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ELSEVIER SCIENTIFIC
PUBLISHERS IRELAND
Mechanisms of Ageing and Development 71 (1993) 1-12
Age-dependent modification of lipid composition
and lipid structural order parameter of rat
peritoneal macrophage membranes
Eloisa Alvarez a, Valentina Ruiz-Guti6rrez b,
Consuelo Santa Maria *a, Alberto Machado a
aDepartamento de Bioquimica, Bromatologia y Toxicologia, Facultad de Farmacia, Universidad de
Sevilla, Calle Prof. Garcia Gonz6lez s/n, 41012 Sevilla, Spain,
blnstituto de la Grasa y sus Derivados, Avenida Padre Tejero 4, 41012 Sevilla, Spain
(Received 8 October 1992; revision received 10 May 1993; accepted 25 May 1993)
Abstract
The effect of aging on the lipid composition and fluidity of rat peritoneal macrophage membranes has been determined using young (3 months), mature (12 months) and aged
(24 months) Wistar rats. In the aged animals, total phospholipid decreased significantly
(P < 0.05), whereas cholesterol increased (P < 0.01), with an age-dependent increase in the
molar ratio of cholesterol/phospholipid. The most marked change in phospholipid content
was the significant (P < 0. 001) age-dependent increase of phosphatidylserine and cardiolipin
and the significant decrease of phosphatidylcholine and phosphatidylinositol. During aging
there was a considerable decrease in arachidonic acid and docosapentanoic acid (about 50%
in both cases). In contrast, an increase in the levels of oleic, linolenic and docosahexanoic acid
was observed. Steady-state fluorescence polarization using 1,6-diphenyl-l,3,5-bexatriene as
the probe was used to estimate the lipid structural order parameter of macrophage membranes. There was a highly significant (P < 0.001) age-dependent increase in the lipid structural order parameter, which correlated well with the increased molar ratio of cholesterol/
phospholipid in the membranes isolated from aged animals. The data suggests alteration in
membrane lipid-protein interactions in aging, and are consistent with the hypothesis of the
aging process.
Key words: Lipid composition; Membrane fluidity; Macrophage; Aging
* Corresponding author.
0047-6374/93/$06.00 © 1993 Elsevier Scientific Publishers Ireland Ltd. All rights reserved.
SSDI 0047-6374(93)01365-F
2
E. Alvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
1. Introduction
Many investigations have suggested that age-dependent deterioration in cell functions can be related to molecular and functional changes in the properties of biological membranes [1-3]. The immune function is one of the systems that decrease with
age [4-6], and alteration in membrane fluidity of lymphocytes has been closely
associated with the decline in lymphocyte function during aging [7,8].
In addition to lymphocytes, macrophages play a critical role in the immune response. They appear to be involved in antigen presentation as well as in the secretion
of factors that stimulate lymphocytes. Macrophages also have the capacity to kill
microbes and tumor cells by generating reactive oxygen species when phagocytic
stimuli or soluble agents interact with macrophage membrane, directly or through
receptors [9-12]. The stimulus-membrane interaction activates a metabolic pathway
called the respiratory burst. This is a co-ordinated series of metabolic events that
leads to the production of oxygen-free radicals [13,14].
Taking into account the importance of membrane integrity in the phagocytic function of macrophages, the purpose of the present paper is to characterize the effect
of aging on fluidity and lipid composition of peritoneal macrophage membranes in
Wistar rats. Total phospholipid and cholesterol content and phospholipid and fatty
acid composition were determined using young (3 months), mature (12 months) and
old (24 months) rats. Fluorescence polarization with 1,6-diphenyl-l,3,5 hexatriene
as the probe was used to evaluate the effect of aging on the lipid order parameter
of peritoneal macrophage membranes.
2. Materials and methods
2.1. Animals
Young (3 months), mature (12 months) and old (24 months) female Wistar rats
were used. They were maintained on a standard laboratory diet with free access to
food and water.
