Biosynthetic origin and longevity in vivo of α-D-mannopyranosyl-(1→4)-α-D-glucuronopyranosyl-(1→2)-myo-inositol, an unusual extracellular oligosaccharide produced by cultured rose cells

A non-reducing trisaccharide, alpha-D-mannopyranosyl-(1 --> 4)-alpha-D-glucuronopyranosyl-(1 --> 2)-myo-inositol (MGI) accumulated in the spent medium of cell-suspension cultures of 'Paul's Scarlet' rose (Rosa sp.) predominantly during the period of rapid cell growth. This trisaccharide was also produced by cultures of sycamore (Acer pseudoplatanus L.) but not by those of the graminaceous monocots maize (Zea mays L.) and tall fescue grass(Festuca al arundinacea Schreb.). When added to cultured Rosa cells, [C-14]MGI was neither taken up by the cells nor bound to the cell surface and was not metabolised-extracellularly. When D-[6-C-14]glucuronic acid was fed to cultured Rosa cells,extracellular [C-14]MGI started to appear only after a 5-h lag period, compared with a 0.5-h lag period for labelling of extracellular polysaccharides. Furthermore, [C-14]MGI continued to accumulate in the medium for at least 20 h after the accumulation of C-14-polymers had ceased. These observations indicate that: extracellular MGI was produced from a slowly turning-over pool of a preformed intermediate. Structural considerations indicate that the intermediate could be a glucuronomannan or a phytoglycolipid (glycophosphosphingolipid). No Rosa polysaccharides could be found that generated MGI in the presence of living Rosa cells. We therefore favour phytoglycolipids as the probable biosynthetic origin of MGI.