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Degradation of pyrene-labelled phospholipids by lysosomal phospholipases in vitro. Dependence of degradation on the length and position of the labelled and unlabelled acyl chains.

The hydrolysis of pyrenylacyl phosphatidylcholines (PyrnPCs)(n indicates the number of aliphatic carbons in the pyrene-chain) by crude lysosomal phospholipases in vitro was investigated. PyrnPCs consist of several sets in which the length of the pyrene-labelled or the unlabelled acyl chain, linked to the sn-1 or sn-2 position, was systematically varied. Lysophosphatidylcholine and fatty acid were the only fluorescent breakdown products detected, thus indicating that PyrnPCs were degraded by A-type phospholipases and lysophospholipases. Of these, mainly A1-type phospholipases appear to be involved, as determined from the relative amounts of labelled fatty acid and lysolipid released from the positional isomers. Based on the effects of the length and position of the pyrene-labelled and unlabelled chains it is suggested that (1) the lysosomal A-type phospholipases acting on PyrnPCs recognize the carboxy-terminal part of the lipid acyl chains and (2) the relevant part of the binding site is relatively narrow. Thus phospholipids with added bulk in the corresponding region, such as those that are peroxidized and polymerized, may not be good substrates for the lysosomal phospholipases mentioned. The impaired hydrolysis of the most hydrophobic PyrnPCs indicates that lysosomal phospholipases may not be able to penetrate significantly into the substrate interphase, but upward movement of the lipid may be required for efficient hydrolysis. Finally, the rate of hydrolysis of many pyrenyl derivatives was found to be comparable to that of a natural phosphatidylcholine species, both in micelles and in lipoprotein particles, indicating that these derivatives can be used as faithful reporters of lysosomal degradation of natural lipids in vivo and in vitro.