Serum albumin binds beta- and alpha-monoolein in vitro

We investigated the interaction of bovine serum albumin (BSA) and monoolein (MO) and estimated the number of BSA binding sites for the alpha- and beta-isomers of MO. The turbidity of increasing concentrations of aqueous dispersions of alpha-MO and beta-MO in the presence and absence of BSA was measured in triplicate by absorption spectrophotometry. Aqueous dispersions of [13C(1)]MO and [13C(1)]MO/BSA mixtures at molar ratios of 1:1, 3:1 and 5:1 were analyzed in duplicate by [13C]nuclear magnetic resonance (NMR) at pH 7.4 and 36 degrees C. BSA bound significantly more beta-MO than alpha-MO at 15 min: 5.4 +/- 0.42 and 3.3 +/- 0.60 mol MO/mol BSA, respectively (P: < 0.05). [13C]NMR spectra of the 1:1 molar ratio of [13C(1)]MO /BSA exhibited a single carbonyl peak at 175.19 ppm, whereas spectra of 3:1 and 5:1 molar ratios exhibited three peaks between 172 and 174 (ppm), each distinct from carbonyl resonances of either [13C(1)]MO dispersed in water, 176.72 (ppm) or BSA alone. The intensities of individual peaks, but not their chemical shift values, varied between 3:1 and 5:1 molar ratios, indicating that BSA has at least three MO binding sites and may bind up to five molecules of MO per molecule. This study confirms that serum albumin binds MO in vitro and supports the theory that albumin transports monoglycerides produced by lipoprotein lipase hydrolysis of triglyceride.