Abstract
An accelerated degradation of phospholipid is the likely basis of irreversible cell injury in ischemia, and the membranes of the endoplasmic reticulum of the liver are a convenient system with which to study the effect of such a disturbance on the structure and function of cellular membranes. In the present report, electron spin resonance spectroscopy has been used to evaluate changes in the molecular ordering of microsomal membrane phospholipids in the attempt to relate the loss of lipid to alterations in membrane structure. The order parameter, S, was calculated from spectra reflecting the anisotropic motion of 12-doxyl stearic acid incorporated into normal and 3-h ischemic microsomal membranes. Over the temperature range 4-40 degrees C, the molecular order (S) of ischemic membranes was increased by 8-10%. This increase was reproduced in the ordering of the phospholipids in liposomes prepared from total lipid extracts of the same membranes. In contrast, after removal of the neutral lipids, liposomes prepared from phospholipids of ischemic and control membranes had the same molecular order. There were no differences in the phospholipid species of control and ischemic membranes or in the fatty acid composition of the phospholipids. In the neutral lipid fraction of ischemic membranes, however, triglycerides and cholesterol were increased compared to control preparations. There were no free fatty acids. The total cholesterol content of the liver was unchanged after 3 h of ischemia. The cholesterol-to-phospholipid ratio of ischemic membranes, however, was increased by 22% from 0.258 to 0.315 as a consequence of the loss of phospholipid. Addition of cholesterol to the control total lipid extracts to give a cholesterol-to-phospholipid ratio the same as in ischemic membranes resulted in liposomes with order parameters similar to those of liposomes prepared from ischemic total lipids. It is concluded that the degradation of the phospholipids of the microsomal membrane results in a relative increase in the cholesterol-to-phospholipid ratio. This is accompanied, in turn, by an increased molecular order of the residual membrane phospholipids.
Highlights
An accelerated degradation of phospholipid is the Disturbances in membrane function in general and in the likely basis of irreversible cell injury inischemia, and plasma membrane in particular characterize the loss of rethe membranes of the endoplasmic reticulum of the versibility of the cell injury in ischemia
Membranes of the smooth endoplasmic reticulum were isolated from postmitochondrial supernatants of control and ischemic livers by centrifugationin CsClZutilizing the increased density of rough microsomal membranes [13]
ESR Spectra of Spin-labeled ischemic MicrosomalMembranes-The molecular ordering of the phospholipids in the ischemic smooth microsomal membranes was examined with the use of a spin-labeled fatty acid
Summary
Membranes of the smooth endoplasmic reticulum were isolated from postmitochondrial supernatants of control and ischemic livers by centrifugationin CsClZutilizing the increased density of rough microsomal membranes [13]. This agrees with the previously reported accelerated degradation of phospholipid with liver ischemia [8]. ESR spectra (Fig. 1)of control and ischemic membranes labeled with 12-doxy1stearic acid were indicative of rapid anisotropic motion of the label's oxazolidine ring typical of a fatty acid residing within a phospholipid bilayer. The decreased fluidity of ischemic microsomal membranes was reproduced in the ordering of the phospholipids in liposomes prepared from the total lipids extracted from control and ischemic membranes. With liposomes prepared from the lipids of both control and ischemic mem-
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