Chemical aspects of saponin bioactivity and ovine photosensitization diseases
A general review of saponin chemistry, bioactivities and of chemical aspects of saponin-associated hepatogenous photosensitization of ruminants (sheep, goats, and cattle) is presented. A field trial was undertaken to explore the genin dose/response hypothesis that saponins produced by the plant Narthecium ossifragum may be the direct cause of the toxicity leading to the photosensitization disease alveld seen in Norwegian lambs grazing N. ossifragum containing pastures. The genin content of N. ossifragum leaves from 2 control and 2 outbreak pasture areas, in a region of Norway known for periodic livestock outbreaks of alveld, ranged from 4553 to 10879 mg/kg DM. No significant statistical differences in the mean sapogenin content from the 2 control and 2 outbreak pasture areas were found. Total faecal sapogenin levels determined for lambs grazing the 2 control and 2 outbreak areas ranged from 973 to 36314 mg/kg DM. No obvious relationships between faecal sapogenin levels of lambs exhibiting external alveld symptoms and lambs not exhibiting symptoms were identified. The data does not support the hypothesis that saponins are the sole causative agent for the development of alveld disease symptoms in lambs grazing pastures containing N. ossifragum. Genin levels in Scottish collections of N. ossifragum plant material, gathered from pasture on which sheep were photosensitized, were determined. Elevated levels of sarsasapogenin and smilagenin containing saponins were found in flower heads (ca. 18000 mg/kg DM). Significant levels of free genins were found in plant roots (832-1184 mg/kg DM). The percentage contribution (ca. 45-65%) of smilagenin (a 25R-genin) in the Scottish samples was typically 2-4 times greater than is the case for Norwegian collections of N. ossifragum (typically ca. 10-15%). A field trial involving the daily intraruminal administration of a 1:4 mixture of [2,2,4,4-2H4]sarsasapogenin and [2,2,4,4-2H4]episarsasapogenin to 4 ewes, their twin lambs, and to 3 weaned lambs over a 21 day period was undertaken in Norway. The results showed that the isotope-dilution methodology can be applied under field conditions to estimate the uptake of sapogenins from plant material under conditions characteristically associated with the development of al veld. Mother ewes, on average, consumed 2-3 times the daily weight of N. ossifragum (ca. 700-750 g DM/day) consumed by the lambs (ca. 200-250 g DM/day). Percentage deuterium ratios, determined using SIM GC-MS protocols, while lower that anticipated, were consistent across the animals involved in the trial (between ca. 2-3%). Average N. ossifragum intakes, calculated relative to live-weights, were determined for all sheep involved in the trial. No correlation between the quantity of N. ossifragum consumed and live-weight (g DM day/kg) and the development of alveld was identified. The synthesis of sarsasapogenin β-D-galactoside, episarsasapogenin β-D-glucoside, episarsasapogenin β-D-galactoside, betulin 3-(β-D-glucoside) and betulin 3,28-(β-D-diglucoside), was performed via Koenig-Knorr coupling of sarsasapogenin, episarsasapogenin, or betulin with 2,3,4,6-tetra-0-acetyl-α-D-glucopyranosyl bromide, or the corresponding galactoside, followed by hydrolysis of the resulting tetra-0-acetyl-β-Dglycosides.Structures of intermediates, and product glycosides were established using a combination of ES-MS and one- and two-dimensional NMR spectral data. The synthesized sarsasapogenin, episarsasapogenin and betulin saponins, 4-methoxyfuran-2(5H)-one 5-(β-D-glucoside), Yucca schidigera 70° Brix extract, a methanol conjugate extract of N. ossifragum leaves and the active triketone fraction (triketone oil) from East Cape Manuka oil were subjected to a bioactivity evaluation against a range of bacterial and fungal organisms using the agar gel well diffusion technique. A possible structure-activity relationship amongst the synthesized saponins is discussed. The activity of triketone oil against two fish-pathogenic strains of Saprolegnia parasiticawas evaluated. The evaluation, performed in vitro, required the use of small percentages of water-miscible organic solvents and phosphate buffer solutions to obtain homogenous aqueous solutions. Inhibition of S. parasitica growth was seen at triketone oil concentrations of: (i) 150 and 200 ppm using aqueous solutions containing 0.5% and 1.0% pyrrolidinone, each containing 0.5% Tween 80 and (ii) at 150 and 200 ppm, at both pH 7 .0 and 7.5 of a 0.25 molL-1 phosphate buffer solution. An in vivo toxicity trial found that East Cape triketone oil at 200 ppm was acutely toxic to Atlantic salmon fry, with a 50% mortality rate after a 2 h single dose static bath exposure.
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