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Sulfur Amino Acid Activity of d- and l-Homocysteine for Chicks 1

Summary Young male chicks were fed crystalline l-amino acid diets in assays designed to evaluate the capacity of d-and l-homocysteine to supply methionine and cysteine. l-Homocysteine had a cystine-sparing value of 100% and a methio-nine-sparing value of 28% at the level fed. d-Homocysteine was utilized less efficiently than was l-homocysteine, having a cystine-sparing value of 68% and a methionine-sparing value of only 7%. The methionine activity of l-homocysteine varied when evaluated in a cystine-adequate diet, being more efficiently utilized at low levels of intake. However, at every level fed, l-homocysteine was inferior to l-methionine in supporting growth. The presence of additional choline in the diet did not improve the performance of chicks fed l-homocysteine as a source of methionine. Performance of chicks fed dl-homocysteine as a source of total sulfur amino acids was equal to that of chicks fed dl-homocysteine as a source of methionine per se. The utilization of the d and l isomers of homocysteine appears to be limited by the methionine activity they can replace. The results of these assays clearly indicate that young chicks fed a crystalline l-amino acid diet cannot efficiently convert either d- or l-homocysteine to methionine via transmethylation.