Distribution of unusual fatty acids in the mesocarp triacylglycerols of maturing sea buckthorn fruits

ISSN 1021�4437, Russian Journal of Plant Physiology, 2010, Vol. 57, No. 6, pp. 852–858. © Pleiades Publishing, Ltd., 2010.Original Russian Text © E.I. Kuznetsova, V.P. Pchelkin, V.D. Tsydendambaev, A.G. Vereshchagin, 2010, published in Fiziologiya Rastenii, 2010, Vol. 57, No.6, pp. 911–917.

852

INTRODUCTION

Previously, by using GLC and MS techniques, wehave determined the structure of unusual FAs esteri�fied in the oil triacylglycerols (TAGs) of the water�richmesocarp of mature sea buckthorn fruits. These FAsinclude n�cis�Δ9�hexadecenoic (H), n�cis�Δ9,12�hexadecadienoic (palmitolinoleic, Pl), and n�cis�Δ11�octadecenoic (cis�vaccenic, V) acids; the sum of theseFAs and oleic acid (n�cis�Δ9�octadecenoic acid, O)was designated as “X�FAs”. Total TAGs of the oil,

including all X�TAG species and all TAG species, whichdo not contain X�FAs, were designated as T�TAGs.Moreover, we have determined the patterns of distri�bution of the X�FA individual species among separatepositions of the glycerol residue in mature mesocarpX�TAGs and found this distribution to be very nonuni�form [1].

Earlier, the dynamics of qualitative and quantita�tive composition of reserve TAGs was mostly studiedin the oil�bearing seeds, which desiccate during matu�ration [2]. We are aware of only two communications,where this problem was solved by using the water�sat�urated oil�bearing mesocarp of maturing avocado [3]and oil palm [4] fruits.

The aim of this work was to investigate the changesin the X�TAG composition during maturation of seabuckthorn mesocarp. The results obtained here maycontribute to understanding the formation mecha�nism of the final oil TAG composition in this reserveplant organ.

Distribution of Unusual Fatty Acids in the Mesocarp Triacylglycerols of Maturing Sea Buckthorn Fruits

E. I. Kuznetsova, V. P. Pchelkin, V. D. Tsydendambaev, and A. G. VereshchaginTimiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, Russia;

fax: 7 (495) 977�8018; e�mail: [email protected]: December 21, 2009

Abstract—The pattern of distribution of X�FAs among individual positional species of the X�triacylglycerols(X�TAGs) in the sea buckthorn (Hippophaë rhamnoides L.) mesocarp oil by the 80th, 90th, and 105th dayafter pollination (DAP) was investigated. The X�FAs comprised oleic acid (O) as well as unusual FAs, viz.hexadecenoic (H), palmitolinoleic (Pl), and cis�vaccenic acid (V). During mesocarp growth, the number ofX�TAG species decreased dramatically as a result of their metabolism, but their proportion in the total TAGsremained constant. H�TAGs predominating in the oil were similar to total TAGs in their composition and thepattern of their dynamics during maturation. As regards minor Pl�TAGs, there was a tendency for an increasein the level of species including palmitolinoleic acid in the middle (sn�2) position of their molecules.Throughout the entire process of fruit maturation, oleic acid was by 98.0–98.8% concentrated in the sn�2position of O�TAGs. At the same time, cis�vaccenic acid, a Δ11�positional isomer of oleic acid, was predom�inantly incorporated in the side positions of V�TAGs, and its 98.1% concentration in these positions wasachieved only by the 105th DAP. A virtually absolute selectivity of the distribution of oleic and cis�vaccenicacids in TAGs was found here for the first time. Discussed are the possible physicochemical and biochemicalfactors of a highly selective incorporation of these FAs in the sea buckthorn TAGs, the pathways of enzymaticbiosynthesis of O� and V�TAGs, the metabolic role of discrimination of incorporation of some FA species inthe glycerol residue of glycerolipids, and the causes for the disappearance of some TAG species during mat�uration.

Keywords: Hippophaë rhamnoides, mesocarp, maturation, triacylglycerols, unusual fatty acids, pathways ofbiosynthesis.

