Methods for kinetic analysis of fatty acid incorporation and turnover in vivo: A steady-state kinetic radiotracer approach

For many years, fatty acids and phospholipids were thought to be of little biological importance, serving only as a source of energy and a primary component of the cellular membrane. These concepts were largely supported by early kinetic analyses measuring the turnover of lipid in cellular membranes in which the ef uxes of radiolabeled components were measured in vivo [1,2]. These analyses suggested that the rates of turnover of fatty acids and phospholipids in biological membranes occurred on the order of days to weeks. The seemingly long half-lives of membrane components lead to a paradox, which suggested that biological membranes were metabolically inactive despite containing known precursors to important signaling molecules. At this time, known lipid-derived signaling components included the polyphosphoinositides that coupled to extracellular signaling receptors [3], arachidonic acid (20:4n-6) that was shown to be the biosynthetic precursor of prostaglandins whose formation is involved in multiple in ammatory processes [4], and platelet-activating factor, the rst phospholipid metabolite to demonstrate known biological activity [5]. Since then, it is greatly appreciated that lipid metabolism is closely related to multiple signaling pathways distinct from its function either as a source of energy or its role as a simple constituent of the unit membrane [6].