2.2. Chemicals
Protease inhibitors were obtained from Sigma Chemical Company (St Louis,
USA). 1,6-diphenyl-l,3,5-hexatriene was obtained from Fluka Chemie AG (Buchs,
Switzerland). Fatty acid methyl ester standards (FAMES) were obtained from
Larodan Fine Chemicals (Maim6, Sweden). The internal standard solutions were
prepared by dissolving 200 mg of tricosanoic acid methyl ester (C23:0) in 100 ml of
hexane. Calibration solutions were prepared by dissolving known amounts of
FAME standards in hexane containing 2,6-ditertbutyl-p-cresol (butylated hydroxytoluene, BHT), obtained from Sigma. Other chemicals were of analytical grade from
Merck (Darmstadt, Germany).
2.3. Preparation of peritoneal macrophages
Peritoneal macrophages were elicited from Wistar rats according to the
Tsunawsky and Nathan method [15]. Female Wistar rats, allowed free access to food
and water, were injected intraperitoneally 4 days before harvest with 5 ml of 6% sodi-
E. Alvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
3
um caseinate. Animals were killed by decapitation, and immediately the peritoneal
cavity was washed with 10 ml of saline solution. Cells were pelleted by centrifugation, resuspended in KRB and immediately used for experiments. Viability, determined by trypan-blue exclusion, was always greater than 95%.
2.4. Membrane isolation
Macrophage membranes were obtained by the method described by Bromberg
and Pick [16].
After the isolation of macrophages by the general method, the pellets were
suspended in Hepes 5 mM, pH 7.5 with a cocktail of protease inhibitors. The
peritoneal macrophages were incubated at 4°C for 15 min to provoke the hypoosmotic rupture of the cells. Samples of the cell suspension were sonicated at 4°C
for 2 x 15 s. The cell rupture was evidenced by trypan-blue. Centrifugition was carried out at 1600 x g for 10 min to precipitate the unruptured cells. The supernatant
was centrifuged at 30 000 x g for 30 min, and the resultant pellet was resuspended
in Hepes 20 mM, pH 7.5 with phenyl-methyl-sulfonyl-fluoride (PMSF).
2.5. Extraction and separation o f lipids
Quantitative extraction of total lipids from macrophage membranes was carried
out following the method of Folch et al. [17] in the presence of BHT as antioxidant.
Tissue dissociation was achieved by homogenization in ice-cold chloroformmethanol 2:1 (v/v) containing 0.01% BHT using an UltraTurrax model Type
TP-18-1.
The lipid extract was quantified gravimetrically and kept in a stoppered vessel
under nitrogen atmosphere at -30°C until the assays. Lipid and phospholipid compositions were obtained by means of the Iatroscan TLC/FID technique [18,19]. The
Iatroscan MK-5 was used in combination with Chromarods S, having a precoated
active silica thin layer. Three #1 of total lipids or phospholipids was spotted on each
rod, using a 10-#1 Hamilton syringe. To separate total lipids, rods were developed
in hexane/diethyl ether/formic acid (90:10:2, v/v/v). The phospholipids were resolved
in two steps, starting with an initial development of rods in chloroform/methanol/acetic acid/water (67:28:2:3, v/v/v/v), drying at 70°C for 10 min, and a second
development in hexane/diethyl ether/formic acid (90:10:2, v/v/v). Rods were scanned
under the following conditions: hydrogen flow, 150 ml/min; air flow, 1750 ml/min;
scanning speed, 47 mm/s; chart speed, 42 mm/min. An Iatrocorder TC-11 integrator
was used for recording and area integration.
2.6. Fatty acid analysis
Fatty acids were analyzed by gas chromatogrphy (GC), as previously described
[20-22]. The samples were saponified by heating for 5 min with 5 ml of 0.2 M sodium methylate and heated again at 80°C for 5 min with 6% (w/v) H2SO4 in
anhydrous methanol. The fatty acid methyl esters thus formed were eluted with hexane and analyzed in a Hewlett-Packard 5890 series II gas chromatograph equipped
with flame ionization detector and using an Omegawax 320 fused silica capillary column (30 m x 0.32 mm i.d., 0.25 /~m film). The initial column temperature was
200°C, which was held for 10 min, then programmed from 200-230°C at 2°C/min.