DOI: 10.1134/S1021443710060142

RESEARCH PAPERS

Abbreviations: DAP—days after pollination; FAs (fatty acids) andtheir residues in TAGs: L—linoleic; Le—linolenic; P—palmitic;St—stearic; the residues of all FAs including X�FAs, saturated,and unsaturated FAs indicated at the middle of an abbreviatednotation of a given TAG positional species or a TAG positionaltype are located in the sn�2 position, and the FA residues indi�cated at its sides can be located both in the sn�1 and in the sn�3positions of a TAG molecule; PSC—positional�species composi�tion; S—total saturated FAs; sn—stereospecific numeration ofcarbon atoms in the glycerol molecule; TAG—triacylglycerol;T�TAGs—total TAGs of an oil including all X�TAG species andall TAG species, which do not contain X�FAs; U—total unsatur�ated FAs; X�FAs—hexadecenoic (H), oleic (O), palmitolinoleic(Pl), and cis�vaccenic (V) FAs; X�TAGs—TAG species con�taining X�FAs.

RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 57 No. 6 2010

DISTRIBUTION OF UNUSUAL FATTY ACIDS IN THE MESOCARP 853

MATERIALS AND METHODS

Sea buckthorn (Hippophaё rhamnoides L., cv. Yan�tarnaya) fruits were collected in the Main BotanicalGarden of the RAS by the 80th, 90th, and 105th dayafter pollination (DAP). The mesocarps were fixed inthe boiling isopropanol. Total lipids were extracted withchloroform containing 0.001% butylated hydroxy�toluene [1]. Extracted lipids were separated by prepar�ative TLC on Silica Gel in a n�hexane : diethyl ether =8 : 2 system, and the TAGs were eluted from the adsor�bent by a chloroform : methanol mixture (96 : 4). TheTAGs thus obtained were more than 98% pure. Quali�tative and quantitative FA compositions of TAGs,positional�species composition (PSC) of TAGs, selec�tivity factor of FA incorporation in TAGs, the values ofFA carbon atom number (mi), double bond number(ei), and relative lipophilicity (Li) in separate TAGfractions (ith fractions), as well as the respectiveweighted�average values (m, e, and L), were calculatedas described previously [1, 5].

RESULTS

The determination of FA composition in the sn�2position of T�TAGs showed that the total saturated

FA (S) content in this position did not exceed 3%; there�fore, the calculated values of their PSC were accurate [1].

A total of 95 T�TAG species were found at all DAP.Eighty species out of them (with a concentration of0.1% or more) were X�TAGs (Table 1), and more than0.25 of them (21 species) remained minor (no morethan 0.1%) until the end of maturation. In the courseof mesocarp growth, a total number of X�TAG speciesdecreased considerably; only 32 different species weredetected at all DAP, while many species (31 species)were found at an only one certain date (Table 2). Bythe 90th DAP, the number of TAG species includingH�, O�, and V�FAs (H�, O�, and V�TAGs, respec�tively) increased somewhat, but by the 105th DAP itfell dramatically, mainly at the expense of disappear�ance of X�TAGs of enhanced lipophilicity (Table 1).Thus, fruit maturation was always accompanied byconsiderable changes and simplifications in the meso�carp X�TAG qualitative composition.

On the contrary, X�TAG relative quantitative con�tent in the T�TAGs at all DAP remained almost thesame, comprising 90.0 ± 1.5%. This was caused by thefacts that the H�TAG concentration remained con�stant, while a decrease in the O�TAG level was accom�panied by an increase in that of V�TAGs (Table 2).The concentrations of Pl�TAGs at the 80th, 90th, and

Table 1. Positional�species composition of X�TAGs (in the order of ascending lipophilicity) in the sea buckthorn mesocarpoil at the 80th, 90th, and 105th DAP (mol % of total content of T�TAGs)

