BIOSYNTHESIS OF PREGNANE DERIVATIVES IN SOMATIC EMBRYOS OF DIGITALIS-LANATA

Activities of the enzymes cholesterol monooxygenase (side chain cleavi ng) (SCCE), Delta(5)-3 steroiddehydrogenase/Delta(5)-Delta(4)-ketoster oid isomerase, proesterone 5 beta-reductase, 3 beta-hydroxysteroid 5 b eta-oxidoreductase, 3 alpha-hydroxysteroid 5 beta-oxidoreductase, prog esterone 5 alpha-reductase and 3 beta-hydroxysteroid 5 alpha-oxidoredu ctase which are involved in biosynthesis and transformation of pregnan e derivatives were determined in different developmental stages of som atic embryos of Digitalis lanata. All enzymes were found to be present in proembryogenic masses (PEMs) as well as in globular and bipolar em bryos. Most SCCE activity was found in the mitochondria fraction. Chol esterol, sitosterol and stigmasterol, precursors of the pregnanes, occ urred in somatic embryos in amounts of about 1 mu g mg(-1) protein. Pr egnenolone was found in traces only (about 20 ng mg(-1) protein). Feed ing of progesterone caused an increase of the contents of 5 alpha- and 5 beta-pregnandione, progesterone, 5 beta-pregnane-3 beta-ol,2O-one a nd 5 alpha-pregnane-3 beta-ol,-2O-one. In contrast, administration of cholesterol caused a small increase of pregnenolone only. These result s indicate that the rate limiting step in pregnane (and probably in ca rdenolide biosynthesis) is compartmentation of CSSE in the mitochondri a of the somatic embryos of D. lanata. (C) 1997 Elsevier Science Ltd.