4
1£. Alvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
The injection and detector temperatures were 250°C and 260°C, respectively. The
flow rate of helium was 2 ml/min, the column head pressure was 250 kPa, and the
detector auxiliary flow rate was 25 ml/min. Peak areas were calculated by a HewlettPackard 3990A recording integrator. Individual fatty acid methyl esters were identified on isothermal runs by comparison o f their retention time against those of standards. Fatty acid methyl esters were quantified by internal standardization
(tricosanoic methyl ester, 23:0), using peak-area integration. Fatty acid methyl esters
for which no standard was available were quantified using calibration tables of relative response ratios constructed according to carbon number (using gas
chromatography-mass spectrometry, GC-MS). GC-MS was done on a Konik KNK2000 chromatograph interfaced directly to an AEJ MS30/70 VG mass spectrometer,
using the electron impact mode. The ion source temperature was maintained at
200°C, multiplier voltage was 4.0 kV, emission current was 100 #A and electron
energy was 70 eV. The data were processed with a VG 11/250 data system.
2. 7. Measurement of fluidity
The steady-state fluorescence polarization and fluorescence anisotropy were determinated according to the method of Vorbeck et al. [23], utilizing the lipid-soluble
fluorescent probe 1,6-diphenyl-l,3,5-hexatriene.
Membranes were labeled at a protein concentration of 50-150 tzg/ml in a 1000fold dilution of 2 mM DPH stock solution (tetrahydrofuran as solvent) containing
0.25 M sucrose and 10 mM Tris-HCl pH 7,4 and incubated for 45 min.
Measurements were made at 25°C and 37°C using a Perkin-Elmer 650-40 fluorescence spectrophotometer equipped with a polarizing filter. The excitation and emission wavelengths were 365 and 430 nm, respectively.
The corrected steady-state fluorescence polarization (p) was calculated as
p = (Ivv - Ivh / Iw + Ivh)
where Ivv and Ivh are observed intensities measured with polarizers parallel to and
perpendicular to, respectively, the vertically oriented polarizer exciting beam.
The steady-state fluorescence anisotropy (rO was calculated from the ratio
rs = 2p/(3-p)
where p is the corrected fluorescence polarization.
The limiting fluorescence polarization (roo) and the lipid structural order parameter (SDPr0 were calculated according to Pottel et al. [24]:
ro, = 4/3 r s - 0.11 (where 0.13 < rs < 0.28)
SDPH = (rMr0)l/2 (where roo/r0 = 4/3 rs/r0 - 0.28)
Light scattering effects were non-detectable in our experimental conditions, since
it was observed that dilutions of the samples from adult, mature and aged animals
did not affect steady-state fluorescence polarization values.
E. Alvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
5
2.8. Statistical methods
Student's test was utilized to evaluate differences between means, and the 0.05
level of probability was used as the criterion of significance. All values reported are
means ± S.D. of at least five animals.
3. R ~ d ~
3.1. Lipid composition
The effect of age on phospholipid and free cholesterol content of peritoneal macrophage membranes is shown in Table 1. Although free cholesterol showed a significant age-related increase, it is clear that there was also a significant age-related
decrease in phospholipid. The levels of cholesterol increased from a mean of 1.45%
for young animals to a mean of 2.15% for the 24-month-old animals. In contrast,
the content of phospholipids decreased from a mean of 98.62% for young animals
to a mean of 97.84% for those of 24 months. In the mature animals the levels found
were intermediate between young and old animals. No differences were found
with respect to those of 3 months; however, in both cases (free cholesterol
and phospholipids) the differences versus 24-month-old animals were significant
(P < O.O1).
The inverse relationship between the change in phospholipid and free cholesterol
content and age results in an increase in the molar ratio of cholesterol/phospholipid.
To determine whether the phospholipid composition of macrophage membrane
was altered with age, the major phospholipid species were quantified by means of
the Iatroscan TLC/FID technique. The results of these analyses are given in Table 2.
