CONCLUSIONS AND HYPOTHESES

New Zealand sheep meat is associated with a characteristic flavour and cooking odour known as pastoral flavour. Pastoral flavour is perceived as a desirable ' full bodied' taste by traditional consumers however consumer preference for food flavours are diverse and a dislike for pastoral flavours is associated with consumer groups, particularly in potential Asian markets, who are unaccustomed to pasture raised sheep meats.

The extensive use of low input highly digestible pasture species is the basis of the international competitiveness of New Zealand agriculture. Improvements in the nutritive value of pasture cultivars, in particular perennial ryegrass and white clover, through selective breeding, increased use of nitrogen fertilisers and intensive grazing management have both improved productivity and reduced the efficiency with which ingested protein is used by the productive ruminant animal.

In the pasture fed ruminant an asynchrony is believed to exist between the rapid rate of plant and microbial mediated proteolysis relative to energy generation via microbial celluloysis in the rumen. Under these conditions bacterial growth is limited by energy supply and free amino acids released into the rumen fluid are rapidly deaminated to yield ammonia and other non-protein compounds.

The synthesis of skatole in the gastrointestinal tract requires bacterial deamination of the amino acid L-tryptophan, hence its rate of production can be manipulated by reducing rumen degradation of plant proteins or decreasing deamination of free amino acids within the rumen. In grazing ruminants skatole production is positively correlated with the intake of rumen degradable protein. Ruminants grazing New Zealand improved

CHAPTER 1 : Literature Review 56 temperate pasture species, in particular white clover, ingest sufficient rumen degradable protein to allow post-prandial skatole formation at a rate greater than the first-pass detoxification capacity of the liver. Skatole is rapidly absorbed from the rumen into blood entering the gastric vein with little outflow to the lower digestive tract. Unless metabolised in the liver skatole enters the peripheral circulation where it is rapidly deposited into adipose tissue and imparts a pastoral flavour to the meat products of grazing ruminants.

Reducing the formation of skatole has been investigated as a means of reducing the concentration of skatole in meat and milk products. These approaches have included reducing the rumen degradability of forage proteins through the use of condensed tannins, modification of the rumen bacterial population or improving microbial protein synthesis by correcting the protein:energy imbalance within the rumen. All are effective in reducing the extent of tryptophan derived skatole formation. However to date these practices have been of limited practical application in pastoral agriculture. This is due to the cost of growing forages with sufficient tannin content due to their low persistence under grazing relative to pasture, the high cost of energy rich supplements and the opposition of export markets to antibiotic use in food producing livestock.

The mechanisms of skatole metabolism in ruminants are not well understood, however high concentrations of skatole in peripheral circulation precedes the onset of acute pulmonary odema and emphysema in goats, cattle and sheep. Skatole itself is not a pulmonary cytotoxin however it is activated to an alkylating metabolite by cytochrome P450 enzymes in bronchiolar epithelial cells. Skatole mediated pulmonary toxicity can be reduced via pre-treatment with sub-clinical doses of skatole in mice and goats,

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indicating that induction of alternative non-toxic skatole detoxification pathways 1s possible in these species.

The concentration of skatole deposited within the adipose tissue of sheep displays significant inter-individual variation however it is not known if this variation arises due to differences in skatole formation in the rumen or rate of elimination via hepatic or post-hepatic metabolism. Pigs also display significant inter-individual variation in the accumulation of skatole in adipose tissue. In pigs the liver is the primary site of skatole metabolism and hepatic enzyme activity is thought to control skatole accumulation in this species. The role of the liver in ruminant skatole clearance is not known however given the role of the liver in detoxification of hydrophobic xenobiotics and its ability to eliminate compounds prior to their entry into peripheral circulation, its capacity for skatole metabolism is also likely to be a key factor affecting accumulation of skatole in sheep.

Due to the current lack of economic methods to reduce rumen skatole formation in pasture fed sheep the obj ective of this study was to undertake a transcriptional profile of hepatic tissue using DNA microarrays to identify gene induction in response to skatole treatment in sheep. A second objective was to determine the effect of steady state intrarumen skatole administration upon skatole concentration in rumen and peripheral plasma pools and to determine the rate of skatole elimination in sheep.

It was therefore hypothesised that under conditions of minimal endogenous skatole production a 72 hour controlled administration of exogenous skatole will result in an increase in rumen skatole to a plateau concentration and induce differential gene expression in the ovine liver. These hypotheses have been tested using a continuous

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intrarumen infusion o f skatole, measurement of skatole concentration in rumen fluid, peripheral plasma and inter-muscular fat and transcriptional analysis of hepatic tissue using DNA microarrays.

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