X�TAGsDAP

X�TAGsDAP

X�TAGsDAP

X�TAGsDAP

80 90 105 80 90 105 80 90 105 80 90 105

HHPl 0.1 0.2 0 PHH 18.0 20.5 17.6 PPH 0.9 0 0 OLV 0 0.1 0

HHLe 0.2 0.1 0.2 PHL 3.3 2.1 2.3 PHP 16.3 17.2 28.8 VHV 0.1 0.5 0.5

HPlH 0.1 0 0.1 PPlP 0.2 0.1 0.7 PHO 0.2 1.5 0 VLV 0 0.1 0.1

HLPl 0 0.1 0 PPlV 0 0 0.2 PHV 2.1 5.7 7.4 PPV 0.1 0 0

HLeH 0.2 0.1 0 POLe 0.1 0.1 0.1 POH 6.7 5.3 3.6 PHSt 1.8 0.1 0.3

PHPl 0.2 0.4 0.1 PLH 4.6 5.9 4.7 POL 1.2 0.5 0.5 PStH 0 0.5 0.4

PHLe 0.3 0.2 0.5 PLeV 0.1 0.1 0.1 PVH 2.6 0.5 0 POP 6.1 4.3 5.8

PPlH 0.2 0.1 0.4 HPH 0.3 0 0 PVL 0.5 0 0 POO 0.1 0.3 0

PPlL 0 0 0.1 HHO 0.1 0.8 0 PLO 0 0.4 0 POV 0.8 1.4 1.5

PLeH 0.7 0.5 0.3 HHV 1.2 3.4 2.3 PLV 0.5 1.6 2.0 PVP 2.4 0.4 0

HHH 5.0 6.2 2.7 HOH 1.9 1.6 0.5 HPV 0.1 0 0 PVV 0.3 0.1 0

HHL 1.8 1.2 0.7 HOL 0.7 0.3 0.1 HHSt 1.0 0 0.1 HStV 0 0.1 0

HPlV 0 0 0.1 HVH 0.7 0.2 0 HStH 0 0.2 0.1 HOSt 0.4 0 0

HOLe 0.1 0 0 HVL 0.3 0 0 HOO 0 0.2 0.2 HVSt 0.1 0 0

HVPl 0 0 0.2 HLO 0 0.2 0 HOV 0.4 0.9 0.5 StOL 0.1 0 0

HVLe 0.1 0.2 0.1 HLV 0.3 1.0 0.6 HVV 0.2 0.1 0 VOO 0 0.1 0

HLH 1.3 1.8 0.7 OHL 0 0.1 0 HLSt 0.3 0 0 VOV 0 0.1 0.1

HLL 0.5 0.3 0.2 VHL 0.2 0.3 0.3 StHL 0.2 0 0 PStV 0 0.2 0.1

HLeV 0 0.1 0 VLL 0.1 0.1 0.1 OHO 0 0.1 0 POSt 0.7 0 0.1

LHL 0.2 0.1 0 LOL 0.1 0 0 OHV 0 0.3 0 PVSt 0.3 0 0

854


KUZNETSOVA et al.

105th DAP were very low and did not exceed 0.7, 1.0and 1.9%, respectively.

Although the total H�TAG content in the T�TAGsdid not change in the course of maturation (Table 2),there were considerable shifts in their composition andstructure (Tables 3, 4; Figs. 1, 2). During this process,the extent of affinity (a selectivity factor) of hexade�

cenoic acid to the middle (sn�2) position of the glyc�erol residue [5] increased from 0.84 to 1.00. As a result,the concentrations of (– Х –) and monounsaturated(еi = 1) H�TAGs, i.e., SUS�H�TAGs, as well as that ofPHP as their major component (Table 1), increasedmore than 1.5�fold. Correspondingly, by the 105thDAP, the increases in the weighted�average lipophilicityof H�TAGs and in the levels of H�TAGs with mi = 48 andLi = 46 were accompanied by a disappearance of18 minor (– – Х)� and (Х – Х) H�TAG species (Table 1).At the same time, there was a decrease in all otherindices of composition and structure of H�TAGs,including the concentration of the (Х Х Х) form,which was totally absent from all other X�TAGs (Table 4).As a whole, H�TAGs predominating in the oil weremore similar to T�TAGs in their composition and inthe pattern of their changes during maturation thanthe rest of X�TAGs (Fig. 2).

