Utilization of D asparagine by Saccharomyces cerevisiae

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 source of nitrogen, 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 nitrogen starved suspensions of these strains exhibit high activity toward the D isomer. Nitrogen starvation of strain X2180 A2 results in coordinate increase of D- and L asparaginase activity; the specific activity observed for the D isomer is 20% greater than that observed for the L isomer. It was observed, in studies with cell extracts, that hydrolysis of D asparagine occurred only with extracts from nitrogen 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 and apparent K(m) of 0.23 mM and a V(max) of 38.7 μmol/min per mg of protein. D Asparagine was a competitive inhibitor of L asparagine hydrolysis and the K(i) determined for this inhibition was approximately equal to its K(m). These data suggest the D asparagine is a good substrate for the external yeast asparaginase but is a poor substrate for the internal enzyme.