Buletinul AŞM Nr.2 (311) 2010 Genetică generală şi moleculară atellus) with high speci city by RT-PCR. J. Virol. Meth- ezawa, H., Hibino, H. and Omura, T., 1993. Detection of ods 112, 115-120. Rice tungro bacilliform virus by polymerase chain reac- 11.Notomi, T., Okayama, H., Masubuchi, H., Yonekawa, tion for assessing mild infection of plant and viruliferous T., Watanabe, K., Amino, N. and Hase, T., 2000. Loop- vector leafhoppers. Phytopathology 83, 655-659. mediated isothermal ampli cation of DNA. Nucleic Acid 15.Varga, A. and James, D., 2006. Use of reverse tran- Res. 28, e63. scription loop-mediated isothermal ampli cation for the 12.Periasamy, M., Niazi, F.R. and Malathi, V.G., 2006. detection of Plum pox virus. J. Virol. Methods 138, 184- Multiplex RT-PCR, a novel technique for the simultane- 190. ous detection of the DNA and RNA viruses causing rice 16.Wang, H., Qi, M., and Culter, A.J., 1993. A simple tungro disease. J. Virol. Methods 134, 230-236. method of preparing plant samples for PCR. Nucleic Ac- 13.Takahashi, Y., Omura, T., Shohara, K. and Tsuchizaki, ids Res. 31, 4153–4154. T., 1991. Comparison of four serological methods for 17.Zhang, X., Wang, X. and Zhou, G., 2008. A one-step practical detection of ten viruses of rice in plants and in- real time RT-PCR assay for quantifying rice stripe virus sects. Plant Disease 75, 458-461. in rice and in the small brown planthopper (Laodelphax 14.Takahashi, Y., Tiongco, E. R., Cabauatan P. Q., Kogan- striatellus Fallen). J. Virol. Methods 151, 181-187. RECENT ADVANCES IN CARDENOLIDE RESEARCH Müller-Uri Frieder, Kreis Wolfgang Friedrich-Alexander-University Erlangen-Nürnberg, Department Biology, Erlangen, Germany Introduction Cardenolide glycosides are still valuable drugs involved in pregnane and cardenolide metabo- in the medication of patients suffering from lism has elucidated this pathway further. This cardiac insuf ciency. Digitalis species are the knowledge may now open the route for ma- major sources of the cardiac glycosides fre- nipulating cardenolide biosynthesis in plants. quently employed in medicine. Recent devel- Protein isolation and puri cation. A opments in the plant secondary metabolism in compilation of the earlier research was pub- general are summarized by Hartmann (2007). lished in the monograph on Digitalis by Luck- The progress in the investigation on differ- ner & Wichtl (2000). The following enzymes ent elds, from the genome to the proteome of the early steps in cardenolide biosynthesis or metabolom, will open new insights into the are studied in more detail: 3β-hydroxysteroid secondary metabolism pathway. Up to now dehydrogenase (3β-HSD), ketosteroid iso- only little is known about how plant metabolic merase (KSI), 5β-progesterone reductase (5β- pathways are regulated. However, the analysis POR), malonyl-coenzyme A:21-hydroxypreg- of the different pathways is ongoing and will nane 21-O-malonyltransferase (MHPMT). give more information to study the regulation As far as the formation of the butenolide in the future (Grotewold, 2005). ring is concerned, it is supposed that the con- The Digitalis cardenolides are charac- densation of 5β-pregnane-3β,14β,21-triol-20- terized by a steroid nucleus with its rings one with a dicarbon unit yields digitoxigenin. connected cis-trans-cis, possessing a 14β- When 5β-pregnane-14β,21-diol-20-one-3β-O- hydroxyl group, and substituted at C-17β with acetate was incubated together with malonyl- an unsaturated ve-membered lactone ring. coenzyme A in a cell-free extract of Digitalis At position 3β a sugar side chain with up to lanata leaves, a product was formed which ve carbohydrate units is attached, containing was identi ed as the malonyl hemiester of the glucose and various rare 6-deoxy, 2,6-dideoxy substrate (Stuhlemmer and Kreis, 1996). The and 6-deoxy-3-methoxy sugars, such as D- enzyme catalyzing this reaction was termed fucose, D-digitoxose or D-digitalose ( g. 1). malonyl-coenzyme A:21-hydroxypregnane Reviews concerning the studies on cardeno- 21-O-malonyltransferase (MHPMT). The lide structure have been published frequently compound decarboxylates at higher tempera- (Kreis et al., 1998; Gershenzon and Kreis, tures (above 120°C) and forms two products, 1999; Luckner & Wichtl, 2000, Kreis and namely 5β-pregnane-14β-ol-20-one-3β-O-21- Müller-Uri 2010). The new text book biosyn- O-diacetate and digitoxigenin 3-O-acetate. thetic pathway leading to the cardenolides is Thiol reagents stimulated the activity of the outlined in g. 2. A more recent identi cation MHPMT. The major part, almost 70 % of the and the characterization of various enzymes MHPMT was found to be soluble, 30 % were 45 Buletinul AŞM Nr.2 (311) 2010 Genetică generală şi moleculară Fig. 1. Cardenolide structures of plant origin. Fig. 2. Pathways leading to nal cardenolide structures: routes including pregnane in- termediates (I), norcholanic acids (II) or cardenolides (III) according to Schebitz et al., 2010. 