

fasting

plasma

however, During

B-6 depletion

described

urinary

increased 1 10-fold during upon vitamin B-6 repletion observed the range

subjects.

by us on

Methods). 3 illustrates the effect

Figure

(39).

as determined

age (see

of similar

were deterabove (40).

Results

Figure repletion

the

xanthurenic

coefficients (data not shown, during vitamin B-6 depletion

pattern within

Human

During

urinary

as reflected by load for 10 of B-fl depletion acid. The scale 8-6 depletion repletion phase

concentration or any of the individual homocysteine concentrations for these 10 subjects rise out ofthe normal range (3.86-

B-6 concentration

at 6 and

activity cantly

mean

to 2230.17

phase

Other

B-6-depletion

bitartrate, from

homocysteine concentrations ofAraki and Sako as described statistically

of mix

(47).

monitored

collected

Institute vitamin

purchased

vitamin

by analysis

weights

plasma by the method

mix

The

confirmed

2 d. Blood

regimen after Total fasting

B-6 (46),

vitamin

(Teklad).

was

Analyses. take

vitamin

AIN-76A

diets

$0

IV

repletion phases, the mean decreased precipitously,

tamin B-6 repletion. concentration did

in the

70

In

10 subjects

chemical, Nutrition

mercial

80

see below.)

± 3.64

pyridoxal-5’-phosphate doxic concentrations,

Cleveland), 5.0; corn oil, 5.0; American (AIN) mineral mix (45), 3.5; AIN-76A

II

the

The components of the diets were purchased individually (Teklad, Madison, WI), then mixed. The diets consisted of the following (in g/l00 g diet): vitamin-free casein, 25.0; DL-methionine, 0.3; cornstarch, 1 5.0; sucrose, 45.0; Cellufil (US Bio-

or without

50

FIG 2. Total fasting plasma homocysteine (imol/L) urinary xanthurenic acid (mol/24 h) after a tryptophan 1 1 elderly volunteers who underwent experimental vitamin and repletion. i ± SE. El, homocysteine; #{149} xanthurenic at the bottom represents the phase ofthe protocol: I, vitamin phase; II, vitamin B-6 repletion phase 1; III, vitamin B-fl 2; and IV, vitamin B-6 repletion phase 3.

tation).

The

40

Days

in rats

or because

pies for analysis,

0.2.

30

did

homocysteine not

the depletion

follow phase

concentration the

pattern

for this

ofthe

other

the homocysteine

10

concen-

tration increased from a baseline value of 16.0 zmoh/L to a high value of 34.7 tmoh/L. Then, during the vitamin B-6-repletion phases, baseline

the

homocysteine

concentration.

concentration

decreased

back

to the


previously

with

20

colorimetric

proved by the Institutional Animal Care and Use Committee ofTufts University. In the original study, Fischer 344 male rats of four different ages were used. For the purposes of this investigation, only the 3- and 23-mo-old rats were studied. The rats were grouped by age and weight and then were randomly assigned to one of two dietary groups: ad libitum vitamin B-6--deficient diet, or 2 pair-fed vitamin B-6-replete diet containing 7 mg pyridoxine hydrochloride/kg diet. Each of the four dietary groups began with 10 rats. Because ofdeath and illness unrelated to the

four pairs groups.

V 0

et al (44).

Animals and diets. The housing conditions before

dietary

0

(1 ± 2 SD) and is con-

normal

24-h

0

E C

in 61 healthy

for an unrelated

was 3.86-24.06

ranges

excretion

scribed

concentrations

C%J

B-6

DEFICIENCY

AND

1157

HOMOCYSTEINE

30 .C

cJ

0

C

0

0

E

C,) >1

C 0

0 0

V U

0

0

0

E

I

U

0 I

20

E

‘C C 0

E

Cs

(I)

E

Cu

C Cs

U)

Q.

x

Cs

a.

C

Cs

C

10

C

Cfl

(5 LI.

U)

Cs U.

