Facilitated Uptake of a Bioactive Metabolite of Maritime Pine Bark Extract (Pycnogenol) into Human Erythrocytes

Abstract

Many plant secondary metabolites exhibit some degree of biological activity in humans. It is a common observation that individual plant-derived compounds in vivo are present in the nanomolar concentration range at which they usually fail to display measurable activity in vitro. While it is debatable that compounds detected in plasma are not the key effectors of bioactivity, an alternative hypothesis may take into consideration that measurable concentrations also reside in compartments other than plasma. We analysed the binding of constituents and the metabolite δ-(3,4-dihydroxy-phenyl)-γ-valerolactone (M1), that had been previously detected in plasma samples of human consumers of pine bark extract Pycnogenol, to human erythrocytes. We found that caffeic acid, taxifolin, and ferulic acid passively bind to red blood cells, but only the bioactive metabolite M1 revealed pronounced accumulation. The partitioning of M1 into erythrocytes was significantly diminished at higher concentrations of M1 and in the presence of glucose, suggesting a facilitated transport of M1 via GLUT-1 transporter. This concept was further supported by structural similarities between the natural substrate α-D-glucose and the S-isomer of M1. After cellular uptake, M1 underwent further metabolism by conjugation with glutathione. We present strong indication for a transporter-mediated accumulation of a flavonoid metabolite in human erythrocytes and subsequent formation of a novel glutathione adduct. The physiologic role of the adduct remains to be elucidated.

Profiling a gut microbiota-generated catechin metabolite's fate in human blood cells using a metabolomic approach

“…The formation of an intracellular conjugate of M1 and glutathione (GSH) was already known [10] and therefore anticipated …”

“…Moreover, it was demonstrated that M1 accumulates in endothelial and blood cells in vitro after http://dx.doi.org/10.1016/j.jpba.2015.04.042 0731-7085/© 2015 Elsevier B.V. All rights reserved. facilitated uptake [9,10]. Recently, M1 was also quantified in human blood cells in vivo after intake of Pycnogenol ® by a human volunteer [11].…”

“…Recently, M1 was also quantified in human blood cells in vivo after intake of Pycnogenol ® by a human volunteer [11]. To our knowledge, the absolute stereochemistry of this bacteria-generated compound has not been clarified yet, although it can be assumed that only a single enantiomeric form is generated by the gut bacteria [10].…”

“…Although it has been pointed out that not all xenobiotics with carbon-carbon double bonds are readily subjected to a Michael reaction with, e.g. glutathione [12] indeed a glutathione conjugate of M1 was detected in human blood cells [10]. Further degradation of M1 by metabolizing gut microbiota, resulting in, e.g.…”

Pycnogenol® bei Erkrankungen des venösen Systems - eine systematische Übersicht

“…Der Metabolit M1 reichert sich in Erythrozyten an und bildet dort ein Addukt mit Glutathion [9]. Diese Anreicherung in Erythrozyten kann man als eine Form der Depotbildung auffassen.…”