Lactic acid enantiomers, normally found in fermented food, are absorbed into the blood and interact with plasma carrier protein human serum albumin (HSA). Unveiling the effect on the function and structure of HSA during chiral interaction can give a better understanding of the different distribution activities of the two enantiomers. Multi-spectroscopic methods and molecular modelling techniques are used to study the interactions between lactic acid enantiomers and HSA. Time-resolved and steady-state fluorescence spectra manifest that the fluorescence quenching mechanism is mainly static in type, due to complex formation. Binding interactions, deduced by thermodynamic calculation, agree with the docking prediction. Docking results and kinetic constants represent chiral-recognizing discriminations consistently. The bindings of lactic acid enantiomers lead to some microenvironmental and slight conformational changes of HSA as shown by circular dichroism (CD), synchronous and three-dimensional fluorescence spectra. This investigation may yield useful information about the possible toxicity risk of lactic acid enantiomers to human health.

Introduction

Lactic acid affects the rheological and sensory qualities of milk and gives good storage properties to fermented products. As a nutrient, it provides 3.6 kcal/g or 15.2 kJ/g of energy (Alm, 1982). Lactic acid is chiral, consisting of two optical enantiomers (Fig. 1). One is L-(+)-lactic acid (L-Lac) and the other is D-(−)-lactic acid (D-Lac). Lactic acid enantiomers are characteristic in all fermented dairy products (wines, sake and milk products) which are generated by both homo- and heterofermentative microbes. In fermented milk (Alm, 1982), the amount of total lactic acid is about 0.6–۱٫۲%, and L-Lac is the major enantiomer formed. The amount of D-Lac is about 0–۱۰% of the total lactic acid in acidophilus milk. In yogurt, about 40% of the total lactic acid is D-enantiomer. Physiological experiments in man and animals showed that lactic acid enantiomers were absorbed from the human intestinal tract (Duran, Van Biervliet, Kamerling, & Wadman, 1977). L-Lac can promote calcic absorption. However, the rate of metabolism of the D-enantiomer was considerably lower than that of L-Lac (Flemström, 1971). However, after intake of large quantities of D-Lac, enhanced Ca2+ was secreted in the urine (Alm, 1982). Restricted consumption of products which contain a high concentration of D-Lac is worth advocating. Infant formulae containing D- or DL mixture should be avoided (Organization, 1974). It is known that ligand–protein interactions affect the distribution, free concentration, and metabolism of various small molecules in the bloodstream. When lactic acid enantiomers are absorbed into the blood, they may bind to plasma proteins and subsequently change the structure and function of the protein. Yet, no reports have so far examined the chiral effects of lactic acid enantiomers at the molecular level, and the possible effect on plasma proteins is still poorly understood. Nutrition and safety of foods are concerns around the world, and thus information is needed to fill in this gap. HSA is the major protein component of human blood plasma. The physiological and pharmacological functions of HSA are maintaining osmotic pressure of blood, buffering pH, and serving in the transportation and distribution of a variety of nutrients and medicines. HSA is a monomeric protein which contains 585 residues. It contains 3 structurally similar α-helical domains (I–III), and each domain can be divided into subdomains A (containing 6 α-helices) and B (containing 4 α-helices) (He & Carter, 1992). Drugs or compounds mainly bind to one of the two primary binding sites on the protein, known as Sudlow’s sites I (warfarin binding site) and II (benzodiazepine binding site).