0

Days

I

II

6Wks

Ill

Length

1V

either

the

3- or 23-mo

No differences in final body weights and daily food intakes between vitamin B-6-deficient rats and their pair-fed controls were observed for both the 3- and 23-mo-old age groups after 10 wk ofthe dietary regimen (Table 1) (39). Animals maintained B-6-deficient

diet

were

clearly

partate

aminotransferase

ures total

4 and

5 illustrate

fasting

plasma

wks

of the

dietary

mocysteine

activity the regimen.

concentrations

B-6-deficient

animals

TABLE 1 Body weights,

were and

6 wk,

between pair-fed

and indices

of vitamin

Fig-

Vitamins

on

metabolism.

the

6 and 9

teine

in hovitamin

controls

for

B-6 status

for rats

cobalamin

Final food intaket

g

g/d

266 277

±

B-fl deficient controls

347 349

±

20

±

11

I I

± 7

group. However, B-6-deficient an-

significant elevation in mean concentration as compared

controls

(P

=

t

Adapted

from

reference

39. 1

±

SE; n

=

9 (3-mo

±

12.3 12.3

±

1.5

±

1.5

groups),

n

=

5 (23-mo

0. 1 0.1

groups).

t After 10 wk of the dietary regimen. :1:After 4 wk of the dietary regimen. § PLP, pyridoxal-5’-phosphate. II eAST,

erythrocyte aspartate different from

#{182} Significantly

total with

0.037).

B-12,

and

As shown

folate

in Figure

the synthesis

are involved

in homocysteine

1, the catabolism

ofcystathionine

ofPLP, whereas the remethylation requires vitamin B-12 in the and

folate

in the form

Plasma

PLP

of homocys-

requires

vitamin

67 801

± ±

511 39

aminotransferase. Values are activity coefficients. pair-fed rats of the same age, P 0.01.

75 ± 1211

495

±

29

B-

of homocysteine form of methyl-

ofmethyltetrahydrofolate.

Be-

eASTflI

1.28 1.03

± ±

0.0811 0.01

23 mo

Vitamin Pair-fed

no

the deficient

nmo//L

9.3 9.3

±

B-6,

through

6 in the form to methionine

Final body weightt

B-6 deficient controls

still were

between

Discussion

1)(39).

differences

detected

Age group

3 mo Vitamin Pair-fed

no

replete

9 wk there

in the 23-mo-old 9 wk, the vitamin

a statistically homocysteine

pair-fed

After

concentrations

B-6-de-

after

respective

their

food consumption,

B-6

concentrations

After

their

deficiency

(Table

of vitamin

homocysteine

imals exhibited fasting plasma

as indicated by deterand erythrocyte as-

coefficients

effect

vitamin

age groups.

in homocysteine

animals and their controls in the 3-mo-old group after

Rat study

vitamin

Regimen

FIG 4. The effect of vitamin B-6 deficiency on total fasting plasma homocysteine (mol/L) in 23-mo-old rats (n = 4 pairs) after 6 and 9 wk. I ± SE. 0, vitamin B-6--deficient animals; U, pair-fed controls.

differences

ficient after 4 wk of the dietary regimen minations of plasma PLP concentrations

of Dietary

1.21 ± 0.0111

1.08

±

0.01


FIG 3. Total fasting plasma homocysteine (tmoh/L) as reflected by urinary xanthurenic acid (zmol/24 h) after a tryptophan load for the one volunteer not included in Figure 2. El, homocysteine; #{149}, xanthurenic acid. The scale at the bottom represents the phase of the protocol: I, vitamin B-6 depletion phase; II, vitamin 8-6 repletion phase I; III, vitamin B-6 repletion phase 2; and IV, vitamin B-6 repletion phase 3.

on the

9Wks

1158

MILLER

C

,

40

0

ET

AL

(37),

which

tions

in vitamin

with

likely

of these

explanation

6-dependent

E

synthase

0

that

sufficient

though

it would

inhibit

homocysteine

comes

from

0.

Os U) (5 U.

6Wks of

Dietary

Regimen

from

replete

controls. possible

strated

errors increases

of homocysteine

metabolism

in homocysteine

concentrations

are

(18-27).

The

effect

concentrations, No

studies

plasma

of vitamin

however, have

assessed

the effect

homocysteine

studies

suggest

B-6 deficiency

has not been

vitamin

elucidated.

