Purification and Partial Characterization of a Dipeptidase from Barley

Abstract

A peptidase hydrolyzing the dipeptide Ala-Gly optimally at pH 8 to 9 was purified about 3500-fold from germinated grains of barley (Hordeum vulgare L.). According to disc electrophoresis in the presence of sodium dodecyl sulfate, the preparation was about 90% pure.The enzyme preparation hydrolyzed all of the 15 neutral dipeptides tested, but showed no activity on Lys-Gly or Asp-Ala. Three tripeptides and four synthetic aminopeptidase substrates were hydrolyzed at less than 0.1% of the rate for Ala-Gly; inhibition and inactivation tests indicated that these reactions were due to some contaminating aminopeptidase(s). The results suggest that the purified enzyme is a specific dipeptidase. Km values were determined for six dipeptides at pH 8.8; they varied from 0.3 to 15.8 mm. The enzyme was strongly inhibited both by metal chelators and by sulfhydryl reagents. The elution volumes of the enzyme in gel chromatography on Sephadex G-150 and Ultrogel AcA 22 corresponded to molecular weight 130,000 and 175,000, respectively. The migration rate in sodium dodecyl sulfate disc electrophoresis indicated a subunit molecular weight of 50,000.The barley enzyme is remarkably similar to several mammalian and microbial dipeptidases.