Food Lipids: Chemistry, Nutrition, and Biotechnology

phatidylinositol (PI) and diacylglycerol and glyceroldiphosphate are formed from
diphosphatidylglycerol. Total hydrolysis into each of the component parts of all phospholipids
can be accomplished by strong acid (HCl, H 2SO 4) catalysis in 6 N aqueous
or 5–10% methanolic solutions [23]. Kinetics and mechanism for hydrolysis in 2 N
HCl at 120�C have been described by DeKoning and McMullan [24]. Deacylation
occurs first, followed by formation of a cyclic phosphate triester as an intermediate
to cyclic glycerophosphate and choline. Eventually an equilibrium mixture of �- and
�-glycerophosphates is formed.
Mild alkaline hydrolysis of ester bonds in phospholipids at 37�C (0.025–0.1
M NaOH in methanolic or ethanolic solutions) leads to fatty acids and glycerophosphates.
In contrast, phosphosphingolipids are not affected unless subjected to strong
alkaline conditions. Some selectivity is seen in the susceptibility of phosphoglycerides
to hydrolyze with diacyl > alk-1-enyl, acyl > alkyl, acyl. With more vigorous
alkaline hydrolysis, the glycerophosphates are apt to undergo further hydrolysis because
the phosphoester bond linking the hydrophilic component to the phospholipid
moiety is not stable enough under alkaline conditions and splits, yielding a cyclic
phosphate. When the cycle opens up, it gives a 1:1 mixture of 2- and 3-glycerophosphates.
Both state of aggregation and specific polar group have been shown to affect
the reaction rates for alkaline hydrolysis of phospholipids [25]. Higher activation
energies were observed for hydrolysis of phospholipids in membrane vesicles than
when phospholipids were present as monomers or Triton X-100 micelles. Alkaline
hydrolysis of PC, on the other hand, was three times faster than hydrolysis of PE.
B. Enzymatic Hydrolysis
Selective hydrolysis of glycerophospholipids can be achieved by the application of
phospholipases. One beneficial aspect to application of phospholipase is improved
emulsifying properties to a PC mixture [26]. Unfortunately, while these enzymes
may be isolated from a variety of sources, in general they are expensive.
Several phospholipases exist differing in their preferential site of attack. The
ester linkage between the glycerol backbone and the phosphoryl group is hydrolyzed
by phospholipase C while the ester linkage on the other side of the phosphoryl group
is hydrolyzed by phospholipase D. Hydrolysis of the acyl groups at the sn-1 and sn-
2 position of phospholipids is carried out by phospholipases A1 and A 2, respectively.
While phospholipase A 2 binding to membrane phospholipids has been enhanced
10-fold by the presence of calcium [27], membrane surface electrostatics
dominated phospholipase A 2 binding and activity in the absence of calcium [28]. A
highly cationic enzyme (pI > 10.5), phospholipase A 2, has a marked preference for
anionic phospholipid interfaces. Thus, phosphatidic acid and palmitic acid promoted
the binding of phospholipase A 2 to the bilayer surface [28,29]. Perturbations and a
loosening of the structure associated with the presence of these hydrolysis products
were suggested as the properties contributing to enhanced binding [30]. The presence
of phospholipid hydroperoxides has also facilitated enhanced binding of phospholipases
through a similar mechanism [31].
VII. HYDROGENATION OF PHOSPHOLIPIDS
Hydrogenation of fats involves the addition of hydrogen to double bonds in the
chains of fatty acids. While hydrogenation is more typically applied to triacylglyc-
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