The significance of the biosynthetic pathway, cholesterol -> 20a-hydroxy cholesterol -*• pregnenolone -* progesterone -* cardenolides, in plants has been examined by the simultaneous administration of a mixture of [7-3H]20a-hydroxycholesterol and [4-1 ^cholesterol (3H/I4C ratio 8.68) to a Digitalis lanata plant. The in vivo transformation of the administered steroids to cardenolides was observed. In addition, [7-^progesterone totally devoid of 14C was isolated. The varia-T Ahe biosynthesis of cholesterol from mevalonic acid in Digitalis purpurea (Jacobsen and Frey, 1967) has been demonstrated. The metabolism of cholesterol to pregnenolone (II) (Chart I) (Caspi et al., 1966) and the incorporation of the latter to cardenolides has been proved (Tschesche and Lilienweiss, 1964; Tschesche and Brassat, 1966;Caspi and Lewis, 1967). Subsequently, the conversion of pregnenolone (II) into progesterone (III) (Caspi and Lewis, 1967) and of the latter into cardenolides was demonstrated in Digitalis lanata (Caspi and Lewis, 1967; Bennet et al., 1968). The sequence of steps from pregnenolone to the C23 products requires the intermediacy of a 3-ketone, presumably of progesterone (Caspi and Hornby, 1968).The degradation of the C2? precursor to C21 intermediates in the plant has been considered to be analogous to that incident in animals (Heftmann, 1967;Frantz and Schroepfer, 1967). Consequently, the sequence cholesterol -* 20a-hydroxycholesterol (I) -pregnenolone (II) -* progesterone (III) -* cardenolides was implied to be the operating pathway in the biosynthesis of cardiac aglycones. The present work was undertaken with a view to examine the validity of this assumption.The rationale of our approach was to administer a mixture of [4-14C]cholesterol and [7-3H]20a-hydroxycholesterol to a D. lanata plant and to evaluate the relative incorporation of the two tracers into cardenolides. Experimental SectionChromatography. Silica gel (Merck HF264) was used for thin-layer chromatography in the indicated solvent

No abstract