The ability of human serum albumin (HSA) to bind fatty acids (FA) in multiple sites has been revealed by many studies. Here we detect and characterize nine individual binding sites by two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy of 18-[(13)C]-oleic acid (OA) complexed with HSA. We characterize site-specific FA binding by addition of (i) different FA molar ratios (from 1:1 to 4:1 OA:HSA) to observe the order of filling and occupancy of binding sites; (ii) methyl-β-cyclodextrin, as a FA acceptor, to observe the dissociation of FA; and (iii) drugs (with known binding sites in the crystal structure) to reveal the correspondence of three NMR peaks with sites in the crystal structure. At 1:1 and 2:1 OA:HSA ratios, three sites were shown to fill sequentially. These high-affinity sites were well resolved from additional sites (one medium-affinity and five low-affinity) observed at 3:1 and 4:1 OA:HSA ratios. Methyl-β-cyclodextrin extracted OA from individual sites in the reverse order of filling. FA bound in three low-affinity sites were displaced by drugs shown to bind in crystalline HSA to FA sites 7 and 3 (Sudlow's drug sites I and II, respectively) and FA site 6. With this strategy, 2D NMR spectral analysis permits site-specific characterization of the binding of drugs and FA and provides a sensitive probe of the mutual effects of FA and ligand binding.

Liver fatty acid binding protein (L-FABP), a cytosolic protein most abundant in liver, is associated with intracellular transport of fatty acids, nuclear signaling, and regulation of intracellular lipolysis. Among the members of the intracellular lipid binding protein family, L-FABP is of particular interest as it can i), bind two fatty acid molecules simultaneously and ii), accommodate a variety of bulkier physiological ligands such as bilirubin and fatty acyl CoA. To better understand the promiscuous binding and transport properties of L-FABP, we investigated structure and dynamics of human L-FABP with and without bound ligands by means of heteronuclear NMR. The overall conformation of human L-FABP shows the typical β-clam motif. Binding of two oleic acid (OA) molecules does not alter the protein conformation substantially, but perturbs the chemical shift of certain backbone and side-chain protons that are involved in OA binding according to the structure of the human L-FABP/OA complex. Comparison of the human apo and holo L-FABP structures revealed no evidence for an "open-cap" conformation or a "swivel-back" mechanism of the K90 side chain upon ligand binding, as proposed for rat L-FABP. Instead, we postulate that the lipid binding process in L-FABP is associated with backbone dynamics.

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Storage of lipid in ectopic depots outside of abdominal visceral and subcutaneous stores, including within the pericardium and liver, has been associated with obesity, insulin resistance, and cardiovascular risk. We sought to determine whether anatomically distinct ectopic depots were physiologically correlated and site-specific effects upon cardiovascular function could be identified. Obese subjects (n = 28) with metabolic syndrome but without known atherosclerotic disease and healthy controls (n = 18) underwent magnetic resonance imaging (MRI) and proton MR spectroscopy (MRS) to quantify pericardial and periaortic lipid volumes, cardiac function, aortic compliance, and intrahepatic lipid content. Fasting plasma lipoproteins, glucose, insulin, and free-fatty acids were measured. Pericardial and intrahepatic (P < 0.01) and periaortic (P < 0.05) lipid volumes were increased in obese subjects vs. controls and were strongly and positively correlated (P ≤ 0.01) but independent of BMI (P = NS) among obese subjects. Intrahepatic lipid was associated with insulin resistance (P < 0.01) and triglycerides (P < 0.05), whereas pericardial and periaortic lipid were not (P = NS). Periaortic and pericardial lipid positively correlated to free-fatty acids (P ≤ 0.01) and negatively correlated to high-density lipoprotein (HDL) cholesterol (P < 0.05). Pericardial lipid negatively correlated to cardiac output (P = 0.03) and stroke volume (P = 0.01) but not to left ventricular ejection fraction (P = 0.46). None of the ectopic depots correlated to aortic compliance. In conclusion, ectopic storage of lipid in anatomically distinct depots appeared tightly correlated but independent of body size. Site-specific functional abnormalities were observed for pericardial but not periaortic lipid. These findings underscore the utility of MRI to assess individual differences in ectopic lipid that are not predictable from BMI.