The yeast genome contains two genes, designated as PLB2 and PLB3, that are 67% and 62% identical, respectively, to PLB1, which codes for a phospholipase B/lysophospholipase in yeast (Lee, S. K., Patton, J. L., Fido, M., Hines, L. K., Kohlwein, S. D., Paltauf, F., Henry, S. A., and Levin, D. E. (1994) J. Biol. Chem. 269, 19725-19730). Deletion and overexpression studies and in vivo and in vitro activity measurements suggest that both genes indeed code for phospholipases B/lysophospholipases. In cell free extracts of a plb1 plb2 plb3 triple mutant, no phospholipase B activity was detectable. Upon overexpression of PLB2 in a plb1 plb3 mutant background, phospholipase B activity was detectable in the plasma membrane, periplasmic space extracts and the culture supernatant. Similar to Plb1p, Plb2p appears to accept all major phospholipid classes, with a preference for acidic phospholipids including phosphatidylinositol 3,4-bisphosphate and phosphatidic acid. Consistent with a function as an extracellular lysophospholipase, PLB2 overexpression conferred resistance to lyso-phosphatidylcholine. Deletion of Plb2p function had no effect on glycerophosphoinositol or glycerophosphocholine release in vivo, in contrast to a deletion of Plb3p function, which resulted in a 50% reduction of phosphatidylinositol breakdown and glycerophosphoinositol release from the cells. In vitro, Plb3p hydrolyzes only phosphatidylinositol and phosphatidylserine and, to a lesser extent, their lysoanalogs. Plb3p activity in a plb1 plb2 mutant background was observed in periplasmic space extracts. Both Plb3p and Plb2p display transacylase activity in vitro, in the presence or absence, respectively, of detergent.Phospholipases B catalyze the hydrolytic cleavage of both acylester bonds of glycerophospholipids. Products of phospholipase B activity are fatty acids and water-soluble glycerophosphodiesters. In yeast, soluble degradation products are released to some extent into the culture medium, and are thus an indicator of cellular phospholipase B activity. The only acylester-hydrolyzing enzyme from yeast characterized so far at the molecular level is a phospholipase B encoded by the PLB1 gene (1). The highly glycosylated enzyme of about 220 kDa (73 kDa for the protein part, predicted from the sequence) is enriched in the yeast plasma membrane but was also found in the periplasmic space and in the culture supernatant. The lysophospholipase activity of Plb1p greatly exceeds the activity catalyzing the first step of hydrolysis; thus, lyso-phospholipids do not accumulate as intermediate products of Plb1p activity (2-4). In addition, this enzyme has transacylase activity, catalyzing the synthesis of phosphatidylcholine (PtdCho) 1 from two molecules of lyso-phosphatidylcholine. The physiological function of yeast phospholipase B is still unclear; neither disruption of the PLB1 gene nor its overexpression result in detectable growth phenotypes. However, the amount of glycerophosphocholine (GroPCho) and glycerophosphoethanolamine released into the culture su...

