Influence of Optical Activity on Utilization of Lysine for Growth

Abstract

Recent studies on the relative value for growth of the optical isomers of 2 essential amino acids have yielded divergent results. Thus, dl-tryptophane promotes growth as readily as the natural l form 1 and the d form is likewise as efficient as the l form 2 , 3 . On the other hand, i-cystine is less effective than l-cystine 4 and neither d-cystine 5 nor one half of the meso form 6 is available for growth. Several years ago McGinty, Lewis and Marvel 7 , in studying the replaceability of lysine by related hydroxy compounds, fed control rats both natural (d) lysine and synthetic (dl) lysine. In the one experiment in which equal amounts of the 2 compounds were employed, the natural amino acid promoted somewhat better growth. As an initial step in evaluating the growth-promoting ability of l- and d-lysine, we undertook a more exact comparison of the d isomer and the dl mixture.

d-Lysine was prepared as the dihydrochloride by the method of Cox, King, and Berg 8 and racemized essentially as directed by Fisher and Weigert 9 . The (dl-lysine was isolated as the dihydrochloride. [α]²⁰ _D for the former dihydrochloride (3% solution in water) was +15.64, for the latter 0.0. Analyses for N and Cl agreed with the calculated values. Zein served as the lysine-deficient protein and was prepared essentially according to the method of Brazier 10 . The diet was composed of zein, 14.6; tryptophane, 0.2; cystine, 0.2; starch, 39.5; sucrose, 15.0; Crisco, 19.0; cod liver oil, 5.0; salt mixture 11 , 4.5; and agar, 2.0%. In each case the lysine supplement replaced an amount of zein. In addition, 200 mg. of yeast was supplied separately to each animal daily. Twelve rats were divided into 4 groups of 3 each. One group received no lysine supplement; the second, 0.125%; the third, 0.25%; and the fourth 0.5%, in the form of an equivalent amount of the dihydrochloride.