LYSOBISPHOSPHATIDIC ACID (BIS(MONOACYLGLYCERO) PHOSPHATE)

STRUCTURE, OCCURRENCE AND BIOCHEMISTRY

Lysobisphosphatidic acid is the widely used trivial name for for the lipid that correctly should be termed bis(monoacylglycero)phosphate, first isolated from rabbit lung but now known to be a common if minor constituent of all animal tissues. This is an interesting lipid from several standpoints, although it is only superficially related to phosphatidic acid per se. For example, its stereochemical configuration differs from that of other animal glycero-phospholipids in that the phosphodiester moiety is linked to positions sn-1 and sn-1’ of glycerol, rather than to position sn-3. All the initial studies from the first publication in 1967 until recently suggested that positions sn-3 and 3’ in the glycerol moieties were esterified with fatty acids.

On the other hand, there is an increasing school of thought to the effect that the fatty acids are esterified to position sn-2 and sn-2’ in the native molecule. Certainly, fatty acids in a lipid with the latter structure would be expected to undergo rapid acyl migration when subjected to most extraction and isolation procedures, resulting in the most thermodynamically stable form with fatty acids in the primary positions.

Isomerization of lysobisphosphatidic acid

Synthetic studies have shown how readily this can occur. Acyl migration might also be expected to take place under the acidic conditions in lysosomes (see below). Further evidence for this hypothesis comes partly from biosynthetic considerations, partly from the fact that a specific phospholipase A₂ has been found that degrades lysobisphosphatidic acid at the physiological pH in lysosomes, and partly from improved chromatographic conditions, which show three peaks for the lipid. None of this is conclusive, but together with doubts about some of the early NMR data, it suggests that a re-evaluation of the basic structure of this molecule is necessary. Certainly, those most active in the study of this lipid favour the structure illustrated below.

Probable structure of dioleoyl lysobisphosphatidic acid

Whatever the positions of the fatty acids on the glycerol molecule, their compositions can be distinctive with 18:1(n-9) and 18:2(n-6), 20:4 and 22:6(n-3) being abundant, although this is highly dependent on the specific tissue, cell type or organelle (see Table 1). For example, the testis lipid contains more than 70% 22:5(n 6); to my knowledge, this is not a major component of any other natural lipid. In contrast, baby hamster kidney (BHK) fibroblast cells contain more than 80% of oleate. Such distinctive compositions suggest quite specific functions, some of which have yet to be revealed.

	rat liver lysosomes	Human liver	Rabbit lung macrophages	Rat uterine stromal cells	Rat testis	BHK cells

Table 1. Fatty acid composition (wt% of the total) of lysobisphosphatidic acid from various tissues.


16:0	3	6	4	6	5	4
18:0	1	5	6	3	3	trace
18:1	5	57	47	30	5	83
18:2	6	10	29	2	1	6
20:4		6	4	4
22:4				6	5
22:5(n-6)	} 4		} 2	3	70
22:5(n-3)	} 4		} 2	8	trace
22:6(n-3)	69	9	1	36	5

Reference	1	1	2	3	3	4

1, Wherrett, J.R. and Huterer, S. Lipids, 8, 531-533 (1973); 2, Huterer, S. and Wherrett, J. J. Lipid Res., 20, 966-973 (1979); 3, Luquain, C. et al., Biochem. J., 351, 795-804 (2000); 4, Brotherus, J. and Renkonen, O. Chem. Phys. Lipids, 13, 11-20 (1974).

Lysobisphosphatidic acid is usually a rather minor component of animal tissues, although it is easily misidentified as phosphatidic acid in many chromatographic systems. It is found at low levels in plasma, where it is associated both with the lipoprotein fractions (40%) and the lipoprotein-deficient compartment (60%).

Scottish thistle However, it is highly enriched in the lysosomes of liver and other tissues, where it can amount to 15% or more of the membrane phospholipids, and it is now recognized as a marker for this organelle. Lysosomes are the digestive organelles of the cell and are rich in hydrolytic enzymes at an acidic pH. Cellular constituents, including excess nutrients, growth factors and foreign antigens are captured by receptors on the cell surface, for uptake and delivery to lysosomes. Within the cell, receptors such as the mannose-6-phosphate receptor bind and divert hydrolytic enzymes from biosynthetic pathways to the lysosomes. These molecules pass through an intermediate heterogeneous set of organelles known as endosomes, which act as a kind of sorting station where the receptors are recycled before the hydrolases and other materials are directed to the lysosomes. There, the hydrolases are activated and the unwanted materials are digested. It is the mature or ‘late’ endosomes and the lysosomes that contain the unique lipid, lysobisphosphatidic acid. Indeed, there appear to be internal membranes of the late endosomes that contain as much as 70% of the phospholipids as lysobisphosphatidic acid.