Several phospholipids increased with age. The most marked change was the significant increase (approximately 3-fold) of lisophosphatidylcholine in the 24-monthold animals. (The levels in 3- and 12-month-old animals were similar.) The age
increase in phosphatidylserine and cardiolipin (approximately 102%) was observed
in 12 and 24-month-old animals. A progressive age increase was observed in the
sphingomyelin content. In contrast, a significant decrease was found in the phosphatidylcholine and phosphatidylinositol content in 12 and 24-month-old animals.
Although phosphatidylethanolamine (the other major phospholipid) also decreased,
the difference was not statistically significant.
Table 1
Effect of age on lipid composition of peritoneal macrophage membranes
Age
(months)
3
12
24
Percentage (w/w)
Free cholesterol
phospholipids
Ratio of
cholesterol/
Phospholipids
1.45 ± 0.127
1.73 ± 0.042
2.15 ± 0.021 a'b
98.62 ± 0.262
98.01 ± 0.013
97.84 ± 0.014 a'b
0.015
0.018
0.022
The values represent the mean ± S.D. from at least five separate experiments. Statistical significance:
ap < 0.05 versus 3 months; bp < 0.01 versus 12 months.
6
E. AIvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
Table 2
Effect of age on major phospholipid content of peritoneal macrophage membranes
Phospholipids
PS + CL
PI
PE
PC
SM
LPC
Percentage
(w/w)
3 months
12 months
24 months
12.54
6.16
27.47
48.15
3.48
2,13
25.76
4.29
26.63
37.14
5.05
2.15
25.37
3.82
22.17
33.39
7.86
6.77
444q44-
3.11
0.19
3.21
8.14
1.14
0.21
44+
4:t:
4-
2.98 a
0.09 a
4.05
2.25 a
0.52 a
1.44
± 2.72 c
± 0.11 c
4- 4.73
± 3.72 c
-4- 0.38 b.c
4- 2.32 b,c
The values represent the mean q- S.D. from at least five separate experiments. PS: phosphatidylserine;
CL: cardiolipin; Ph phosphatidylinositol; PE: phosphatidylethanolamine; PC: phosphatidylcholine; SM:
sphingomyelin; LPC: lisophosphatidylcholine. Statistical significance: ap < 0.01 versus 3 months; bp <
0.01 versus 12 months; cp < 0,01 versus 3 months.
3.2. Fatty acid content
A comparative analysis of fatty acid levels was made on peritoneal macrophage
membranes from rats of different ages (3, 12 and 24 months). The results obtained
are given in Table 3. Palmitic acid (16:0) was the predominant fatty acid found in
peritoneal macrophage membranes in each of the age groups examined (between 22
and 24% of the total fatty acids). Lauric acid (12:0) comprised the smallest part (between 0.3 and 0.5%) of the total fatty acids.
During aging there was an appreciable and progressive decrease in arachidonic
acid (20:4) (from approximately 12% in young, to 9% in mature and 6% in aged animals). A decrease in the levels of docosapentaenoic (22:5), docosatetraenoic (22:4)
and tetracosenoic (24:1) acid was also apparent during aging. The decrease in the
24:1 and 24:4 was significant even in rats of 12 months. In contrast, we observed an
ifmrease in the levels of oleic (18:1), linolenic (18:3) and docosahexanoic (22:6) acid
during aging (the levels in 12-month-old rats were similar to those in 3-month-old
rats). The levels of the other fatty acids changed relatively little between the ages
studied.
One important change found in this study was the increase of total (n-3) fatty acids
(mainly 22:6) accompanied by a decrease in total (n-6) fatty acids (mainly 20:4) in
the aged animals. As a consequence, there was a reduction in the ratio (n-6)/(n-3)
in these animals.
3.3. Membrane fluidity
The steady-state fluorescence polarization and fluorescence anisotropy data for
DPH-labeled macrophage membrane preparations from young, mature and aged
animals are given in Table 4. There was a highly significant (P < 0.001) increase in
fluorescence polarization between 25°C and 37°C. Fluorescence anisotropy also increased in preparations from the 24-month-old animals (P < 0.001). In mature animals the levels observed were intermediate (and statistically significant) between 3
and 24 months.
E. Alvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
Table 3
Effect of age on major fatty acid composition of peritoneal macrophage membranes
Fatty acids
C12:0
C14:0
Cl4:l,n. 7
C16:0
Ci6:l,n. 7
C16:4,n.3
Cl8:0
Ci8:l,n. 9
Cis:l,n. 7
Ci8:2,n.6
C18:3,n.3
C20:4,n.6
C22:4,n.6
C22:5,n.6
C22:6,n.3
C24:1
C24:1
Saturated
Monoenoic
Polienoic
Total n-6
Total n-3
n-6/n-3
Monoenoic/
saturated
C2o:4/Ci8:2
Percentage (w/w)
3 months
12 months
24 months
0.315
2.570
1.460
22.600
6.220
3,500
13.415
13.950
2.505
4.975
3.075
12.200
0.700
3.915
5.670
1.590
1,290
40.490
25.420
34.030
21.790
12.240
1.780
0.627
0.392
3.125
1.430
23.902
6.767
3.076
12.836
14.265
3.065
5.840
2.935
9.420
0.575
3.935
5.810
2.020
0.515
42.275
26.042
31.591
19.770
11.821
1.672
0.616
0.410
2.995
1.420
22.435
6.815
2.760
12.515
17.420
3.165
5.970
6.060
6.290
0.570
2.075
8.620
1.340
0.510
39.690
29.690
32,340
14.900
17.440
0.850
0.748
2.450
4- 0.035
4. 0.269
4- 0.014
q- 0.948
± 0.042
4- 0.290
4- 0.587
:t: 0.580
± 0.007
4- 0.247
± 0.304
q- 0.255
4- 0.028
4- 0.120
q- 0.382
4- 0.580
4. 0.099
4- 2.419
q- 0.742
4- 1.626
4- 0.650
4- 0.979
1.613
4- 0.039
4. 0.150
4- 0.315
4. 0.824
4- 0.450
4- 0.178
q- 0.405
q- 0.460
4- 0.245
± 0.825
4. 0.276
4- 0.141 a
4- 0.014 a
± 0.148
4- 0.141
q- 0.245
4. 0.021 a
4- 2.013
4- 1.425
4- 2.592
4- 0.521
4- 1.051
± 0.057
± 0.016
4- 0,350
4- 1.082
+ 0.926
4- 0.010
4- 0.040
4- 0.550 b'¢
4- 0.658
4- 1.245
4. 0.085 b'c
4- 0.495 b'c
4- 0,028 c
4- 0.092 b'c
4- 0.962 b'c
4. 0.042
4. 0.010 c
4- 1.237
4- 2.485
4- 4.152
4- 2.072 b'c
4- 2.080 b'c
1.053
The values represent the mean 4- S.D. from at least five separate experiments. The sum of the saturated,
monoenoic and polyenoic (poli.) was calculated for all positively identified fatty acids. Statistical significance: ap < 0.05 versus 3 months; bp < 0.05 versus 12 months', cp < 0.05 versus 3 months.
Pottel et al. [24] have presented an empirical relationship between steady-state fluorescence anisotropy and the limiting fluorescence anisotropy. From this relationship they estimated a lipid structural order parameter (reciprocal of fluidity) directly
from simple steady-state fluorescence polarization measurements of a variety of
membranes. Using the same relationship, we estimated the lipid structural order parameter in macrophage preparations. As shown in Table 4, there was a significant
increase in the lipid order parameter in macrophage membranes from the 12 and 24month animals at the two temperatures studied.
4. Discussion
Results from the present study show that peritoneal macrophage membranes isolated from young, mature and aged rats differ considerably in both lipid content and
fluidity.
8
E. Alvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
Table 4
Steady-state fluorescence polarization, steady-state fluorescence anisotropy and lipid order parameter of
peritoneal macrophage membranes.