46 Buletinul AŞM Nr.2 (311) 2010 Genetică generală şi moleculară associated with the microsomal fraction. Mal- ty (Acc.-Nr. AY 789449-53, AY 844959-960). onyl-coenzyme A and acetoacetyl-coenzyme 16`-O-glycohydrolase (CGH). Shi and A were accepted as co-substrates. No ester Lindemann (2006) expressed the cardenolide formation was observed when acetyl-CoA or 16`-O-glycohydrolase from D. lanata, earlier succinyl-CoA was added to the incubation reported by Schöniger et al. (1998), cloned mixture. CoA inhibited the malonylation reac- and characterized by Framm et al. (2000), in tion. 5β-pregnane-14β,21-diol-20-one-3β-O- Cucumis sativus L. hairy roots after transfor- acetate and 5β-pregnane-3β,14β,21-triol-20- mation using Agrobacterium rhizogenes. Gly- one were the most suitable substrates for the colytic activity of the transgenic CGH I was transferase reaction. 21-Hydroxypregnenolo- measured by HPLC using lanatoside glyco- ne, 21-hydroxyprogesterone, 5β-pregnane-21- sides as substrate. ol-3,20-dione and 5β-pregnane-3β,21-diol- Peripheral-type benzodiazepine recep- 20-one, were no highly speci c substrates. tor/inhibitor. The role of the peripheral-type Spontaneous butenolide ring formation at benzodiazepine receptor and the polypeptide 120°C was only seen with 14β-hydroxylated diazepam binding inhibitor was discussed by pregnane 21-O-malonyl hemiesters. So far, Papadopoulos et al. (1997) In analogy to the the enzyme could only be detected in cardeno- animal system an orthologous gene/protein lide-producing plants (Stuhlemmer and Kreis, was suggested for plants. Lindemann et al. 1996). (2000, 2004) reported about a novel protein Kuate et al. 2007 reported about the pu- that functions like a peripheral-type benzo- ri cation and characterization of malonyl- diazepine receptor in Arabidopsis thaliana. coenzyme A: 21-hydroxypregnane 21-O-mal- The molecular mass and some characteristics onyltransferase (Dp21MaT) from leaves of point to the involvement of this protein in the Digitalis purpurea L. The enzyme catalyzes steroid import into mitochondria. Lindemann the transfer of the malony moiety from malo- et al. (2004) demonstrated that the described nyl-coenzyme A to 21-hydroxypregnane sub- protein is present not only in Arabidopsis and strates. A simple and versatile method for the Digitalis species, but showed a ubiquitous chemical synthesis of 21-hydroxypregnane distribution in the plant kingdom. Binding 21-O-malonyl hemiesters (intermediates of studies showed a ligand-dependent uptake of the cardenolide biosynthesis) and other puta- cholesterol (and protoporphyrin IX) into bac- tive substrates were described (Padua et al., terial, plant and animal mitochondria. From 2008; Kuate et al., 2007). Optimized fresh this study it was concluded that a similar pro- plant material (leaves) has been used for the tein complex may exist as described by Papa- puri cation protocol including four column dopolous et al. (2006) for the animal system. chromatography steps. The enzyme was well Progesterone 5β-reductase (5β-POR). A characterized and showed a speci c substrate full-length cDNA clone that encodes proges- pattern. terone 5β-reductase (5β-POR) was isolated, Expression of recombinant enzymes/ cloned and characterized from D. lanata proteins. Molecular studies have been carried leaves. The recombinant gene was function- out not only for biosynthetic but also for mod- ally active. Only 5β-pregnane-3,20-dione but ifying proteins. The corresponding genes were not its α-isomer was formed when progester- cloned and the recombinant proteins were pro- one was used as the substrate by the fusion duced and characterized, for review see Kreis protein. Kinetic constants and other values and Müller-Uri, 2010. were determined. A compilation of the bio- Δ5-3β-hydroxysteroid dehydrogenase chemical characterization of the recombinant (3β-HSD). Molecular cloning and heterolo- enzymes from D. lanata and Isoplexis canar- gous expression of the Δ5-3β-hydroxysteroid iensis (now described as D. canariensis, see dehydrogenase (3β-HSD) from D. lanata below) are found in Herl et al., 2006a,b. The was reported by Herl et al. (2007). Pregne- 5β-POR like genes have been identi ed rst nolone and other 3β-hydrogypregnanes but from Arabidopsis thaliana (Herl et al., 2009) not cholesterol was 3β-oxidized by the re- and from several angiosperm species (tab. 1) combinant enzyme when NAD was used as (Bauer et al., 2010), some of them containing co-substrate. Testosterone was converted to cardenolide, some not. The suggested ubiq- 4-androstene-3,17-dione indicating that also a uitous occurrence of this enzyme class by 17β-dehydrogenase activity was obtained. For Perez-Bermudez et al. (2010) has therefore comparison, 3β-HSD genes from another six been proven experimentally within the an- Digitalis species were isolated and sequenced. giosperms. They share a high degree of sequence similari- Ernst et al. (2010) demonstrated that both 47 Buletinul AŞM Nr.2 (311) 2010 Genetică generală şi moleculară Table 1.