Human liver fatty acid-binding protein (L-FABP) has been efficiently expressed in Escherichia coli. The cDNA encoding human liver FABP was under the control of T7 RNA polymerase promoter in the expression vector pET-3b. Expression required overnight induction with isopropyl beta-D-thiogalactopyranoside in the presence of the bacterial RNA polymerase inhibitor, rifampicin. The protein could be purified by (NH4)2SO4 fractionation, anion-exchange and gel filtration chromatography, and was recognized by anti-(human L-FABP) antiserum. The binding characteristics of delipidated recombinant human L-FABP and muscle FABP (M-FABP) for fatty acids of different chain length and saturation grade, and for various hydrophobic ligands, were determined by radiochemical analysis and also by fluorescence for L-FABP. The apparent binding affinity of the ligands was calculated by using displacement curves of oleic acid and dansylamino-undecanoic acid (DAUDA). L-FABP showed a preference for the binding of long-chain saturated and unsaturated fatty acids up to C24:1, whereas the M-FABP has a preference for unsaturated fatty acids, especially those with 18 C atoms. L-FABP also binds palmitoyl derivatives and many other hydrophobic ligands--however, generally with a lower affinity than fatty acids. M-FABP binds--besides with fatty acids--only with oestradiol and testosterone with high affinity. Fatty acids with fluorescent reporter groups are also more tightly bound by L-FABP. A direct assay and displacement study of oleic acid gave the same Kd value of DAUDA for L-FABP. Fluorescence enhancement and displacement studies indicate that the binding of fluorescent fatty acids is determined by both the fluorescent reporter group and the acyl carbon chain.
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