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Chemical aspects of some ovine hepatogenous photosensitization diseases

Abstract
Current understandings of the chemistry, aetiology of saponin-associated hepatogenous photosensitization diseases of ruminants (sheep, cattle, goats and deer), the distribution of saponins in causative plants (Agave lecheguilla, Brachiaria decumbens, Narthecium ossifragum, Nolina texana, Tribulus terrestris and some Panicum species), and the ovine metabolism of saponins implicated in the development of hepatogenous photosensitization diseases, are reviewed. Sarsasapogenin β-D-glucoside and 3β-D-epismilagenin β-D-glucoside were synthesised from sarsasapogenin and α-acetoglucosyl bromide via three and five step reaction sequences respectively. The structures of intermediates and the product glucosides were established using a combination of GC-MS, ES-MS and 400 MHz one- and two- dimensional NMR spectral data. 20,23,23-D₃-sarsasapogenin was prepared from sarsasapogenin using deuterioacetic acid, and the stability (exchangeability) of the deuterium atoms was determined in a series of pH 1-8 buffer solutions at 37°C, and in acetic acid at room temperature and 37°c. Administration of an ethanol-water suspension of sarsasapogenin β-D-glucoside to a sheep afforded a maximum level of episarsasapogenin in bile samples 12 hours after dosing. Elevated levels of free sarsasapogenin and low levels of conjugated sarsasapogenin, free and conjugated episarsasapogenins and free and conjugated sarsasapogenones were detected in a rumen sample. Administration of mixtures of sarsasapogenin β-D-glucoside and 20,23,23-D₃-sarsasapogenin to two sheep afforded a maximum level of episarsasapogenin in bile samples 7-16 hours after dosing. Unexpected loss of deuterium atoms occurred during ovine metabolism. Analyses of mass spectral ion ratio data, determined for the mixed dosing experiment (using free and conjugated sarsasapogenins) and an experiment in which only 23,23,23-D₃-sarsasapogenin was dosed, showed that when administered as an ethanol-water suspension sarsasapogenin β-D-glucoside was ca 1.7 times more bioavailable than sarsasapogenin. Dosing of 3β-D-epismilagenin β-D-glucoside to two sheep showed that oxidation and reduction at C-3 was a reversible process, and that epismilagenin (α-OH), rather than smilagenin (3β-OH), was the favoured reduction product of smilagen-3-one. Deuterium loss occurred at different rates for free and conjugated epismilagenins in different metabolic regions (e.g. the rumen and the jejunum). The ion ratio profile of jejunum epismilagenin conjugates was similar to that of bile epismilagenin conjugates. Six saponins (faroecin, asiaticin, asiaticioside, narthecin, narthecioside and 22-methoxy-narthecioside) were isolated from a Norwegian collection of N. ossifragum and their structures were elucidated using ES-MS and one- and two-dimensional NMR spectral data. Three of these saponins, faroecin, asiaticioside and narthecioside, were identified as new compounds. Elevated levels of sarsasapogenin and smilagenin glycosides were found in some N. ossifragum samples (up to 2121 and 557 mg/kg respectively). The furostenol tetrasaccharide, (25R/S)-furost-5-ene-3β,22α,26-triol 3-O-{O-α-L-rhamnopyranosyl-(1→2)-O-[O-α-L-rhamnopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranosyl}-26-O-β-D-glucopyranoside, was isolated from a USA collection of P. virgatum. Variable levels of diosgenin and yamogenin glycosides were found in 13 USA P. virgatum collections (up to 288 and 42 mg/kg of diosgenin and yamogenin glycosides respectively). The furostenol tetrasaccharide, (25R/S)-furost-5-ene-3β,22α,26-triol 3-O-{O-α-L-rhamnopyranosyl-(1→2)-O-[O-α-L-rhamnopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranosyl}-26-O-β-D-glucopyranoside and the corresponding spirostenol tetrasaccharide, (25R/S)-spirost-5-en-3β-ol 3-O-{O-α-L-rhamnopyranosyl(1→2)-O-[O-α-L-rhamnopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranoside}, were isolated from a New Zealand collection of P. miliaceum. Hydrolysis of the ethanol-water extracts of a USA collection of N. texana afforded three dihydroxy sapogenins, ruscogenin, neoruscogenin and spirosta-5,25(27)-diene-1β,3β-diol, in a ratio of ca 1:1.2:2.5. Three monoglycosylated saponins, spirosta-5,25(27)-diene- 1β,3β-diol 1-O-α-D-fucopyranoside, spirosta-5,25(27)-diene-1β,3β-diol 1-O-α-L-arabinopyranoside and spirosta-5,25(27)-diene-1β,3β-diol 1-O-β-D-galactopyranoside, were identified as constituents of the N. texana ethanol-water extract. The levels of saponins in 82 Papua New Guinea collections of B. decumbens and 4 Iranian collections of Tribulus terrestris were determined. Variable levels of diosgenin and yamogenin glycosides (B. decumbens samples), and diosgenin, smilagenin, sarsasapogenin, tigogenin, neotigogenin, hecogenin and neohecogenin glycosides (T. terrestris samples) were identified.
Type
Thesis
Type of thesis
Series
Citation
Date
1999
Publisher
The University of Waikato
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