No abstract

We synthesized enantiomeric 1-O-alkyl-2,3-diacyl-sn-glycerol and 3-O-alkyl-1,2-diacyl-sn-glycerol containing pyrene as a fluorescent reporter and the trinitrophenylamino residue as a fluorescence quencher; both reporter groups were covalently bound to the omega end of the acyl chains at positions sn-2 and sn-3(1), respectively. The fluorescence of the intact substrate molecules was very low. Chemical or enzymic release of the fatty acyl chains lead to fluorescence dequenching. The rate of lipolysis could be measured from the time-dependent increase in fluorescence intensity. We used the respective substrates for the continuous determination of activity and stereopreference of four different microbial lipases from Chromobacterium viscosum, Candida rugosa, Pseudomonas sp., Rhizopus arrhizus, as well as cutinase from Fusarium solani and lipoprotein lipase from bovine milk. The stereopreference of the lipases depended, in general, on how the substrate was solubilized in the reaction medium. All lipases under investigation preferentially hydrolysed the sn-1 acyl ester bond, if the lipid analog was dispersed in albumin-containing Tris/HCl buffer in the absence of detergent or organic solvent. In mixtures of 1:1 (by vol.) water/ethanol, the enzymes showed higher activity toward the sn-3 acyl ester bond, except for lipoprotein lipase which preferred the sn-1 acyl isomer under all conditions tested. Different stereopreferences were observed with the different lipases if the substrate was solubilized by amphiphiles (micelles of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate). C. rugosa lipase and F. solani cutinase showed high stereopreference for the sn-3 acyl ester, whereas Pseudomonas sp. lipase and C. viscosum lipase hydrolysed both enantiomers at similar rates. From spectroscopic studies, it can be inferred that the conformation of the fluorescent lipids is probably similar in water, mixtures of water and organic solvents, and in micelles. The possible effects of reaction conditions on substrate accessibility and enzyme conformation on stereoselectivity of the respective lipases are discussed.

No abstract

Fluorescent triacylglycerol analogs were synthesized as covalent inhibitors of lipase activity. The respective 1(3),2-O-dialkylglycero-3(1)-alkyl-phosphonic acid p-nitrophenyl esters contain a fluorescent pyrenealkyl chain and a long-chain alkyl residue bound to the sn-2 and sn-1(3) positions of glycerol, respectively. The phosphonic acid p-nitrophenyl ester bond is susceptible to nucleophilic substitution by the active serine residue in the catalytic triad of a lipase, leading to inactivation of the enzyme. The fluorescent dialkylglycerophosphonates contain two chiral centers, the sn-2 carbon of glycerol and the phosphorus atom. The (1-O-hexadecyl-2-O-pyrenedecyl-sn-glycero)-O-(p-nitrophenyl)-n-hexyl-phosphonate, first peak during HPLC separation and the (3-O-hexadecyl-2-O-pyrenedecylsn-glycero)-O-(p-nitrophenyl)-n-hexyl-phosphonate, second peak during HPLC separation were found to be potent lipase inhibitors. After incubation of an equimolar amount of these isomers with lipase from Rhizopus oryzae complete inactivation was observed. Stable conjugates containing a 1 : 1 molar ratio of lipid to protein were formed. The spatial proximity of the fluorescently labeled sn-2 alkyl chain of the inhibitor and tryptophan residues of the lipase was assessed by fluorescence resonance energy transfer. The extent of tryptophan fluorescence quenching and the concomitant increase in pyrene fluorescence upon excitation of lipase tryptophans was found to be similar for the above-mentioned isomers. Thus, the (labeled) sn-2 alkyl chains of a triacylglycerol analog are likely to interact with the same binding site of the R. oryzae lipase, irrespective of their steric configuration. However, it was shown that the extent of resonance energy transfer is strongly influenced by the reaction medium, indicating conformational changes of the lipase in different environments.Keywords: fluorogenic triacylglycerol analogs; microbial lipases; phosphonate inhibitors; pyrene and tryptophan fluorescence; resonance energy transfer.In many cases the lipase-catalyzed hydrolysis of triacylglycerols is a stereoselective process, the extent of stereoselectivity depending on the enzyme [1], the substrate [1,2], the solvent [3±7] and the physicochemical properties of these systems. For example, studies on monolayers at the air±water interface have demonstrated that lipase activity and stereoselectivity are influenced in opposite ways, depending on the surface pressure applied [8]. Accordingly, lipase function depends critically on the reaction medium, which influences the structure of the protein and its interaction with the substrate. Knowledge about molecular mechanisms of protein±lipid interaction is essential for an understanding of lipase stereoselectivity. X-ray crystallography studies on lipase±inhibitor complexes provided detailed information about lipase±substrate interaction at a submolecular level [9±13]. However, to date no direct experimental evidence is available about enzyme±substrate interaction in solution.Inhibitors are useful tools fo...