Effects of substitution of methionine, leucine, phenylalanine, or valine by their alpha-hydroxy analogs in the diet of rats

Abstract

Weanling male rats were fed diets containing essential amino acids as the sole source of nitrogen at a level of either 6 or 15% by weight of the diet. When L-valine was replaced isonitrogenously by calcium L-alpha-hydroxyisovalerate in the 15% amino acid diet, growth ceased and urea excretion increased; urea clearance was unaffected. When L-leucine was replaced isonitrogenously by calcium L-alpha-hydroxyisocaproate in this diet, growth and feed efficiency fell approximately 60% and urea excretion rose, but urea clearance doubled. L-Phenylalanine was replaced both isonitrogenously and without supplemental glutamate by calcium L-phenyllactate in both the 6 and 15% amino acid diets and was also added in twice equimolar quantities to the 6% amino acid diet. When the 15% amino acid diet was fed, growth was severly reduced, but when the 6% diet was fed, the double dose of phenyllactate reduced growth and feed efficiency only 25%. Urea clearance was not consistently altered. When D,L-methionine was replaced by calcium K,L-alpha-hydroxy-gamma-thiobutyrate in the 15% amino acid diet, growth and feed efficiency improved whether or not glutamate was added to make diets isonitrogenous, and also whether or not 0.1% neomycin was included. Urea clearance did not differ among these groups but was significantly higher in rats receiving the hydroxy analog or D,L-methionine than that in controls receiving L-methionine. Thus, the ability of these hydroxy analogs to replace the corresponding amino acids in the diet of rats varies from complete (methionine) to partial (leucine and phenylalanine) or virtually nonexistent (valine).