Remodeling of rat hepatocyte phospholipids by selective acyl turnover

• Published in: The Journal of biological chemistry Vol. 266; no. 21; pp. 13690 - 13697
• Main Authors: SCHMID, P. C, JOHNSON, S. B, SCHMID, H. H. O
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Biochemistry and Molecular Biology 25-07-1991
• Subjects: acyl; Analytical, structural and metabolic biochemistry; Animals; Biological and medical sciences; cell membranes; characterization; Fatty Acids - chemistry; Fatty Acids - metabolism; Fundamental and applied biological sciences. Psychology; Glycerolipids, phospholipids; hepatocytes; Lipids; liver; Liver - metabolism; Male; Mass Spectrometry; Other biological molecules; Oxygen Isotopes; Phosphatidylcholines - chemistry; Phosphatidylethanolamines - chemistry; phospholipids; Phospholipids - metabolism; Rats; Rats, Inbred Strains; remodeling; Time Factors; Triglycerides - chemistry; turnover; Composition; Radiolabelling; Rat; Rodentia; Phospholipid; Biosynthesis; Triglyceride; Fatty acids; Molecular species; Vertebrata; Mammalia; Hepatocyte; Turnover
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)92754-7

Acyl turnover of rat hepatocyte phospholipids and triacylglycerols was assessed by incubating the cells in media containing
40% H2(18)O and measuring the time-dependent incorporation of 18O into ester carbonyls by gas chromatography-mass spectrometry
of hydrogenated methyl esters. Incorporation of 18O into 22-carbon acyl groups was low in phosphatidylcholine, phosphatidylinositol,
and phosphatidylserine, whereas in phosphatidylethanolamine, it was about the same as in the other acyl groups. Incorporation
of 18O into individual molecular species of phosphatidylcholine and phosphatidylethanolamine was determined after phospholipase
C hydrolysis, derivatization to dinitrobenzoates, and separation by high-performance liquid chromatography. In most molecular
species, acyl groups at the sn-1 and sn-2 positions became 18O-labeled at drastically different rates, indicating remodeling
through deacylation-reacylation. Molecular species expected to arise de novo from acylation of glycerophosphate exhibited
similar rates of 18O incorporation at the sn-1 and sn-2 positions. The data suggest that hepatocyte phospholipids are continually
synthesized, remodeled by deacylation-reacylation at specific turnover rates up to 10-15%/h, and degraded. This acyl turnover
probably does not involve the majority of intracellular unesterified fatty acids whose 18O incorporation was found to be very
low. In contrast, the oxygens of extracellular unesterified fatty acids were readily exchanged with the media. This exchange
was enzyme-catalyzed, possibly by lipases released into the media from damaged cells. Incorporation of 18O into exogenously
added fatty acids was also rapid and resulted in enhanced uptake of 18O-labeled fatty acids into cellular lipids, primarily
triacylglycerols and phosphatidylcholine, without drastic change of the intracellular free fatty acid pool.