Two prophospholipase A fractions, having a similar amino acid composition but different isoelectric points, have been isolated from ox pancreas. Upon tryptic activation both zymogens release an identical heptapeptide located at the N‐terminal part of the chain giving rise to the formation of two isoenzymes.
From sheep pancreas also two prophospholipases A were purified. The latter zymogens, however, differ only in the amino acid composition of the activation peptide and they produce the same active enzyme upon tryptic activation.
The ruminant phospholipases show a strong resemblance in amino acid composition and kinetic behaviour, but they differ markedly from the porcine enzyme as regards pH optimum, specific activity and affinity for lipid/water interfaces.
Egg yolk fermented with pancreatic phospholipase A2 has been shown to be a more potent emulsifier for mayonnaise than untreated egg yolk. The mayonnaise withstood heating at 100°C for 30 min without the emulsion breaking. The treatment also effected a considerable thickening of the product. Animal feeding trials with lysolecithin and fermented egg yolk suggested that the enzymic hydrolysis does not pose any toxicological hazards.
Upon tryptic activation of pure human prophospholipase A2, a heptapeptide is released from the N‐terminal part of the protein yielding active phospholipase A2 (EC 3.1.1.4). Both the kinetics of the activation process and the amino acid sequence of the activation peptide strongly resemble those of pancreatic zymogens of other mammalian sources.
The kinetic properties of human phospholipase A2 and its zymogen are compared with those of the corresponding porcine enzyme using substrates present as micelles, molecular dispersed solutions or as monomolecular surface films. The most obvious difference between the human and porcine phospholipase A2 is the low enzyme activity of the former protein at pH 8.0 as compared to pH 6.0, both against micellar and monomeric substrates. Neither the Ca2+ binding properties nor the inhibition kinetics of the human enzyme using haloketones can easily explain this different pH optimum.
The sequence analysis of the N‐terminal region of the first 40 residues is reported.
During collagen-induced blood platelet aggregation, arachidonic acid is set free from membrane phospholipids and subsequently converted into 12-hydroxyeicosatetraenoic acid by arachidonate lipoxygenase and into thromboxane A2, 12-hydroxyheptadecatrienoic acid (HETE) and malondialdehyde by cyclooxygenase and thromboxane synthase. Lipoxygenase and cyclooxygenase have optimal activity at neutral to basic pH, while the thromboxane synthase is pH-independent between 5 and 9. These enzymes are membrane-bound. The cyclooxygenase is rapidly inactivated upon membrane disruption by nonionic detergents or phospholipid degradation with phospholipase A2. It was found that platelet phospholipase A2 preferentially splits off fatty acid with four double bonds. Eicosatetraynoic acid was used to investigate the physiological function of the arachidonate lipoxygenase during collagen-induced aggregation of rat blood platelets. This fatty acid is a more efficient inhibitor of lipoxygenase than of cyclooxygenase. At an inhibitor concentration of 0.6 microgram/ml, platelet aggreation, 12-hydroxyeicosatetraenoic acid production as well as 15-hydroxytryptamine release are completely inhibited, while there is an apparent stimulation of the cyclooxygenase. These results indicate that arachidonate lipoxygenase is essential for irreversible blood platelet aggregation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.