Are Lipid Phase Transitions Responsible for Chilling Damage in Human Platelets?

Abstract

In previous studies we have proposed that the well-known chilling-induced activation of human blood platelets can be ascribed at least in part to a thermotropic phase transition in membrane lipids. The evidence that this is the case is reviewed and amplified in this review, followed by an examination of the available physical data concerning phase transitions in lipid mixtures that mimic the mixture found in platelet membranes. Assuming complete mixing at all temperatures and equal contributions of the members of the mixture to the phase transition, the lipid mixture found in platelets should give values forTmranging from about 1°C to about 16°C, depending on the isomers present in the mixture. (The former value is not in agreement with the observedTm, but the latter is in excellent agreement.) However, examination of the phase diagram for a binary pair of lipids found in platelet membranes shows that ideal mixing almost certainly does not occur; instead of a linear phase diagram, a convex one was obtained. This shape for the phase diagram, which would displaceTmto an unexpectedly elevated temperature, is in agreement with previously published phase diagrams for mixtures of this type. The prediction, based on thermodynamic properties of lipids found in the platelets, is thatTmwill be displaced upward in more complex mixtures of the composition found in platelets, a prediction that requires experimental testing.