Lead complexation in wines with the dimers of the grape pectic polysaccharide rhamnogalacturonan II

Abstract

Wine is believed to be a significant source of lead in the human diet even though the lead content of wines has decreased considerably over the last thirty years. Nevertheless, the lead content of wines must be reduced to a minimum since this heavy metal is highly toxic.

The plant cell wall pectic polysaccharide rhamnogalacturonan Il (RG-II) is a predominant anionic molecule in red wine. RG-Il exists as a dimer (dRG-Il-B) that is cross-linked by a 1:2 borate-diol ester and forms complexes in vitro with lead and other selected di- and tri-valent cations. One mole of dimer binds at least 1 mole of Pb²⁺. We have now determined the amount of lead in wines that is bound to dRG-II-B since previous studies have suggested that most of the lead in wine is bound to an anionic macromolecule.

Seven wines, with lead concentrations between 30 and 110 μg/l, were obtained from different grape varieties harvested at different vintages and vinified by different procedures. Two chromatography steps, adsorption on a polystyrene- divinylbenzene resin and size-exclusion on a Superdex® 75-HR column, have been used to purify a d-RG-II-B-lead complex which contained at least 85 p. cent of the total lead of each wine. The dRG-II-B-Pb complex is stable at the pH of wine and is present in a wine that was procluced in 1988. The dRG-II-B present in red (~ 100 mg/l) and white (~ 20 mg/l) wines can bind at least ten-fold more Pb²⁺ than is typically present in wine.

Our study is the first to show that in wine most of the lead is complexed with a pectic polysaccharide that is not degraded during vinification. dRG-II-B is also known to form complexes with other cations, including strontium and barium. However, it is not known what role dRG-II-B has in determining the metabolic fate in humans of toxic cations present in wine.