Page 85 - VHSA - Onderstepoort 100 Years - Part 3

J.J. Quin, in a remarkable piece of research, had identified phylloerythrin, a photodynamic porphyrin, as the agent responsible for the photosensitivity in geeldikkop. Basically, geeldikkop results from the failure of the liver to excrete phylloerythrin derived by the degradation of chlorophyll by micro-organisms in the rumen. When phylloerythrin comes into contact with sunlight, for example in the exposed unpig- mented parts of the body, such as the face of a Merino sheep, it reacts with sunlight to cause severe photosensitization.
In 1983, Coetzer and his co-workers suggested that in geeldikkop, the retention of phylloerythrin (and thus, photo- sensitivity) resulted from occlusion of the bile ducts by crystals. They based their hypothesis, amongst others, on the fact that the lesions in geeldikkop were consistent with those of biliary occlusive disease in humans, and that some bile ducts were seen to be occluded by crystals. Whether occlusion of bile ducts by crystals is responsible for the re-
tention of phylloerythrin in geeldikkop or not, the hypothesis has over the past two decades exerted considerable influence on research on crystal-associated hepatoge- nous photo-sensitizations across the world.
Our research now, not surprisingly,
centred on the composition and origin
of the crystals. To this end we invited
two accomplished organic chemists from
New Zealand, namely C.O. Miles (Ruakura
Agricultural Research Centre) in 1992 and
A.L. Wilkins (University of Waikato) in
1994, to visit Onderstepoort. Working in
collaboration with our staff, notably with
Erasmus, they determined that the crystals
were composed of Ca++ glucuronides of
epismilagenin and episarsasapogenin. The
steps proposed for the formation of the
crystals included hydrolysis of a steroidal
saponin from T. terrestris to the aglycone (diosgenin) in the rumen, followed by reduction of the double bond at C-5 and epimerization at C-3 (3β-OH to 3α-OH). The resultant epismilagenin and episarsasapogenin are conjugated with glucuronic acid in the liver to form glucuronides which precipitate as insoluble calcium salts or crystals in the bile ducts.
Significantly, and consistent with the hypothesis, the crystals were derived from steroidal saponins in T. terrestris. Previously Miles and his co-workers had shown that the composition of the intrabiliary crystals induced by a Panicum grass in New Zealand were essentially similar to those of geeldikkop, thus clarifying the connection between the two diseases. Somewhat disappointingly, a limited attempt to induce either photosensitivity or intrahepatic crystals in a sheep by drenching it with high doses of a pure diosgenin- derived saponin from T. terrestris had no effect. The reason for this failure and many other questions must now to be investigated.
ONDERSTEPOORT 100
Studies on the clinical signs and cardiac dynamics of gousiekte by P.J. Pretorius and M. Terblanche in 1967, and Pretorius and co-workers in 1973, and the subcellular effects on energy production and the contractile system of the myocardium by Snyman and co-workers in 1982, led them to the belief, correctly it transpired, that the primary lesion in gousiekte was inhibition of the contractile mechanism of the myocardium. And so matters rested until the 1990s, when Fourie and co-workers launched an ambitious project to isolate the cardiotoxin and to explain the pathogenesis of the disease. The project was ambitious because gousiekte is a very challenging disease to investigate; amongst others, be- cause only ruminants are affected, extremely large amounts of plant material (of notoriously variable toxicity) are required to induce the disease, and there is a latent period of up to 6 weeks with which to contend. In a most accomplished piece of
research, Fourie isolated a pure toxin from P. harborii, later identified by R. Vleggaar of the University of Pretoria as a polyamine, pavetamine. Not only have Vleggaar and Moira Bode elucidated the structure of pavetamine, they have also succeeded in synthesizing it, a remarkable achievement. Bode continued with this research after her departure from Onderstepoort to join the CSIR in 1999.
Perhaps a word on polyamines is in
order. Polyamines are highly biologically
active compounds affecting many func-
tions in the body, including cell growth.
Studies in rats by Schultz and her co- workers confirmed that pavetamine, in common with other polyamines, inhibits 175 the synthesis of proteins. This property interestingly also has potential therapeutic application as artificial analogues of poly-
amines are being investigated abroad for the treatment of cancer and HIV/AIDS.
More importantly from a veterinary point of view, ongoing investigation of the pathogenesis of the disease by Schultz and her co-workers indicates that pavetamine acts by inhibiting the synthesis of the contractile protein, myosin, in myocardial cells. The heart is believed to function normally during the latent period while myosin is being depleted; at a critical point, when myosin cannot be synthesized quickly enough to replace that degraded during normal physiological protein turnover, sudden cardiac failure ensues. This observation is consistent with the clinical findings of Schultz using a cardiopulmonary flow index (CPFI), a procedure developed at the Potchef- stroom University for CHE, for monitoring heart function. Using the CPFI, she was able to confirm that in gousiekte cardiac function is maintained almost to the very end. At the same time L. (Leon) Prozesky, the pathologist on the team, was able to study the full range of lesions induced by gousiekte under controlled conditions, a boon for diagnosticians.
“Perhaps a word on polyamines is in order. Polyamines are highly biologically active compounds affecting many functions
in the body, including cell growth. Studies in rats by Schultz and her co-workers confirmed that pavetamine, in common with other polyamines, inhibits the synthesis of proteins.”
Toxicology
1908-2008
Years