If the reported presence of lysobisphosphatidic acid in some alkalophilic strains of Bacillus species can be confirmed, this will be the only known exception to the rule that this lipid is strictly of mammalian origin and not present in prokaryotes, yeasts and higher plants.

There is good evidence that lysobisphosphatidic acid is synthesised from phosphatidylglycerol, primarily in the endosomal system. Although the later steps have still to be demonstrated experimentally, the scheme outlined below is believed to be the primary route. In the first step, a phospholipase A₂ removes the fatty acid from position sn-2 of phosphatidylglycerol. In the second, the lysophosphatidylglycerol is acylated on the sn-2’ position of the head group glycerol moiety to yield sn-3:sn-1’ lysobisphosphatidic acid, by means of a transacylase reaction with lysophosphatidylglycerol as both the acyl donor and acyl acceptor. The third step has still to be adequately described but must involve removal of the fatty acid from position sn-1 of the primary glycerol unit and a rearrangement of the phosphoryl ester from the sn-3 to the sn-1 position. Finally position sn-2 of the primary glycerol unit is esterified, probably by a transacylation reaction with another phospholipid as donor (thence the distinctive fatty acid compositions).

Biosynthesis scheme for lysobisphosphatidic acid

Other biosynthetic routes may be possible, but cardiolipin appears to have been ruled out as a potential precursor.

The function of lysobisphosphatidic acid in lysosomes is under active investigation. It may simply have a structural role in developing the complex intraluminal membrane system, aided by a tendency not to form a bilayer. It is a cone-shaped molecule, like cardiolipin, and it encourages fusion of membranes at the pH in the endosomes. Alternatively, its unique stereochemistry means that it is resistant to phospholipases, so it will hinder or prevent self digestion of the lysosomal membranes. For example, it is not touched by the main phospholipases that hydrolyse phosphatidylcholine and phosphatidylethanolamine. The fatty acid constituents may turn over rapidly by transacylation, but the glycerophosphate backbone is stable. A further possibility is that this lipid may associate with specific proteins in membrane domains, functionally similar to rafts.

Scottish thistle It has been suggested that that the characteristic network of lysobisphosphatidic acid-rich membranes contained within multivesicular late endosomes regulates cholesterol transport by acting as a collection and re-distribution point. When lysosomal membranes are incubated with antibodies to lysobisphosphatidic acid, cholesterol tends to accumulate. Lysobisphosphatidic acid is known to greatly stimulate the enzymes involved in the degradation of glycosylceramides, such as the sphingolipid activator proteins like the saposins. In this instance, it may simply function to provide a suitable environment for the interaction of the glycosphingolipid hydrolases and their activator. In addition, it has a dynamic role in the provision of arachidonate for eicosanoid production in alveolar macrophages.

Lysobisphosphatidic acid accumulates in lysosomal sphingolipid storage diseases, such as Niemann-Pick disease, and in drug-induced lipidoses. In these circumstances, its composition tends to change to favour molecular species that contain less of the polyunsaturated components. It is an antigen recognized by autoimmune sera from patients with a rare and poorly understood disease known as antiphospholipid syndrome, so it is obviously a factor in the pathological basis of this illness.

Semilysobisphosphatidic acid, i.e. with only one mole of fatty acid per mole of lipid, is occasionally found in tissues also. In particular, it is concentrated in the Golgi membranes, where the relative amount varies in different regions, but can attain as much as 15% of the total phospholipids in those compartments that are most active biologically. It would not be at all surprising if this lipid were found to have a distinctive role in the Golgi complex, but at the moment this is a matter of speculation.

The fully acylated lipid, bis-phosphatidic acid, has been found in lysosomes from cultured hamster fibroblasts (BHK21 cells).

Modern mass spectrometric methods involving electrospray ionization appear to be well suited to the analysis of lysobisphosphatidic acid, especially when used in conjunction with liquid chromatography.

The Lipid Library

LYSOBISPHOSPHATIDIC ACID (BIS(MONOACYLGLYCERO) PHOSPHATE)

STRUCTURE, OCCURRENCE AND BIOCHEMISTRY

Table 1. Fatty acid composition (wt% of the total) of lysobisphosphatidic acid from various tissues.

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