As noted above, in this particular sea buckthornvariety, the mesocarp oil was characterized by a verylow Pl�TAG level (Table 1). A total of 10 Pl�TAG indi�vidual species with Li = 40–44 were found, and theircontents amounted to 0.1 – 0.7% of T�TAGs each. Asit was also the case in mature fruits [1], all these speciesbelonged to the m, e, and L types, and (– Х –) pre�dominated. There was no definite pattern in changingthe concentrations of these positional types during

Table 2. The number of X�TAG positional species in their var�ious groups and the contents of these groups at the 80th, 90th,and 105th DAP

Indices of X�TAG compositionDAP

80 90 105

Total number of X�TAG species 63 59 45

The number of X�TAG species (31 at all) found only at a given DAP

17 10 4

Number of positionalspecies

H�TAGs 44 42 31

O�TAGs 16 20 11

V�TAGs 21 24 17

Total concentrationof positional species, mol % of total content of T�TAGs

H�TAGs 76.3 80.6 76.5

O�TAGs 19.7 17.2 13.0

V�TAGs 13.5 17.1 16.2

Table 3. Quantitative distribution of X�FAs in the sea buckthorn mesocarp X�TAGs at the 80th, 90th, and 105th DAP

Indicesof X�TAG

composition

Content of an ith fraction of X�TAGs, % of total X�TAGs

H�TAGs O�TAGs V�TAGs

80 90 105 80 90 105 80 90 105

mi

48 54.5 56.2 65.0 0 0 0 0 0 0

50 39.8 39.2 31.7 63.8 79.1 75.6 67.5 59.2 62.5

52 5.7 4.6 3.3 34.6 20.3 23.7 31.7 39.0 35.6

54 0 0 0 1.5 0.6 0.7 0.8 1.8 1.9

ei

1 25.0 20.5 38.5 34.5 27.9 45.4 20.7 2.9 0.7

2 41.3 42.5 37.6 41.6 47.1 39.2 44.4 54.7 52.5

3 24.2 28.6 18.5 18.8 22.7 13.8 26.7 32.0 39.7

4 7.6 7.1 4.7 4.1 1.7 1.5 6.7 8.1 5.7

5 1.7 1.2 0.7 1.0 0.6 0 1.5 2.3 1.4

Li

40 0.9 0.9 0.4 0 0 0 0 0 0

42 13.6 15.1 7.8 0.5 0.5 0 0.7 1.2 0

44 41.5 45.7 37.1 14.7 21.1 5.4 21.5 30.8 22.8

46 40.8 37.3 53.8 43.1 43.2 36.9 48.1 54.1 66.5

48 3.0 0.9 1.0 38.1 35.1 56.9 27.4 13.4 10.1

50 0.1 0 0 3.6 0 0.8 2.2 0.6 0.6

mi—the number of FA carbon atoms, ei—the number of double bonds, Li—relative lipophilicity.



maturation, and the values of m, e, and L were alwaysclose to those found earlier [1]. The Pl�TAG speciesfound were distributed among the (– Х –)� and (– – Х)�positional forms in the 6 : 4 ratio, and, at the 80th,90th, and 105th DAP, their relationships were 3 : 2, 2 : 3,and 6 : 2, respectively. Therefore, it might be suggestedthat there was some tendency for a predominant for�mation of the (– Х –) form of Pl�TAGs by the end ofmaturation (only this form was present in maturemesocarp [1]). It was impossible to accurately deter�mine the maturation dynamics of other indices ofcomposition and structure of individual Pl�TAG spe�cies because of a low content of these species.

When considering such dynamics for the O�TAGs, itwas shown that both the affinity of oleic acid to the sn�2position (Fig. 1) and the concentration of the (– Х –)form of O�TAGs (Table 4) remained at a very high lev�els throughout fruit maturation. Correspondingly, inall 24 O�TAG species found here, almost all (98.5%)oleic acid was concentrated in their middle position.In the course of fruit growth, total (– Х –) species con�tent remained constant in spite of increases in the con�centrations of both the (еi = 1)�O�TAGs (Table 3), i.e.,SUS�O�TAGs (Table 4), and the (mi = 50)� and (Li =48)�O�TAG fractions by the 105th DAP. This was thecase because these increases were compensated bydecreases in the levels of all other mi�, ei�, and Li frac�tions (Table 3), as well as in the indices of positional�type composition (Table 4). In the course of growth,the e value of O�TAGs decreased from 2.0 to 1.7, andthe L value increased respectively (Fig. 2). All thesechanges were brought about by considerable matura�

tion�caused shifts in the FA composition of O�TAGs;however, these shifts involved only the sn�1,3 positions ofO�TAGs and by no means affected the sn�2 position FAs.

