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Effect of abomasal methionine infusion on methionine metabolism in the lamb Strath, Robert Adam


The ruminant with its extensive synthesis of amino acids by symbiotic microorganisms presents unique problems to studies of amino acid nutrition. Protein quality is dependent upon the available amino acids leaving the rumen, rather than those in the ingested diet. Protected proteins or amino acids may be fed to the animals or alternatively nitrogenous compounds may be administered postruminally to study ruminant amino acid nutrition. There is both a theoretical and practical interest in defining ruminant amino acid requirements under various production conditions. The relatively high content of the sulphur amino acids in ruminant products compared to that present in the rumen microorganisms suggests that these amino acids may be a limiting factor in ruminant production. In this study the essential sulphur containing amino acid, D,L-methionine, was infused into the abomasum of two growing lambs. Graded levels of methionine were infused for three days at each level. A jugular blood sample was collected on the third day of each infusion level. Plasma free amino acids were determined for each infusion level. One lamb Initially had an increase in plasma methionine concentration with each increase in the level of abomasal methionine. This indicates that methionine was not limiting for this lamb. For the other lamb an inflection point on the methionine response curve was observed just below 2.0 gm of infused abomasal methionine. This suggested that methionine may have been limiting for this lamb. One lamb developed diarrhoea at the higher infusion levels so a comparison of the two lambs was not permitted. Taurine increased with increasing methionine infusions and cystine rose to a constant level. At this infusion level the non-essential amino acids - serine, glycine and alanine - fell to constant levels. This suggests that methionine conversion to cystine was impaired due to a decreased concentration of serine. Generally, high methionine concentrations depressed the plasma concentrations of most amino acids. A tracer dose of ³⁵S-L-methionine was injected into a jugular vein of one lamb at levels below and above the limiting methionine infusion. Serial jugular blood samples were collected at various intervals to 24 hour post-injection. Total urine was collected once daily for four days post-injection. The radioactivity associated with the plasma proteins and other sulphur containing compounds of plasma were expressed as percentages. A plasma methionine specific activity curve indicated some kinetic parameters of plasma methionine. When methionine was suspected of being limiting, all of the activity of plasma was in the plasma protein fraction after 24 hours. There was no detectable conversion of methionine sulphur to other sulphur containing compounds in deproteinized plasma. Urinary excretion of the administered label was 13% after four days. During the infusion level when methionine was not limiting, only one half of the activity of plasma was in the protein fraction after 24 hours. At this level there was radioactivity in methionine, taurine, cystine, cystathionine and methionine sulfoxides of free plasma. The urinary excretion of radioactivity after four days was one-half of the injected dose. The plasma methionine kinetics indicated that the elevated methionine pool size was not due to an increase in total entry rate but due to a smaller increase in irreversible loss. This agrees favourably with impaired methionine metabolism. It was concluded that a tracer dose of methionine gave a dynamic picture of methionine metabolism. Below its limiting level, methionine is mainly involved in anabolic processes. Above the level of limiting methionine, catabolic metabolism of methionine becomes increasingly important. Therefore, it is suggested that ³⁵S-methiomne tracer techniques may provide a relatively quick and reliable tool for evaluation of the sulphur amino acid nutrition of ruminants.

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