Transamination of 2-oxo-4-[methylthio]butanoic acid in chicken tissues

The keto acid 2-oxo-4[methylthio]butanoic acid (OMTB) is an intermediate in the conversion of synthetic feed grade methionine sources to L-methionine in vivo in poultry and other animals. Because methionine sources are utilized by the chick with considerably less than 100% efficiency as sources of L-methionine, it is important to determine what metabolic process may limit the utilization of these sources. Because OMTB is converted to L-methionine by transamination, a study was conducted to determine which amino acids might serve as nitrogen donors in the conversion of OMTB to L-methionine in the chicken. Dialyzed tissue homogenates, mitochondria, and cytosol from liver, kidney, intestine, and skeletal muscle were incubated with OMTB and individual L-amino acids (isoleucine, leucine, valine, glutamic acid, aspartic acid, alanine, glutamine, asparagine, and phenylalanine) and the methionine that accumulated was determined by ion exchange chromatography. Tissues differed in the conversion of OMTB to methionine: kidney was most active, liver and intestinal mucosa were intermediate, and skeletal muscle had lowest activity. All amino acids supported methionine synthesis. Branched-chain amino acids and glutamic acid were the most effective substrates in tissue cytosols except in intestinal mucosa, in which asparagine was also effective. The preferred substrates in mitochondria were glutamate in liver mitochondria, isoleucine and alanine in kidney mitochondria, and branched-chain amino acids and glutamic acid in skeletal muscle mitochondria. All amino acids except alanine supported methionine synthesis from OMTB in mitochondria of intestinal mucosa. We conclude that a wide variety of amino acids can serve as substrates for transamination of OMTB in the chicken, and that the availability of nitrogen donors is unlikely to be a limiting factor in the conversion of OMTB to methionine.