Biosynthetic studies of marine lipids. 42. Biosynthesis of steroid and triterpenoid metabolites in the sea cucumber Eupentacta fraudatrix

The saponins, conjugated sterols, and free sterols of the sea cucumber Eupentacta fraudatrix were examined. A total of 85 steroids, twelve of them new, were identified in the free sterol, sulfated sterol, and sterol-xyloside fractions. The free sterol fraction contained 4α,14α-dimethylcholest-9(11)-en-3β-ol (6) and 14α-methylcholest-9(11)-en-3β-ol (7) together with 18 minor sterols. Examination of the aglycone moieties of the sterol-β-xyloside fraction afforded 31 different sterols. Cholestan-3β-ol (15) and 24-methylcholesta-7,22-dien-3β-ol (20) were the major sterols in this group. Cholestanol sulfate (74) and cholesterol sulfate (64) were identified as the major components among the 34 different sterol sulfates present. Finally, cucumariosides G 1 (1) , C 1 (2) , C 2 (3) , H (4) , and G 2 (5) were isolated from the saponin fraction. Radiolabeling experiments indicated that ther are two pathways of sterol biosynthesis in E. fraudratix. The first involves transformation of squalene to produce lanosta-9(11),24-dien-3β-ol (parkeol) which is subsequently demthylated to form 4α,14α-dimethylcholest-9(11)-en-3β-ol (6) and 14α-methylcholest-9(11)-en-3β-ol (7) . The second proceeds through squalene to lanosterol which is further metabolized to produce the triterpene saponins, 5α-cholest-7-en-3β-ol (19) and its xyloside (49) .