Changes of aminopeptidase activities in barley grain during storage

We have investigated changes of aminopeptidase (AP) activities in barley grain during one year storage at warehouse conditions (20-25 degrees C, 65% RH). Using various amino acid-2-naphtylamides (2NA) as aminopeptidase substrates we have determined activities of these enzymes in three barley cultivars: Angora, Barun and Rodnik at the start and after a year of storage. Another three barley cultivars: Sladoran, Rex and Martin were tested for AP activities only after one year of storage. Depending on amino acid-2NA used as substrate, it was found that during one-year storage AP activities decreased 20-25% for enzymes specific for Arg- and Met-2NA, and 50-56% for enzymes splitting Phe- and Leu-2NA. This indicates unequal stability of various aminopeptidases present in dry grains. Also, some differences of AP stability in different cultivars were noticed. Comparing relative AP activities after one-year storage, it was observed that extent of enzyme changes is well presented by Arg-2NA/Phe-2NA or Met-2NA/Phe-2NA aminopeptidase activity ratios. The obtained results demonstrate the loss of AP activities after a year of storage under warehouse conditions. They also indicate a possibility to use AP activity ratio for the estimation of age of seeds stored under warehouse conditions. However, for the application of these criteria further standardization of methods and ranking of cultivars is needed.