Front Matter

derived fromsn-glycerophosphates. Fatty acid residues are attached to C 1 or C 2 or

both C-atoms of the glycerol backbone that may vary considerably with respect to the

degree of saturation and chain length. The individual species of the glycerophospho-

lipids differ in the nature of the alcohol esterified to the phosphate group at C 3 .In

case of the amino alcohol choline, the correct name of the resulting compound is 1,2-

diacyl-sn-glycero-3-phosphatidylcholine, normally abbreviated to phosphatidylcho-

line (PC). Other important phospholipids are phosphatidylserine (PS), phosphati-

dylethanolamine (PE), phosphatidylinositol (PI), phosphatidylglycerol (PG), and

phosphatidic acid (PA). The trivial term lecithin is the name for a mixture containing

neutral lipids, polar lipids, and carbohydrates and which is used as a food additive in

Europe and the USA (Schneider, 1997a).

Although phospholipids occur in most membranes of living organisms, vegetable

oil seeds (soybean, rape seed, sunflower seed) are mainly used for the recovery of

phospholipid mixtures, produced as byproducts from vegetable oil refining pro-

cesses. Egg yolk and brain are occasional sources for phospholipids used for phar-

maceutical and cosmetic preparations. The composition of the phospholipid mix-

tures, as well as that of the fatty acid residues attached to the glycerol backbone

of the phospholipid molecules, are characteristic of the respective source.

Naturally occurring phospholipid mixtures are used directly as crude lecithins, or

after processing with methods such as solvent fractionation, deoiling, and chromato-

graphic purification. They may also be modified chemically (hydroxylation, hydro-

genation, acetylation) or by using enzymes, and a multitude of possible applications

results from this. Due to their amphiphilic nature, phospholipids are used as emul-

sifying agents and surfactants in the food industry, as ingredients in cosmetics (ly-

posomes) or for the formulation of pharmaceutical products (drug release). Clinical

studies have produced evidence for a cholesterol-lowering potential of PC, and PS

has proved to be an efficacious drug in the treatment of age-related cognitive dete-

rioration. A comprehensive survey of the applications of phospholipids has been

published by Schneider (1997b).

Glycerophospholipids are substrates forphospholipaseswhich are capable of

modifying them by attacking the molecules at definite points. Phospholipase A 1

(phosphatidylcholine 1-acyl-hydrolase, EC 3.1.1.32; PLA 1 ) and phospholipase A 2

(phosphatidylcholine 2-acyl-hydrolase; PLA 2 ) catalyze the hydrolysis of the carbox-

yl ester groups at positionssn-1 andsn-2, respectively, leading to the corresponding

lyso-phospholipids. Two phospholipase C species (EC 3.1.4.3 and EC 3.1.4.10) cat-

alyzing the cleavage of the phosphate ester bond at C 3 to form diacyl glycerols and

organic phosphate compounds are known – PLC (EC 3.1.4.10) accepts phosphati-

dylinositol as substrate. In the presence of phospholipase D (EC 3.1.4.4), the ester

bond between the phosphate residue and the terminal polar head group is hydrolyzed

to form a phosphatidic acid (PA).

Depending on the source, PLA 1 and PLA 2 may also show a tendency to catalyze a

transesterification reaction, though hydrolysis reactions are normally more efficient.

In contrast, phospholipase D (PLD) is unique with respect to its ability to catalyze

transphosphatidylation reactions. This polar head group exchange was the basis for

the synthesis of many new phospholipids in the recent past (Bornscheuer and Ka-

zlauskas, 1999). In this connection it must be taken into account that – depending on

the reaction conditions and on the source of the applied PLD – this reaction competes

264 13 Preparation and Application of Immobilized Phospholipases