Effects of fluorophore structure and hydrophobicity on the uptake and metabolism of fluorescent lipid analogs

Cellular transport and metabolism of fatty acids are integral components of lipid metabolism, but the mechanisms and regulation involved are poorly understood. A variety of commercially available fluorescent analogs of fatty acids are potentially useful probes for the study of lipid metabolism by such techniques as cell sorting and fluorescence microscopy. We have screened a series of fluorescent fatty acids to identify analogs that would reliably simulate the metabolic behavior of natural fatty acids; i.e., similar kinetics of transport, of intracellular movement, and of metabolic fate. The metaboalic behavior of these analogs was compared with those of some naturally occurring fatty acids in HepG2 cells, which are a good model of some aspects of hepatic function. Fluorescent analogs containing polar fluorophores yielded the lowest rates of cellular uptake and conversion to acylated lipid products. Similarly, fluorescent analogs with the fluorophore located near the carboxylic acid group were poorly metabolized. Fatty acid analogs containing anthracene or pyrene at the n-terminus of the acyl chain were the most extensively incorporated into cellular lipids. The types and amounts of labeled lipid products formed from these analogs and from natural fatty acids were similar. Pyrene-labeled analogs have spectral properties that can be measured fluorometrically at very low concentrations. Therefore, we compared the cellular metabolism of 12-(1-pyrenyl)dodecanoic acid with those of palmitic and oleic acids. These three fatty acids were similar with respect to cellular uptake, the identities of the products formed, and inhibition of lipid synthesis by α-bromomyristic acid. These data show 12-(1-pyrenyl)dodecanoic acid is a reliable metabolic analog of some natural fatty acids that can be used to follow fatty acid transport by fluorescence microscopy.