Front Matter

that found for the electrode working with the biocatalysts bound by adsorption toj-

carrageenan. The linearity ranges were 34 mg l–1to 169 mg l–1and 2.1 mg l–1to 42.4

mg l–1, and the minimum detection limits 16.5 mg l–1and 1.05 mg l–1, respectively.

13.5 Summary and outlook

Phospholipases have a vast potential in terms of their application. In the field of

foodstuffs, PLA 2 is of great importance as a biocatalyst for the modification of le-

cithin mixtures on an industrial scale into the corresponding lyso-compounds by

hydrolysis of the acyl chain insn-2 position. Furthermore,sn-2-lyso-PC can be

used as a starting material in esterification reactions for the synthesis of phospho-

lipids with a new acyl chain composition. PLA 1 , though not yet available commer-

cially, may be substituted by a number of lipases, again opening up new routes of

synthesis. Interesting and actual examples are the lipase-catalyzed incorporation of

PUFAs into naturally occurring phospholipids in the presence of fish oil as substrate

for the manufacture of diets, or the use of immobilized PLA 2 in the treatment of

hypercholesterolemia. Among the known phospholipases, PLD is the most versatile

with respect to substrate specificity and ability to catalyze transphosphatidylation

reactions. Apart from its application in the large-scale production of valuable, albeit

rarely occurring phospholipids such as PG and PS, a multitude of new natural and

unnatural lipids can be prepared with PLD (both in combination with other phos-

pholipases and with lipases which show phospholipase activity) by the PLD-cata-

lyzed polar head group exchange, with possible applications in the fields of food

additives, pharmaceuticals, and cosmetics.

Although the application of phospholipases in their immobilized state offers many

advantages, at present these enzymes are employed mainly in solution. As the pro-

blems concerning the immobilization of phospholipases – and especially of PLA 2 –

have been overcome, and in the knowledge that stable phospholipases (for example

PLDs) of bacterial and other origin have been found in the recent past, it should be

expected that the preparation and application of immobilized phospholipases will be

the subject of much greater attention in the future. Servi (1999) proposed the pro-

duction of organic phosphate and diacylglycerol using PLD, alkaline phosphatase

and PLC immobilized in hollow-fiber membrane bioreactors (see Figure 1). Several

similar arrangements of different phospholipases immobilized in such reactors or

bound to suitable solid supports and used in fixed-bed reactors are conceivable

for the continuous synthesis of defined phospholipids. In addition, with the modular

construction of such a small plant it should be possible to meet the requirements for

the production of a particular phospholipid simply by exchanging one or more phos-

pholipase-containing units.

The current situation in this field may be characterized by the fact that problems

concerning the immobilization of phospholipases, which itself results in highly ac-

tive catalysts, can be regarded as being solved. In contrast, with very few exceptions

immobilized phospholipases exhibit low long-term stabilities, and this is especially

true for phospholipase D. In the future, a major point of emphasis will be placed on

the synthesis of immobilized phospholipases as being suitable for repeated use over a

13.5 Summary and outlook 287