Acylated Steryl Glycoside Synthesis in Seedlings of Nicotiana tabacum L.

In tobacco seedlings (Nicotiana tabacum L.), glucose from supplied uridine diphosphate-[U-(14)C]glucose was first incorporated into steryl glycosides and later into acylated steryl glycosides. However, when [(14)C]cholesterol was used as substrate, the acylated steryl glycosides became labeled earlier than the steryl glycosides. With [(14)C]cholesteryl glucoside as substrate, most of the radioactive label was recovered as free sterol, and the acylated steryl glycosides were not readily labeled; however, palmitoyl [(14)C]cholesteryl glucoside was rapidly converted to steryl glycoside. In feeding experiments with free sterol, an unknown, highly radioactive steroid component was isolated. Incorporation of radioactivity into the unknown occurred before the acylated steryl glycosides were labeled.IT IS POSTULATED THAT TWO PATHWAYS EXIST FOR THE BIOSYNTHESIS OF ACYLATED STERYL GLYCOSIDE: one through steryl glycosides, and the other through an unidentified steroid component. It is the latter pathway which appears to be dominant in the in vivo tobacco system.