L-Asparagainases from Citrobacter freundii

Abstract

Three enzymes which catalyze the hydrolysis of L-asparagine have been identified in extracts of Citrobacter freundii. One of these (asparaginase-glutaminase (EC 3.5.1.1) also shows substantial glutaminase activity. This enzyme is extremely labile, is sensitive to inactivation by p-chloromercuribenzoate, and is not protected by dithiothreitol. A second enzyme (asparaginase B) is also sensitive to mercurials but is protected from inactivation by dithiothreitol. This enzyme has a relatively low affinity for L-asparagine (Km = 1.7-10(-3) M). The third enzyme (asparaginase A) is insensitive to inactivation by mercurials, is stable upon long term storage and has a relatively high affinity for L-asparagine (Km = 2.9-10(-5) M). This enzyme has been purified to homogeneity and has a molecular weight of approx. 140 000; the subunit weight being approx. 33 000. The C. freundii asparaginase A produced significant increases in the survival time of C3H/HE mice carrying the 6C3HED lymphoma tumor.