Abstract:
Animal Production in Australia THE IMPORTANCE OF METHIONINE FOR WOOL GROWTH IN SHEEP P.J. REIS* SUMMARY Mixtures of essential amino acids were infused into the abomasum of sheep, and effects on wool growth were measured. Various mixtures containing methionine stimulated wool growth, whereas mixtures lacking methionine were not effective. In two experiments, the provision of cysteine in place of methionine failed to stimulate wool growth. These results indicate a specific role of methionine in controlling wool growth, other than to provide cysteine via transulphuration. INTRODUCTION The amounts and proportions of amino acids available to wool follicles have marked effects on the rate of wool growth (Reis 1979). The sulphur-containing amino acids, cyst(e)ine or methionine, are particularly effective for stimulating wool growth. The abomasal infusion of mixtures of amino acids simulating their proportions in casein enhanced the rate of wool growth, but wool growth was inhibited when methionine was omitted from these infusions (Reis and Tunks 1978). The present experiments were designed to investigate the importance of methionine for wool growth in various mixtures of essential amino acids, and to see whether cysteine could effectively replace methionine in these mixtures. MATERIALS AND METHODS Mature pylorus, were and pelleted offered once Merino wethers, each fitted with an abomasal cannula near the kept indoors in metabolism cages. The ration, 600 g of a ground mixture of lucerne hay (three parts) and oat grain (two parts), was daily; drinking water was available ad i?ibitm. Effects on wool growth, of infusion of amino acids into the abomasum, were measured in five experiments. Seven different mixtures of amino acids (Table 1) were given for periods of 12 days, and wool growth was compared with that in a control period prior to infusion. In some experiments the effects of two mixtures of amino acids were also compared in two consecutive 12-day periods, in each of which half the sheep received one mixture and half the other. Amino acids were obtained from Tanabe Seiyaku Co. Ltd., Osaka, and Ajinomoto Co. Inc., Toyko. A peristaltic pump was used to maintain a steady rate of infusion of aqueous solutions into the abomasum over about 23 hours per day in a volume of l-2 2. The patterns of infusion of amino acid mixtures in the various experiments are described below: Experiment 1: Mixture 1 (ten essential amino acids) and Mixture 2 (cysteine replacing methionine in Mixture 1) were compared in four sheep. Experiment 2: Mixture 3 (five essential amino acids) and Mixture 4 (Mixture 3 with methionine omitted) were compared in four sheep. Experiment 3: Mixture 3 (five essential amino acids) and Mixture 5 (cysteine replacing methionine in Mixture 3) were compared in two sheep. Experiment 4: Mixture 6 (four essential amino acids) was given to five sheep. Experiment 5: Mixture 7 (six essential amino acids) was given to four sheep. * CSIRO, Division of Animal Production, P-0. Box 239, Blacktown, N.S.W. 2148. Animal Production in Australia TABLE 1 Composition of amino acid mixtures infused into the abomasum (g/day) Responses in wool growth were assessed in two ways. The mass of clean, dry wool grown per day was assessed as described by Reis and Tunks (1978), using a clipping schedule which allowed for the estimated emergence time of wool fibres. Wool was removed from a defined area of skin (100-200 cm2) on the midside of each sheep, prior to infusion and for 8-day periods which corresponded to each infusion period. The autoradiographic technique of Downes et al. (1967) was used to measure changes in diameter, length growth and volume of fibres. Intravenous injections of tracer doses of L-[35S]cystine (c. 60 pCi/dose) were given at intervals of 4 or 6 days prior to infusion, and 4 days apart at the end of each 12-day infusion period (Reis and Tunks 1978), to measure wool growth prior to treatment and during the period days 9 to 12 of treatment inclusive. Fibre measurements and calculations were made as described by Reis and Tunks (1978). Fibres were sampled from four sites along one side of each sheep; a total of 40-80 fibres were measured per sheep with approximately equal numbers from each site. The statistical significance of differences was tested using a t-test for paired variates. RESULTS Abomasal infusion of the ten essential amino acids (Mixture 1) approximately doubled wool growth in Experiment 1, by increasing both fibre diameter and length growth rate (Table 2). However, when methionine was replaced by an equimolar amount of cysteine (Mixture 2), wool growth was not significantly different from control values. In Experiment 2, a mixture of five essential amino acids including methionine (Mixture 3) also appreciably increased wool growth, but when methionine was omitted (Mixture 4) the infusion failed to alter wool growth. The results of Experiment 3 also indicated that the replacement of methionine with cysteine (Mixture 5) in this mixture of five essential amino acids was not effective for stimulating wool growth; these results with only two sheep were not analysed statistically. The infusion of four essential amino acids including methionine (Experiment 4) stimulated wool growth similarly to Mixture 3, but the infusion of the remaining six essential amino acids (Experiment 5) did not stimulate wool growth. In fact, fibre diameter and the amount of wool grown were slightly, but significantly, reduced by the latter treatment (Table 2). 480 Animal Production in Australia TABLE 2 Effect of abomasal infusion of mixtures of amino acids on wool growth. Values are means for two sheep (Experiment 3), four sheep (Experiments 1, 2 and 5) or five sheep (Experiment 4). Values with different superscripts differ significantly at the 5% level of probability DISCUSSION The present experiments have established that a mixture of the ten amino acids essential for growth in the rat is effective for stimulating wool growth, and have confirmed (Reis and Tunks 1978) that methionine is a vital component of The abomasal infusion of l-3 g/day cyst(e)ine supplementing rations the mixture. similar to that given here stimulates wool growth (Reis 1979), and the effect is probably due mainly to the provision of cysteine as substrate for the synthesis of wool proteins. Equivalent amounts of methionine may also provide cysteine In view of the small amount of via the transulphuration pathway (Reis 1979). methionine in wool proteins, and its complete absence from the matrix proteins (see Reis 1979), the requirement for methionine as a substrate for the synthesis of wool proteins would be small. The observation that methionine cannot be effectively replaced by cysteine in Experiments 1 and 3 indicates that methionine must be influencing wool growth These experiments by a mechanism other than acting as a source of cysteine. provide the first demonstration of a situation in which the provision of It may be additional cysteine failed to stimulate wool growth significantly. postulated that, even when cysteine is supplied, without sufficient methionine to perform some specific function, the machinery for synthesis of wool proteins Specific effects of methionine on wool growth cannot function at a high rate. This are most probably related to the formation of S-adenosyl-L-methionine. compound is a methyl donor for many important reactions which'may influence wool It is also an intermediate in the biosynthesis of the growth (see Reis 1979). polyamines, spermidine and spermine, which appear to have a role in nucleic acid and protein synthesis, especially in actively dividing tissues (Tabor and Tabor 1976; Williams-Ashman and Canellakis 1979). 481 Animal Production in Australia The relative importance of cyst(e)ine and methionine in controlling the rate of wool growth would be best determined during experiments in which the animal's total supply of amino acids was controlled, by giving all nutrients either post-ruminally or parenterally. ACKNOWLEDGMENTS Thanks are due to Messrs. W.D. Herrmann and D.A. Tunks for their skilled assis tance in th e conduct of these experiments REFERENCES 2): 2. 11 'ew 421.