Physical properties of the lipid phase of membranes from cultured animal cells

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Abstract
ESR analysis of membranes from cultured animal cells reveals a more complex lipid phase behavior than that displayed by ideal binary lipid systems. When endoplasmic reticulum membranes from LM cells are spin labeled with a nitroxide derivative of decane, 5N10, and scanned by ESR at 1° C‐intervals, the partitioning of 5N10 between the hydrocarbon and aqueous portions of the membrane suspension undergoes thermotropic changes at characteristic temperatures of 9°, 16°, 22°, 32°, and 38° C. Lipids extracted from these same membranes, however, exhibit only two characteristic temperatures, 16° and 35° C, and in this respect resemble binary lipid systems. The phase behavior of lipids in animal cell membranes is suggestive of an organized distribution of lipid which is disrupted by extraction. In support of this, mathematical treatment of the partitioning data indicates that four of these characteristic temperatures can define the boundaries (i.e., the t1 and th ) of two independent phase transitions in endoplasmic reticulum membranes. These results are similar to those of a physical treatment of data from plasma membranes of both mouse and chick cells in which the two monolayers appear to exist as independent physical entities with different physical properties. The most probable phase boundaries for the two monolayers of the endoplasmic reticulum membranes studied here are 16° and 32° C for one monolayer and 22° and 38° C for the other.