Biosynthesis of pregnane derivatives in somatic embryos of Digitalis lanata

Activities of the enzymes cholesterol monooxygenase (side chain cleaving) (SCCE) Δ5-3β-hydroxysteroiddehydrogenaseΔ5-Δ4-ketosteroid isomerase, progesterone 5β-reductase, 3β-hydroxysteroid 5β-oxidoreductase, 3α-hydroxysteroid 5β-oxidoreductase, progesterone 5α-reductase and 3β-hydroxysteroid 5α-oxidoreductase which are involved in biosynthesis and transformation of pregnane derivatives were determined in different developmental stages of somatic embryos of Digitalis lanata. All enzymes were found to be present in proembryogenic masses (PEMs) as well as in globular and bipolar embryos. Most SCCE activity was found in the mitochondria fraction. Cholesterol, sitosterol and stigmasterol, precursors of the pregnanes, occurred in somatic embryos in amounts of about 1 μg mg−1 protein. Pregnenolone was found in traces only (about 20 ng mg−1 protein). Feeding of progesterone caused an increase of the contents of 5α-pand 5β-pregnandione, progesterone, 5β-pregnane-3α-ol,20-one and 5α-pregnane-3β-ol,-20-one. In contrast, administration of cholesterol caused a small increase of pregnenolone only. These results indicate that the rate limiting step in pregnane (and probably in cardenolide biosynthesis) is compartmentation of CSSE in the mitochondria of the somatic embryos of D. lanata.