Age
3 months
12 m o n t h s
24 months
(25°C)
0.282 + 0.003
0.290 + 0.005 a
0.299 + 0.008 b'c
(37°C)
0.220 4- 0.006
0.230 4- 0.002 a
0.234 + 0.003 b'c
(25°C)
0.207 + 0.003
0.213 + 0.005 a
0.222 + 0.006 b.c
(37°C)
0.158 + 0 . 0 0 4
0.166 q- 0.002 a
0.169 4- 0.002 b.c
(25°C)
0.165 4- 0.004
0.174 4- 0.006 a
0.185 4- 0.007 b'c
(37°C)
0.102 4- 0.006
0.111 + 0.003a
0.115 4- 0.003 b.c
(25°C)
0.638 4- 0.007
0.656 4- 0.012 a
0.676 + 0.014 b'c
(37°C)
0.495 4- 0.015
0.522 4- 0.007 a
0.531 4- 0.008 b'c
PDPH 1
rs2
ro~3
SDPH 4
1. Fluorescence polarization (PDPH) = (|vv - lvh)/(Ivv + Ivh)'
2. Fluorescenceanisotropy (rs) = 2P/(3 - P).
3. Limiting fluorescence anisotropy (r®) = 4/3 r s - 0.11 ( w h e r e 0.13 < rs < 0.28).
4. Lipid order parameter (SDPH) = (r~./r0)~/2; (ro./r 0 = 4/3 r J r 0 - 0.28) (where ro = 0.4).
The values represent the mean q- S.D. from at least five separate experiments. Statistical significance:
a p < 0.005 versus 3 m o n t h s ; b p < 0.05 versus 12 m o n t h s ; c p < 0.001 versus 3 months.
An age-related increase in free cholesterol has been found, together with a
decrease in the phospholipid content (Table 1). These changes lead to an increment
in the molar ratio of cholesterol/phospholipid with age. Similar results have been
described in other membranes from aged animals [11.
The increase in the molar ratio of cholesterol/phospholipid observed in peritoneal
macrophage membranes isolated from aged animals is consistent with the increase
in the lipid order parameter (reciprocal of fluidity) that we have described (Table 4).
The molar ratio of cholesterol/phospholipid is considered to be a main determinant
of lipid fluidity of both artificial and biological membranes [25].
Various investigations have suggested that aging might be accompanied by a
reduction in membrane fluidity. An increment in membrane microviscosity as a
result of aging has been demonstrated in human platelets [26], rat intestinal
microvilli [27], mice lymphocytes [28], human erythrocytes [29] and rat brain [301.
The physiological implications of the observed increase in the lipid order parameter with age may relate primarily to modulation of the dynamics of membrane proteins. In erythrocyte membranes, changes in membrane cholesterol content and lipid
fluidity have been shown to alter the availability of protein sulfydryl groups at the
surface of the membrane [31,32]. In liver mitochondria, the consequences of in vivo
E. Alvarez et al./Mech. Ageing Dev. 71 (1993) 1-12
9
or in vitro manipulation of cholesterol content have been correlated with changes
in membrane function [33]. In macrophages, the degree of lipid fluidity influenced
both phagocytosis and pinocytosis [34].
To check whether phospholipid composition changed with age, the major phospholipid species were quantified. The phospholipid content for the control group
was similar to that described by Brouard and Pascaud [35]. The predominant
phospholipids in adult animals were ethanolamine and phosphatidylcholine. These
were also the major phospholipids extracted from macrophage membranes of adult
rabbit [36]. The levels of phosphatidylcholine and phosphatidylinositol declined
with age, whereas sphingomyelin, lisophosphatidylcholine, phosphatidylserine and
cardiolipin increased with age (except in lisophosphatidylcholine, all the changes
were significant in mature animals).
The increase in the relative amount of sphingomyelin is, together with the increment in the cholesterol/phospholipid ratio, another important cause of the rise in
membrane microviscosity [37]. At the same time, the decrease in phosphatidylinositol is particularly relevant, because many stimuli act through hydrolysis of phosphatidylinositol to activate NADPH oxidase, the enzyme responsible for the respiratory
burst [9] and which seems to decrease with age [unpublished results].
