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F. in Nucleic Acid and Protein Sequence Analysis: A Practical Approach TALKING POINT (Bishop, M. J. and Rawlings, C. J., eds), IRL Press at OUP (in press) MARK BORODOVSKY School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA. EUGENE V. KOONIN AND KENNETH E. RUDD National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA. HEPATIC FATrY ACID metabolism has been studied largely through the use of in vitro preparations, such as isolated hepatocytes and the isolated perfused liver. Despite proving extremely useful for studying the effects of individual hormones and the role of specific post- receptor events, such preparations have serious drawbacks. For example, isolated hepatocytes lose the cell po- larity and zonation that exist within the intact liver, while in the perfused liver, parameters such as perfusate flow rate, pressure and efficiency Of oxygenation all determine the balance of fatty acid metabolism i. As with many in vitro sys- tems, perhaps the most serious draw- backs arise through our inability to mimic the infinite combinations of sub- strate, hormonal and neuroendocrine inputs that exist in vivo. Consequently, several controversies have arisen over which system (perfused liver or isolated hepatocytes) most closely resembles the situation in vivo. It was to overcome such problems, specifically with respect to the intra- hepatic partitioning of fatty acids between their different metabolic fates in the liver, that a method was devised 2 to label hepatic fatty acids selectively in vivo. Its development has allowed the noninvasive study of fatty acid par- titioning during metabolic transitions characterized by rapid changes in liver metabolism 3,4. Selective labelling of hepaticfatty acids in vivo The major obstacle to the study of liver-specific fatty acid m.,~,~,abolism in V. A. Zammit and A. M. B. Molr are at the Departmentof Biochemistryand Molecular Biology, HannahResearchInstitute, Ayr,UK KA6 5HL Monitoring the partitioning of hepatic fatty acids in vivo: keeping track of control Victor A. Zammit and Alison M. B. Moir Noninvasive labelling of hepatic fatty acids in conscious, unrestrained rats shows how liver lipid metabolism responds acutely to physiological pertur- bations, such as the starved-to-refed transition. The speed with which the liver switches from fatty acid oxidation to esterification varies widely according to the requirement of the animal for continued synthesis of glu- cose 6-phosphate from three-carbon precursors. Ingestion of a meal also provides a strong signal for the diversion of fatty acids away from triacyl- glycerol synthesis and secretion, but insulin may only play an indirect role in this effect. vivo is the fact that most tissues can metabolize free fatty acids (FFA) di- rectly. Consequently, only a minor portion of any dose of labelled FFA in- jected into an animal ends up in the liver. Moreover, this occurs over a con- siderable length of time, such that se- cretion and re-uptake of labelled products of liver metabolism make quantitative studies impossible over a physiologi- cally relevant timescale. Any method that aims to deliver fatty acids selec- tively to the liver and pulse-label its fatty acid pool(s) has to do so very rapidly and in vast preference to the pool(s) in any other tissue, such that the metabolic products that reach the circulation are known to be of hepatic origin. The method devised to achieve this is to 'encapsulate' the labelled fatty acyl moiety within particles that interact rapidly and specifically with hepatocytes, and from which the label does not escape into the circulation and is not abstracted by other tissues. These conditions are met through the use of cholesteryl esters of fatty acids (labelled in their acyl moiety) incorpor- ated within the hydrophobic core of apolipoprotein C (apoC)-depleted very low density lipoprotein (VLDL)- and chylomicron-remnants. The particles are subsequently injected intravenously into rats chronically implanted with a jugular cannula. Our studies 5 show that cholesteryl esters in these particles are taken up preferentially by the liver before the irreversible uptake of the lipoprotein particles themselves. This effect is presumed to be due to the transfer of cholesteryl ester directly into hepatocytes during the multiple © 1994, Elsevier Science Ltd 0968-0004/94/$07.00 3~l.3