The fatty acid composition of phospholipids is an important determinant of 
membrane function. Although the mitochondria play a pivotal role in skeletal 
muscle function, the fatty acid composition of their individual phospholipids 
has not been examined. The purpose of this study was to determine the fatty acid 
profile of each phospholipid in rat skeletal muscle mitochondria and compare it 
with that of the whole muscle. Lipids were extracted from the gastrocnemius 
muscles of 10 Wistar rats, and phospholipids were separated by thin-layer 
chromatography. The fatty acid composition of each phospholipid was then 
determined by gas chromatography. The same procedure was applied to a 
mitochondrial preparation from these muscles. We found that the fatty acid 
composition of the individual mitochondrial phospholipids (phosphatidyl choline, 
phosphatidyl ethanolamine, cardiolipin, phosphatidyl inositol, phosphatidyl 
serine, sphingomyelin, and lysophosphatidyl choline) and of the total 
mitochondrial phospholipids differed markedly (P < 0.05) from the fatty acid 
composition of the corresponding whole muscle phospholipids. Notably, the 
mitochondrial phospholipids had higher percentages of MUFA [13.9 (2.1) vs. 10.3 
(0.9)] and lower percentages of PUFA [34.8 (4.3) vs. 39.5 (5.2)] and n6 fatty 
acids [25.0 (2.5) vs. 27.6 (2.5)]. Overall, the mitochondrial phospholipids had 
a lower unsaturation index than whole muscle phospholipids [135 (20) vs. 161 
(26)]. Because PUFA are susceptible to peroxidation, unlike saturated fatty 
acids and MUFA, we propose that the low polyunsaturation of mitochondrial 
phospholipids is the result of selective pressure toward membranes that are more 
resistant to oxidative damage by reactive oxygen species produced in their 
vicinity. The negative effect of the low polyunsaturation on membrane fluidity 
may be counterbalanced by the higher percentage of MUFA and the known low 
cholesterol content of mitochondrial membranes.