A highly specific glucosyltransferase is involved in the synthesis of crocetin glucosylesters in Crocus sativus cultured cells

Saffron (Crocus sativus) stigmata contain rare water-soluble carotenoids and the major one is crocin, the crocetin digentiobiosyl-ester. Previous studies indicated that two glucosyltransferases might be involved in the formation of crocetin glucosyl- and gentiobiosyl-esters (Dufresne et al. 1997). A UDP-Glc: crocetin 8,8′-glucosyltransferase involved in the biosynthesis of crocetin monoglucosyl- and diglucosyl-esters was extracted from saffron cell cultures and purified 300-fold by gel filtration chromatography and preparative IEF electrophoresis, with a recovery of 13 percnt;. The purified enzyme preparation was highly specific for crocetin and formed ester bonds between the glucose moiety of UDP-Glc and the free carboxyl functions of crocetin. The enzyme did not add other glucose units to the glucosyl-esters to form crocetin gentiobiosyl-esters. A crude desalted extract of the same material was less specific and formed glucosyl-esters with several other compounds, including abscisic and retinoic acids. The purified preparation was active between pH 4.4 and 4.6. SDS-PAGE revealed a major band at 26 kDa while the native molecular mass determined by gel filtration was in the range of 49 to 55 kDa. The study provides concrete evidence for the hypothesis that more than one glucosyltransferase is involved in the biosynthesis of crocetin glycosyl-esters in saffron.