Proteolytic Activity and Nitrate Reductase Inactivation in Maize Seedlings

Abstract

The main proteinase in the maize root (proteinase I) has been characterized by its degradation of azocasein, inactivation of nitrate reductase and inhibition by phenylmethylsulfonyl fluoride. An enzyme with these properties has been found only at low activity in the maize shoot. Two other proteinase fractions were identified in the maize shoot by degradation of haemoglobin. These enzymes were inhibited by both p-chloromercuribenzoate and phenylmethylsulfonyl fluoride and are similar to a second proteinase fraction (proteinase II) in the maize root.

Root proteinase II, unlike root proteinase I, does not attack azocasein, but both degrade casein in addition to haemoglobin. The highest activity of proteinase II on casein was in the pH range 5.0-6.0, while high activity of proteinase I on this substrate occurred between pH 5.5 and pH 8.0. The endopeptidase substrate α-N-benzoyl-L-arginine-p-nitroanilide was hydrolysed by proteinase II, but not by proteinase I.

Increase in maize root age from 4 to 14 days was accompanied by a rise in the activity per gram fresh weight of proteinase I and carboxypeptidase I, the increase in the former accounting for the accumulation of nitrate reductase inactivating activity. By contrast, a marked decrease in the activity of proteinase II and carboxypeptidase II occurred in the same period of seedling growth.