The time course of the hydrolytic action of porcine pancreatic phospholipase A2 on sonicated dimyristoylphosphatidylcholine liposomes in the presence of variable NaCl concentrations has been studied at temperatures between 17 and 36 degrees C; at these temperatures liposomes are in the gel phase. At a NaCl concentration of 10 mM, the hydrolysis shows a small and constant lag period of 6-8 min at all temperatures within this range. As the temperature is raised into the liquid crystalline range, the latency phase lengthens monotonically so that at 36 degrees C it reaches 55 min. An increase in the NaCl concentration to 1 M makes the lag period longer at all temperatures studied, with the exception of the phase transition range (near 24 degrees C); within this temperature range, a small reduction in the lag time is observed. The increase in the length of the latency period at high salt concentrations may be due to screening of the negative surface charge generated by the nascent fatty acid which seems to be essential for the efficient interfacial binding of the enzyme. In the phase transition range of the lamellae, the unfavorable effect of high salt concentrations on the electrostatic binding of the enzyme appears to be overcome by another type of interaction. Recent findings raise the possibility that this interaction could be hydrophobic in nature.

Animals↗

PubMedDate not supplied

The interfacial calcium ion concentration as modulator of the latency phase in the hydrolysis of dimyristoylphosphatidylcholine liposomes by phospholipase A2.

Analysis of the time course of hydrolysis of dimyristoylphosphatidylcholine liposomes catalyzed by porcine pancreatic phospholipase A2 at 18 degrees C shows that, in the presence of 10 mM NaCl, the length of the latency period in the presteady-state phase increases from 3 to 10.5 min when the CaCl2 concentration is reduced from 15 to 1 mM. This inverse dependence of the lag period on calcium ion concentration is seen more readily at 1 M NaCl, where the induction time changes from 13.5 to 42 min by decreasing the concentration of CaCl2 from 15 to 1 mM. To interpret these results, we took into account the small amount of fatty acid that is produced during the latency phases. The fatty acid generates a negative surface electrostatic potential and makes the interfacial concentration of calcium ions different from the concentration in the bulk solvent. Variations in the analytical concentrations of NaCl and CaCl2 affect both the interfacial calcium ion concentration and electrostatic potential, as estimated theoretically from Grahame and Boltzmann equations. According to these estimates, the length of the latency period diminishes with the increase of the interfacial calcium concentration, but does not show any logical dependence on the change in surface electrostatic potential.

Animals↗

PubMedDate not supplied

Calcium ions reverse a latency period induced by bovine serum albumin in the time course of phospholipase A2 action on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine liposomes.

The time course of hydrolysis of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine liposomes in the gel phase catalyzed by porcine pancreatic phospholipase A2 was studied at 1 mM NaCl and variable CaCl2 concentrations, in the presence of delipidated bovine serum albumin. It has been found that the duration of the latency induced by bovine serum albumin shows an inverse dependence with CaCl2 concentration. As we showed previously, the induction of a lag phase by bovine serum albumin is related to its ability to sequester the fatty acid newly released by hydrolysis. Based on this and on our observation that there is an inverse dependence between the length of the latency period and the interfacial calcium ion concentration, it is interpreted that, while a direct effect of bovine serum albumin is the diminution of the liposome negative surface charge density by sequestration of the fatty acid released during hydrolysis, an indirect effect could be the decrease in the surface Ca2+ concentration. This, in turn, should diminish the enzyme binding to the lipid-water interface. The appearance of a latency phase seems to be the final consequence of these events.

1,2-Dipalmitoylphosphatidylcholine↗

PubMedDate not supplied

[Dental fluorosis].

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Fluorides↗