Unlike oleic acid, cis�vaccenic acid was character�ized, first, by an almost exclusive affinity not for thesn�2, but for the sn�1,3 positions, and, second, by thefact that, in the course of growth, such affinity was notachieved immediately (Fig. 1). Actually, by the 80thDAP, more than 20% of V�TAGs accounted for by their(еi = 1)�fraction (Table 3), i.e., by the sn�2 V�SUS posi�tional type (Table 4). Only by the 105th DAP, the pre�dominance of sn�1,3 V�TAGs (a sum of (– – Х)� and(Х – Х) forms) became virtually total (98.1%, Table 4).More than 99% of these V�TAGs consisted of SUU�and UUU� positional types, and the concentrations ofthese types increased during mesocarp maturation.Besides, mesocarp growth was accompanied by asteady rise in the e value (from 2.2 to 2.6), as well as inthe levels of (еi = 2)� and (еi = 3)�fractions (Table 3),thus causing a decrease in L (Fig. 2). Finally, V�TAGs, likeO�TAGs, consisted almost entirely of the (mi = 50)� and(mi = 52)�fractions, which included, along with vac�cenate, also С16�FA chains (two and one, respec�tively); in the course of mesocarp growth, their con�tents changed relatively little (Table 3).

DISCUSSION

Even in our previous work [1], we have demon�strated a considerable predominance of oleic acid inthe sn�2 position of O�TAGs (85.7%) and of cis�vac�cenic acid in the sn�1,3 positions of V�TAGs (91.1%)of the mature sea buckthorn mesocarp. Such pattern

Table 4. Composition of positional forms and positional types of the sea buckthorn mesocarp X�TAGs at the 80th, 90th, and 105thDAP

Indicesof X�TAGstructure

Concentration of a respective form or type of TAGs, mol % of a sum of X�TAGs

H�TAGs O�TAGs V�TAGs

80 90 105 80 90 105 80 90 105

Positional forms of TAGs

(– – X) 25.4 22.4 14.9 1.5 1.2 0 39.5 87.3 93.8

(– X –) 32.8 29.6 52.5 98.0 98.8 98.5 55.6 7.5 1.9

(X – X) 5.9 6.2 1.7 0 0 0 0.8 4.0 4.3

(– X X) 29.4 32.8 27.3 0.5 0 1.5 4.0 1.2 0

(X X X) 6.6 8.9 3.5 0 0 0 0 0 0

Positional types of TAGs

SUS 23.9 19.7 38.0 34.5 27.9 45.4 20.0 2.3 0

SSU 1.2 0.7 0.5 0 0 0 0.7 0.6 0.6

SUU 53.6 53.3 48.3 48.7 50.0 43.8 51.9 55.0 69.2

USU 0.7 0.4 0.1 0 0 0 0.7 0.6 0.1

UUU 20.7 25.8 13.1 16.8 22.1 10.8 26.7 41.5 30.2

Note: The contents of positional forms represent the sums of contents of X�TAG species, which include a given X�acid only in the sn�1,3 [(– – X)and (X – X)], only in the sn�2 (– X –), as well as in the both of these positions [(– X X) and (X X X)].

856


KUZNETSOVA et al.

was confirmed in the present investigation. Moreover,as regards the sea buckthorn variety studied here, thesevalues, by the 105th DAP, amounted to 98.5 and98.1%, respectively (Table 4). Thus, the oleate andvaccenate affinity parameters became virtually abso�lute. To the best of our knowledge, such characteristicsof TAG composition and structure were neverobserved previously.

