nent. The most important sphingophospholipid is sphingomyelin, bearing ceramide
and choline as ester components of phosphoric acid.
Corresponding to the structure of the diacylglycerophospholipids, several phos-
pholipases are known which differ in their specificity according to the position of the
bond attacked in the phospholipid molecule. As shown in Figure 2, phospholipase A 1
(PLA 1 ) catalyzes the hydrolysis of the ester bond insn-1 position, and phospholipase
A 2 (PLA 2 ) that insn-2 position. Phospholipase C (PLC) catalyzes the hydrolysis of
the phosphodiester bond at the glycerol side, and phospholipase D (PLD) that at the
polar side. Some special types of these enzymes possess an additional specificity
concerning the polar alcoholic component, such as the phosphatidylinositol (PI)-
specific PLC (PI-PLC) or the glycosylphosphatidylinositol (GPI)-specific PLD.
The latter cleaves the GPI anchor with which some proteins are covalently con-
nected and inserted into the plasma membrane. Phospholipases hydrolyzing glycer-
ophospholipids in both thesn-1 and thesn-2 position are termed phospholipase B
(Saito et al., 1991; Oishi et al., 1996; 1999; Merckel et al., 1999). Other related
enzymes are lysophospholipases specific for the hydrolysis of 2-lysophospholipids
or 1-lysophospholipids (Karasawa and Nojima, 1991; Wang et al., 1997; reviewed in
Waite, 1987). Finally, sphingomyelinase is to be regarded as a phospholipase similar
to PLC, hydrolyzing sphingomyelin toN-acylsphingosine and choline phosphate
(Ikezawa et al., 1978; Levade and Jaffre ́zou, 1999).
In addition to the hydrolytic activity, some phospholipases catalyze transesterifi-
cation reactions. Best-known for this property is PLD (see Sections 12.3 and 12.4),
although the physiological function of this activity has not yet been elucidated (Yu et
al., 1996).
12.2 Sources of phospholipases
Corresponding to their importance in the metabolism of phospholipids, phospholi-
pases are widespread among prokaryotes and eukaryotes. In recent years, intensive
research into phospholipases, particularly into PLA 2 , PLC, and PLD, has been com-
menced since their significance in several cell regulating processes such as signal
12.2 Sources of phospholipases 221
Figure 2. Action of different phospholipases on 1,2-diacyl-sn-glycerophosphocholine.