The utilization of D-leucine for growth by the rat.

Experiments in tivo have demonstrated that animals are able to invert certain n-amino acids to their natural forms (l-4). This inversion has been repeatedly confirmed in studies with different species of animals by their ability to utilize the D enantiomorphs for t-he purposes of growth or maintenance of nitrogen equilibrium. In the case of the rat, the D isomers of histidine (5, 6), methionine (7-lo), phenylalanine (ll-13), tryptophan (14-17), tyrosine (18), and, more recently, valine (4, 19, 20) were shown to support growth.’ The unnatural isomers of isoleucine (21-23)) threonine (24), lysine (25-27), and cystine (28), on the other hand, were inactive. The question of the utilization of n-leucine remains unsolved, although it is generally assumed that the D enantiomorph is of no value for the growth of rats. Rose (21) drew this conclusion from earlier, but yet unpublished, studies in his laboratory (S. S. Fierke, thesis, University of Illinois, 1936). This conclusion is at variance with the later demonstration by Ratner, Schoenheimer, and Rittenberg (3), using deuterium and N16, that n-leucine can be converted to the L form by rats. More recently, n-leucine was also shown to be utilized to some extent in maintaining nitrogen equilibrium in the presence of a small but inadequate suppIy of L-leucine in the adult rat (29). In recent studies on the utilization of n-valine for growth of rats, Wretlind (19) observed that 2 per cent of nn-leucine in the diet caused growth inhibition which could be prevented by replacing the racemic amino acid with 1 per cent of n-leucine. Based on this finding, he concluded that the utilization of n-valine was blocked by n-leucine, possibly through competitive inhibition of the deamination of n-valine. In view of the fact that nn-leucine in the above diet was present in twice the concentration of L-leucine, it is possible that the growth inhibition was due to an excess of leucine rather than to the presence of the D isomer. It is well