Specific features of respiration in Dioscorea strains differing in the steroid biosynthesis.

Abstract

The cell culture of Dioscorea deltoidea is a promising source of steroid saponins with diosgenin as aglycon. A mutant Dioscorea strain IFR DM0.5, which has been produced by treating the original strain IFR D1 with the growth-stimulating concentration of the chemical mutagen N-NMU, attracts a great attention of researchers. This strain exceeds the original strain and intact plants in the contents of diosgenin and saponins. When grown on the agar-solidified medium, this strain is characterized by more stable and rapid growth [1]. In the flow culture of the cell suspension under conditions of the chemostat, strain IFR DM0.5 displayed even a higher productivity of the biomass [2]. The suspensioncultured mutant cells had larger cells enriched in various inclusions. The IFR DM0.5 cells had a structure characteristic of the active-secreting cells [3] with an extensive endoplasmic reticulum [4]. However, some aspects of the life activity of Dioscorea strain IFR DM0.5, which eventually result in the stable and high production of saponins and diosgenin, remain unclear. In particular, respiration of this strain is not sufficiently studied, whereas respiration is not only a source of energy but also a source of intermediate metabolites for cell growth and the biosynthesis of steroid compounds. The biosynthesis of terpenoids and steroids, especially of its early stages, is known to require much energy [5]. Beginning from the stage of mevalonic acid, a process is directed toward the biosynthesis of terpenoids, since, in animal and plant cells, reducing OMG-CoA is practically irreversible (Quereshi and Portner, 1981; cit. after [5]). To synthesize six molecules of mevalonic acid, which are required for the synthesis of the sterol carbon backbone, approximately 18 molecules of ATP and 12 molecules of NADHP are necessary. A conversion of cycloartenol into phytosterols is known to occur by its dealkylation (the removal of three ec 3 groups) at e 4 and e 14 atoms, isomerization ( Δ 8 φιΔ 5 ), and double alkylation of the e 24 atom. During the conversions at the stages between squalene and e 29 phytosterol, up to 20 molecules of reduced pyridine nucleotides are spent. The formation of acetyl-CoA from acetate also consumes energy. Thus, many stages of the steroid biosynthesis in D. deltoidea demand an energy expenditure. In this work, we studied the rates of respiration (oxygen uptake) in the cell suspension of D. deltoidea , strains IFR D1 and IFR DM0.5, and the effects of the inhibitors of mitochondrial oxidation, antimycin A (AA) and salicylhydroxamic acid (SHAM), on this process.