Response of Nitrogen Metabolism, Plasma Amino Acids and Insulin Levels to Various Levels of Methionine Infusion in Sheep

Abstract

Adult sheep were prepared to receive total nutriment by dual infusions. Volatile fatty acids were infused intraruminally and glucose, vitamins, and amino acids via jugular cannula. Methionine and glycine were varied to provide isonitrogenous amino acid infusion with levels of 0, 0.75, 1.50, 2.25, 3.00, 3.75, 4.50, 5.25, and 6.00 g of methionine/day. Urinary nitrogen (UN), plasma urea nitrogen (PUN), urinary urea nitrogen (UUN), and nitrogen balance (NB) responded curvilinearly to increased methionine infusions. The point at which a tangent to the curve had a slope of zero was taken as the level at which methionine was no longer limiting. This occurred at infusion levels of 4.81 to 5.00 g/day. Plasma methionine increased sharply at infusion levels of 3.63 g/day. Most plasma amino acids were influenced by levels of methionine infusion with nonessential amino acids being higher when methionine was limiting or in excess. Plasma insulin and plasma glucose were also influenced by level of methionine infusion with plasma insulin responding in a manner similar to PUN, UN, UUN, and NB. J. Nutr. 104: 1646-1656, 1974.