B-6 deficiency

in humans,

and

This

on effect

on plasma homocysteine concentrations depending on the prandial state of the animal (34, 37). In the present study we investigated the effect of vitamin B6 depletion on fasting plasma homocysteine concentrations in humans and rats. In 10 of 1 1 human subjects, fasting plasma homocysteine

range

concentrations

during

23-mo-old

vitamin rats,

not significantly

after

6 or 9 wk ofvitamin rats,

not increase

B-6 depletion.

fasting

were

3-mo-old

did plasma

In vitamin

from

those

B-6 depletion. plasma

homocysteine

B-6-deficient

These

results

accompanied

and

suggest

pair-fed

that

by an elevated

of pair-fed

controls

concentrations

were

observed.

plasma

often

is not

homocysteine

con-

centration. Consequently, a fasting plasma homocysteine concentration seems not to be a good indicator ofvitamin B-6 status. The opposite is true for folate and vitamin B-12 for which a fasting plasma homocysteine concentration is a good indicator ofstatus (28-30). These

results

Linkswiler that fasting vated

are

consistent

(31) and Shin and urinary homocystine

in vitamin

are also consistent

B-6-depleted

with

with

the

studies

Linkswiler (32), concentrations human

the study

subjects

by Smolin

of

Park

which were

and

showed not dc-

(3 1 , 32).

and

This

showed

that

that

cystine depleted

They

Benevenga

and

Shin

possible on

visions

enzyme B-6 depen-

extent vitamin

than was B-6 de-

has a lower B-6

thus

studies (32)

than

the

by Park

and

in which

as opposed

in urine specimens were elevated. based

affinity

the former

depletion

by the

Linkswiler

from

vitamin

on a recently

ofcystathionine

as stemming

from

it was

to homo-

of homocysteinemia

insensitivity

B-6 deficiency

vitamin

and

concentrations,

pathogenesis

the apparent

vitamin

is itself

interpretation,

the

et al demon-

catabolic

/3-synthase,

and

cystathionine

concentrations, human subjects

as proposed

Sturman

to vitamin

B-6-

/3-synthase has a with other vitamin

cystathionine

is supported

in tissue

in vitamin

phenomenon,

‘y-cystathionase

susceptible

(31)

shown

lower

rats than

study,

would

cystathionine

slightly

to a much greater by the experimental

that

possibility

Linkswiler

B-6-

proposed (50),

en-

13-synthase

to

the propensity

of this

enzyme to be activated by S-adenosylmethionine (SAM) (51, 52). SAM is an important intermediate in the synthesis of homocysteine

from

esis, activation ofcystathionine

methionine

(Fig

1). According

conditions

fasting even homocysteine

are not elevated.

which

explains

why

known,

but

of the

synthase

affinity would

ofthe synthase enzyme be consistent with the

possible

Several

possibilities enzyme

above assayed

confounding

from

and Linkswiler centrations nificantly As stated

the

activity that

is not

urinary

availunder activated

cystathionine

con-

B-6 deficiency whereas (3 1 , 32). The mechanism 13-synthase is at present 1)

SAM

increases

or 2) SAM

for substrate,

hoby un-

the affinity increases the

for PLP. Both ofthese possibilities results of the study by Sturman

because in this

endogenous study were

et

SAM concentrations not considered as a

factor.

studies

previously

concentrations are (33-37). A possible derives

include

hypoth-

though cofactor concentrations

Cystathionase fasting

centrations are elevated in vitamin mocystine concentrations are not which SAM activates cystathionine

al (48) described in the tissues

to this

by SAM will induce sufficient residual fl-synthase to catabohize the homocysteine

is normally generated during ability is limited. Therefore, by SAM,

B-6-deficient

B-6 deficiency

fasting

B-6 deficiency

In their

cystathionine

is more

latter.