What factors can cause such a high selectivity ofincorporation? Usually, it is believed that, in plantTAGs, the extent of affinity of the residues of certainFAs to the middle or side hydroxyl groups of the glyc�erol residue is mainly determined by the physicalproperties of these FAs. In fact, both the solid С

≥16 FAs(see Table 1) and, e.g., a high�melting С22 : 1 monoun�saturated FA, m.p. = 34.7°C [6], acylate almost exclu�sively the sn�1,3 positions, from which they can bemore easily hydrolyzed by lipase during seed germina�tion. At the same time, С

≤18�U�FAs including oleicacid, m.p. 13.4°C, which are liquid at room tempera�ture, are incorporated, first of all, in the spatially hin�dered sn�2 position (see below). However, cis�vacce�nic acid, unlike C22 : 1 acid, is very close in its m.p. =14.5°C to oleic acid. Therefore, a radical discrepancybetween these monoenoic octadecenoic FAs in thepattern of their localization in TAGs (Table 4) canhardly be caused by differences in their physical prop�erties.

One can also suggest that such discrepancy iscaused by differences between these FA species in themechanism of their biosynthesis. It is known that both

cis�vaccenic acid and C≥20 monounsaturated acids

characterized by an exclusive affinity to the sn�1,3positions of TAGs (see above) are formed by the samemechanism of elongation, i.e., by a successive C2 chainelongation of n�cis�Δ9�hexadecenoic and oleic acid,respectively. At the same time, the sn�2 position is firstof all esterified with the products of a direct FA desat�uration. These products include oleic acid occurringvia a n�cis�Δ9�dehydrogenation of stearate and, inparticular, the products of further oleate desaturation,viz. linoleic and linolenic acids [1, 7]. As we see, in thesea buckthorn mesocarp, oleic acid was incorporatedonly in the sn�2 position. Earlier, a considerable, butnot exclusive, affinity of oleate to this position wasobserved in the olive mesocarp [8].

At present, there are several possible schemes ofTAG biosynthesis in plants [9]. It was shown [10] that,in avocado mesocarp, this process proceeds accordingto the classical mechanism proposed by Kennedy [11].Taking that the same mechanism is active also in themesocarp of sea buckthorn, oleic acid, in our case,must incorporate only in the sn�1�acyl�glycero�3�phosphate, i.e., lysophosphatidic acid (LPA), which isthereby converted into phosphatidic acid (PA). Thisincorporation is catalyzed by the LPA acyl transferase(AT) [10]. Considering the pattern of oleate distribu�tion in TAGs found here (see above), one may believethat the LPA AT�oleate enzyme–substrate complex ischaracterized by an absolute specificity to the sn�2

2.0

1.5

1.0

0.5

0 1009080

Days after pollination

Sel

ecti

vity

fac

tor,

arb

. un

its

1

2

3

Fig. 1. Affinity of separate X�FA species (1–3) to the sn�2position of sea buckthorn mesocarp X�TAGs at the 80th,90th, and 105th DAP.(1) Hexadecenoic; (2) oleic; (3) cis�vaccenic.

52

48

46

44

0 1009080

Days after pollination

m a

nd

L v

alu

es 1

2

3

42

504

4а

1а

3а

2а

Fig. 2. Weighted�average values of mi (m, 1–4) and Li (L,1a–4a) indices in separate X�TAG fractions (see Table 3)and in T�TAGs at the 80th, 90th, and 105th DAP.(1, 1a) H�TAGs; (2, 2a) O�TAGs; (3, 3a), V�TAGs;(4, 4a) T�TAGs.



position of LPA throughout the entire fruit maturationperiod.

At the same time, LPA AT scarcely reacts with cis�vaccenic acid during the end of maturation (Table 4).In mesocarp TAGs, this FA can be suggested to bepresent either only in the sn�1, or only in the sn�3, or inthe both of these positions of the glycerol residue. In thefirst two instances, its incorporation may be catalyzed bythe sn�3�glycerophosphate AT and by the sn�1,2�dia�cylglycerol (DAG) AT, respectively [2, 5], DAG beingthe product of phosphatase hydrolysis of PA [12]. Thelatter suggestion is favored by finding, at the 105thDAP, minor concentrations of VHV, VLV, and VOVTAG species, the sum of which amounted to 0.7% ofT�TAGs (Table 1). To obtain exhaustive quantitativedata on the localization of vaccenate in the sea buck�thorn mesocarp V�TAGs, one must perform a ste�reospecific analysis of their structure [7].

