Hydrolysis of a fluorescent phospholipid substrate by phospholipase A2 and lipoprotein lipase

The fluorescent phospholipid 1-acyl-2-6-(7-nitro-2,1,3benzoxadiazol-4-yl)amino]-caproyl]phosphatidylcholine (C₆-NBD-PC) was used as a substrate for porcine pancreatic phospholipase A₂ (PA₂) and bovine milk lipoprotein lipase (LpL). Hydrolysis of C₆-NBD-PC by either enzyme resulted in a greater than 50-fold fluorescence enhancement with no shift in the emission maximum at 540 nm; Ca⁺⁺ was required for PA₂ catalysis. Identification of the products of hydrolysis showed cleavage at the $\text{sn}$-1 and $\text{sn}$-2 positions for LpL and PA₂, respectively. For PA₂, but not for LpL, there was a marked enhancement of enzyme catalysis at lipid concentrations above the critical micellar concentration of the lipid. Furthermore, apolipoprotein C-II, the activator protein of LpL for long-chain fatty acyl substrates, did not enhance the rate of catalysis of the water-soluble fluorescent phospholipid for either enzyme.