Utilization of D-asparagine by Saccharomyces cerevisiae

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Abstract
Yeast strains .SIGMA. 1278b and Harden and Young, which synthesize only an internal constitutive form of L-asparaginase, do not grow on D-asparagine as a sole N source, and whole cell suspensions of these strains do not hydrolyze D-asparagine. Strains X2180-A2 and D273-10B, which possess an externally active form of asparaginase, are able to grow slowly on D-asparagine, and N-starved suspensions of these strains exhibit high activity toward the D-isomer. N starvation of strain X2180-A2 results in coordinate increase of D- and L-asparaginase activity; the specific activity observed for the D-isomer is .apprx. 20% greater than that observed for the L-isomer. In studies with cell extracts hydrolysis of D-asparagine occurred only with extracts from N-starved cells of strains that synthesize the external form of asparaginase. Furthermore, the activity of the extracts toward the D-isomer was always higher than that observed with the L-isomer. A 400-fold purified preparation of external asparaginase from S. cerevisiae X2180-A2 hydrolyzed D-asparagine with an apparent Km of 0.23 mM and a Vmax of 38.7 .mu.mol/min per mg of protein. D-Asparagine was a competitive inhibitor of L-asparagine hydrolysis and the Ki determined for this inhibition was approximately equal to its Km. D-asparagine seems to be a good substrate for the external yeast asparaginase but is a poor substrate for the internal enzyme.