For some applications of phospholipids it is desirable to specifically incorporate fatty acyl groups into the sn-2 position of the molecule, e.g. in order to modify the emulsification properties, to increase the stability against oxidation or to improve the physiological or nutritional value of the phospholipid.
It has been described that exchange of acyl groups in diacylphospholipids can be catalyzed by native or derivatized lipase (JP-A 63-185,391, JP-A 63-105,686). These lipases are known to possess activity against primary alcohols or primary esters, so during phospholipid interesterification a significant fraction of the incorporated fatty acids is incorporated into the sn-1 position of the phospholipids.
It is known from U. Z. Muratova et al., Biochemistry (USSR), 52(7), 919-922 (1988) that purified phospholipase A.sub.2 from rat liver mitochondria membranes catalyzes ester exchange between phospholipid and fatty acid, but practical use of this finding has never been considered. The enzyme is very unstable, especially in purified form, it has a low specific activity as a hydrolytic enzyme, and does not appear amenable to economical production.
H. P. Franck et al., Z. Naturforsch. 23b, 439-448 (1968) reported that under appropriate conditions, snake venom phospholipase A.sub.2 could be used to remove the acyl group from the sn-2 position of a phospholipid (to form lysophospholipid) and then to reinsert the fatty acid. However, J. Goerke et al., Biochim. Biophys. Acta, 248, 245-253 (1971) reported that this finding was due to misinterpretation of results.
Apart from these erroneous conclusions of H. P. Franck et al., commercially available phospolipases A.sub.2 from snake or bee venoms or of pancreatic origin have never been described as being capable of interesterifying phopholipids although these enzymes are very well described with regard to hydrolytic properties, of which for instance their strict specificity towards the sn-2 position of phospholipids are well known.
It is the object of the invention to provide a process for modifying a phospholipid by specific exchange of the acyl group in the sn-2 position, using a readily available enzyme with high specific activity, such that a high degree of incorporation of a desired fatty acid can be obtained.