STUDIES ON THE ACTIVATION AND RELEASE OF BOUND LIMIT DEXTRINASE IN MALTED BARLEY

Free and bound forms of limit dextrinase (LD) (EC 3.2.1.41) in malted barley (Hordeum vulgare L.) were studied. A soluble extract of malt co ntains two forms of LD, a free (active) and a latent or soluble bound (inactive) form. Conversion of the bound to the free form in vitro can be achieved using reducing agents or the proteolytic enzyme papain. M alt contains approximately 20% free and 70% soluble bound LD, with the remainder bound to the insoluble grist fraction. Soluble bound enzyme was released by papain but not by dithiothreitol (DTT) after endogeno us proteases had been inactivated by heating, consistent with release of bound enzyme by a proteolytic mechanism. However, inhibition of the release of soluble bound LD by leupeptin (a specific sulfhydryl prote ase inhibitor) was partly reversed by heating to 40 degrees C in the p resence of DTT or papain, indicating that the mechanism of release is not simply via protease activation. Release of soluble bound enzyme va ries with the period of extraction and temperature. The ratio of free to bound enzyme varies between malts prepared from different cultivars , but the majority of the enzyme in four- to five-day malts is bound. Gibberellic acid increases the proportion of free enzyme during maltin g but only when used at levels much higher than those used commerciall y.