Earlier, it has been shown that, in the seed TAGs ofcoriander [13] and Moringa oleifera, an African plant[14], cis�vaccenic acid was concentrated in the sn�1,3positions, while in the two Asclepias species [15],canola [16], and basil [13], it was also found in the sn�2position.

Discrimination of the incorporation of some FAspecies into the glycerol residue of glycerolipids or intocertain positions of this residue takes place not onlyduring TAG formation, as it is the case for saturated,C

≥20 monoenoic, oleic, and cis�vaccenic acids (seeabove), but also during the biosynthesis of polar lipids.It is known that the concentrations of unusual (otherthan P, St, O, L, and Le) and industrially valuable FAs,such as ricinoleic, erucic, vernolic, lauric, petrose�linic, С

≥20, etc., in the seed TAGs of the respectiveplant species can be as high as 80–90%; however, theseFAs are scarcely found in the membrane lipids of theseseeds [2, 17, 18]. According to current views, the dis�crimination with regard of these FAs is caused by thefact that their incorporation into polar lipids disruptsthe structure and functions of cell membranes [19].

So far, a concrete molecular mechanism of the dis�crimination observed here and elsewhere is not estab�lished. Meanwhile, an elucidation of such mechanismwould provide additional insights into the pathways ofTAG and polar lipid biosynthesis, as well as into thefunctioning of cell lipid membranes. Moreover, itwould play a significant role in the genetic modifica�tion of wide�spread oil�bearing crops. Such modifica�tion would make it possible to obtain novel plant formscontaining oils with an unusual FA composition (seeabove) and of a significant industrial interest [17].

As noted above, the final pattern of cis�vaccenicacid distribution was not set up immediately: by the80th and 90th DAP, about 60 and 9% of V�TAG spe�cies, respectively, fell within the sn�2 V�TAGs (Table 4).In the course of mesocarp growth, these species, suchas PVH (2.6% of T�TAGs), PVP (2.4%), etc., almostdisappeared, and the level of sn�1,3 V�TAGs rose cor�respondingly (Table 1). Therefore, it can be suggested

that, in fruits, a considerable portion of V�TAGs wereinitially formed according to the pathway describedabove for oleic acid. By the end of mesocarp matura�tion, an intense breakdown was noted not only forthese sn�2 V�TAG species, but, simultaneously, forother X�TAG species devoid of cis�vaccenic acid(<1%, Table 2). However, in contrast to V�TAGs, all ofthem were initially (by the 80th DAP) the minor ones.

To the best of our knowledge, the distribution ofС16 and С18 FA positional isomers in separate positionsof the TAG molecule as well as the changes in TAGcomposition of the water�rich storage organs of plantswith an oil�bearing mesocarp were little investigated sofar. In the works on avocado and oil palm cited above[3, 4], no notable changes in a qualitative compositionof mesocarp oil TAGs during fruit maturation wereobserved. At the same time, experiments on maturingseeds have repeatedly demonstrated that certain TAGspecies, present in considerable amounts in the matur�ing cotyledons, were absent from the mature plant oil[20]. It was suggested that the irreversible disappear�ance of such TAGs is brought about by their transacy�lation. However, no particular enzymatic mechanismof this process was established up to now [9].

ACKNOWLEDGMENTS

This work was supported in part by the RussianFoundation for Basic Research, project no. 07�04�00083.

REFERENCES

1. Pchelkin, V.P., Kuznetsova, E.I., Tsydendambaev, V.D.,and Vereshchagin, A.G., Distribution of Unusual FattyAcids in the Triacylglycerols of Sea Buckthorn Meso�carp Oil, Russ. J. Plant Physiol., 2006, vol. 53, pp. 346–354.

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Distribution of unusual fatty acids in the mesocarp triacylglycerols of maturing sea buckthorn fruits