Enzymes of asparagine catabolism in the developing maize endosperm

Asparaginase and asparagine transaminase (asparagine-pyruvate and asparagine-α-ketoglutarate) activities have been identified in maize endosperm. In each case there is a low level of activity at day 5, an increase to a maximum, and finally a decline in activity. The peaks for the transaminase activities (asparagine-pyruvate, 241 nmol∙h−1∙endosperm−1, and asparagine-α-ketoglutarate, 223 nmol∙h−1∙endosperm−1) were somewhat later in time than the peak in asparaginase activity (349 nmol∙h−1∙endosperm−1). The initial increase coincides with the increase in zein protein in the endosperm. Both the levels of activity at each stage of development and Km for asparagine (about 10 mM) suggest that either reaction could be responsible for the degradation of asparagine in maize endosperm. The degradation of asparagine in developing seeds is apparently not as significant a reaction in Zea mays as it is in Lupinus or Pisum.