Anthraquinone glycosylation and hydrolysis in Morinda citrifolia cell suspensions: Regulation and function

Abstract

Summary Morinda citrifolia cells grown in suspension are able to accumulate significant amounts of anthraqui-nones (AQ) when grown in a low-auxin medium. As most of these anthraquinones are stored as glycosides, we studied the nature of the sugar moiety and several aspects of the glycosylation and hydrolysis reactions involved in the synthesis and degradation of these secondary metabolites. The sugar moiety of anthraquinones produced by Morinda cell suspensions appeared to be the disaccharide primverose (6-O-β-D-Xylopyranosyl-D-glucose). Extracts of AQ-producing cells contained also free primverose that was probably formed during the extraction procedure by the action of hydrolytic enzymes. This hydrolytic capacity was also present in non-AQ-producing, 2,4-D grown Morinda cells. These non-producing cells were also used for feeding experiments in which the capacity of Morinda cells to process added AQ (aglucons and glycosides) was tested. Addition of the aglucon alizarin (in the presence of DMSO) resulted in uptake and glycosylation; extracts of these cells also contained primverose, indicating that the normal glycosylation machinery was also active in these non-producing cells. Addition of AQ glycosides (a mixture of alizarin-primveroside and lucidin-primveroside) also resulted in uptake and processing of the added metabolites. Successive hydrolysis and glycosylation reactions lead to cells which contain a mixture of AQ aglucons and AQ glycosides after a few days of incubation. Apparently the mechanism for hydrolysis and glycosylation of AQ is also present or rapidly induced in cells that do not produce these compounds themselves, indicating that glycosylation of AQ and subsequent storage in the vacuoles probably is not a crucial step in the regulation of the production of these compounds. The significance of the presence of this hydrolysis and glycosylation machinery for the handling of e.g. xenobiotics is discussed.