It has been stated that aging can alter the fatty acid composition and that this
change may alter the membrane fluidity [38]. We have studied the fatty acid composition of macrophage membranes and our analysis revealed the presence of four
major fatty acids in young animals: palmitic (16:0), stearic (18:0), oleic (18:1) and
arachidonic (20:4), as has also been described for young mouse peritoneal macrophages [39]. However, in aged animals the arachidonic acid decreased to approximately 50% and the fourth major fatty acid was docosahexaenoic (22:6).
Arachidonic acid (20:4) may act as a potent stimulator of superoxide anions,
which might influence cellular functions at a later stage [40].. In addition,
arachidonic acid is the precursor for prostaglandins, leukotrienes and hydroxy fatty
acids which have feedback influence on the functions of phagocytic cells [41,42].
Since arachidonic acid (20:4) may act as a stimulant of superoxide anions, the great
decrease that we have observed in arachidonic acid may bg related to the decrease
in the production of superoxide anions that we have found in aged animals
[unpublished data] and affect the phagocytic function of these cells.
The decline in arachidonic acid (20:4) is not accompanied by a similar fall in
linoleic acid (18:2) in the aged animal; thus there is a reduction in the ratio 20:4/18:2
with age. In other tissues this reduction has been attributed to a reduction in the
desaturation of essential fatty acids produced by an inhibition of the enzyme A6
desaturase [43]. In many mammalian tissues, 18:2 (n-6) is converted to 20:4 (n-6) by
an alternating sequence of A6 desaturase, chain elongation and A5 desaturation. It
has been demonstrated that murine peritoneal macrophages lack A6 desaturase activity [44], have substantial elongase activity [44], and possess a slight A5 desaturase
activity [45]. The lack of A6 desaturase in murine macrophages suggests that the
availability of macrophage 20:4 (n-6) cannot be modulated at the level of local synthesis from 18:2 (n-6) - - its major dietary essential fatty acid antecedent - - and that
the difference between young, mature and aged levels must be due to other causes.
On the other hand, the decrease in arachidonic acid might be related to a greater
10
E. Alvarez et al./Meeh. Ageing Dev. 71 (1993) 1-12
peroxidation of polyunsaturated lipids, which has been reported to increment with
age [46] because of a greater production in free radicals or lower protection [47-50].
This might be responsible for the great incidence of atherosclerosis with age. Most
of the foam cells are derived from monocyte-macrophages [51 ]. These macrophages
accumulate lipids in their cytoplasm, most of which consists of low-density lipoprotein (LDL). However, the L D L must be modified before it will produce cholesterol
ester accumulation. The reaction with malondialdehyde and acetylation are the most
important modifications [52,53].
Docosahexaenoic acid (22:6) increased with age and was the fourth major fatty
acid in the 24-month animals. Docosahexaenoic acid is a strong inhibitor of cyciooxygenase [54] and has been shown to inhibit leukotriene synthesis in mouse
peritoneal macrophages [35].
Another important change described in our study was the increase of total (n-3)
fatty acids (mainly 22:6) accompanied by a decrease in total (n-6) fatty acids (mainly
20:4) with age. As a consequence, there is a reduction in the ratio (n-6)/(n-3) in the
aged animals. Earlier reports have demonstrated that (n-3) fatty acids inhibit the metabolism of (n-6) fatty acids. Thus the reduction in the ratio (n-6)/(n-3) in aged animals might be due to an inhibition of the metabolism of (n-6) fatty acids produced
by (n-3) fatty acids. The balance between (n-6) and (n-3) fatty acids plays an important role in the regulation of the physicochemical nature of the membrane lipid matrix, which in turn might influence the function of a wide variety of intrinsic and
extrinsic membrane-bound enzymes [55]. This may also contribute to altering the
macrophage membrane function with age.
5. Acknowledgement
Eloisa Alvarez is a recipient of a fellowship from the Junta de Andalucia. This
work was supported by grant PB89-0173 from CAI de Ciencia y Tecnologia.
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