these

controls after 6 wk; only difference between vi-

animals

vitamin

normal

concentrations

In vitamin

not significantly different from pair-fed after 9 wk was a statistically significant tamin

ofthe

B-6-deficient

homocysteine

different

fasting

out

enzyme

does

in

can still be state, even

for this interpretation

4.4. 1 . 1), which

suggests

than

fi-

a decrease

vitamin

for this

1 ), was affected 13-synthase

dent (Fig cystathionine

hypothesis

animal

has a differential

(EC

A second

on homocysteine

of vitamin

B-6 deficiency

to

it would be cxor folate should Several studB-l2 and folate

completely

concentrations

that

known

in the blood

(homocysteinemia) and urine (homocystinuria), pected that a deficiency of vitamins B-6, B-12, also lead to elevated homocysteine concentrations. ies demonstrated this to be true for both vitamin

despite

or only

of the

B-

Support

B-6-deficient

activity

most

vitamin

showed

elevated explanation

studies

by Park

(32), who showed

increased in their after administration above, homocysteine

that

in vitamin for these and

plasma

homocysteine

B-6-deficient animals disparate observations

Linkswiler

that urinary

(31)

homocystine

and

Shin

con-

vitamin B-6-depleted subjects sigof an oral dose of methionine. is synthesized from methionine.


inborn

the

the

homocysteine in the fasting

who

same

explanation

y-cystathionase

for PLP large

the

enzymes.

that

ficiency. cause

vitamin

B-6-dependent

FIG 5. The effect of vitamin B-6 deficiency on total fasting plasma homocysteine (zmoh/L) in 3-mo-old rats (n = 4 pairs) after 6 and 9 wk. I ± SE. 0, vitamin B-6 deficient animals; U pair-fed controls. Significantly different from pair-fed controls, P = 0.037.

cause

et al (49), was

a 24-

The

cystathionine

by Sturman et al (49), is that cystathionine relatively high affinity for PLP as compared

9Wks

Length

extracts

that

catabolism.

activity

One

I.-’

be expected

Sturman

/3-synthase

C

activity

Therefore, at least

after

controls.

enzyme

residual

E

concentra-

is that

catabolic

(5

Cu

B-6-replete

observations

homocysteine retains

homocysteine

rats were not elevated

vitamin

its cofactor’s availability. converted to cystathionine,

U)

plasma

B-6-deficient

h fast as compared

0

I

demonstrated

B-6 This

implies

synthase

that

in B-6 deficiency

activity

is not

the residual

sufficient

to handle

DEFICIENCY

AND

cystathionine the

/3-

increase

in ho-

synthesis that is expected to occur after a methionine In the animal studies cited above (33-37), some explicitly state that nonfasting blood samples were used whereas the others do not say that the animals were fasted before blood draw. All ofthe diets used in these studies contained a significant amount of methionine. We suggest that in essence, eating these diets constitutes a methionine load, thus explaining the observed dcvations in plasma homocysteine concentrations in vitamin Bmocysteine

load.

6-deficient

animals.

This

interpretation

is supported

by

the

3-mo-old

rats

after

9 wk

is perhaps

more

apparent;

3 mo-old

rats are in a state of rapid growth, which has been shown to be accompanied by a high rate of vitamin B-6 turnover (53). We suggest

that

vitamin

B-6

depletion

in growing

rats

leads

to a

much more severe vitamin B-6 deficiency than that seen in adult rats who are not in a state of rapid growth and have a much lower rate of vitamin B-6 turnover. Because other indices of vitamin

were dietary is not

B-6

vitamin regimen an initial

deficiency

clearly

indicated

that

B-6-deficient well before (Table 1), it is conceivable indicator

of vitamin

B-6

all depleted

rats

the sixth week of the that homocysteinemia deficiency

but

can

be

a sign of severe deficiency, or, like in the one human subject, a sign of a preexisting condition exacerbated by vitamin B-6 depletion. In summary, this study demonstrated that total fasting plasma homocysteine concentrations are not initially elevated in vitamin B-6-deficient

humans

and

rats,

and

therefore

a fasting

plasma

homocysteine concentration is not a good indicator of vitamin B-6 status. It is proposed that fasting homocysteine concentrations are not elevated because of the ability of SAM to activate the homocysteine catabolic enzyme cystathionine /3-synthase, despite a decrease in availability of this enzyme’s cofactor. L3

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ET