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Metabolism and Enzymology

Release and Activity of Bound -Amylase in a Germinating Barley Grain ¹

Laboratoire de Physiologie des Organes Végétaux, CNRS, 4 ter route des Gardes, 92190 Meudon, France, Biotechnical Laboratory, Technical Research Centre of Finland, Tietotie 2, 02150 Espoo, Finland

In resting grains of Triumph barley (Hordeum vulgare L. cv Triumph) about 40% of the -amylase could be extracted with a saline solution, the remaining 60% being in a bound form. During seedling growth (20°C), the bound form was released mainly between days 1 and 3. When a preparation containing bound -amylase was incubated with an extract made of endosperms separated from germinating grains, release of bound -amylase took place and could be studied in vitro. The release was almost completely prevented by leupeptin and antipain, specific inhibitors of a group of SH-proteinases, but it was not inhibited by pepstatin A or EDTA, which inhibit some other barley proteinases. It is thus very likely that in a whole grain, at least the bulk of the bound -amylase is released by the proteolytic action of one or several SH-proteinases. When the bound -amylase was released by papain, its molecular weight was about 5000 daltons smaller than that of -amylase released by dithiothreitol. This indicates that the release is due to removal of a sequence of -amylase itself. A similar decrease in size took place during seedling growth. Bound -amylase showed some activity against native starch and it hydrolyzed maltotetraose at a rate that was about 70% of the rate the same amount of bound -amylase gave after release. Bound -amylase is thus not inactive and it is likely that the slower rate of hydrolysis is due to steric hindrances which prevent substrates from reaching the active site.

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