Peptidases in germinating barley grain: properties, localization and possible functions.

Barley grain contains about 10% insoluble reserve proteins. When the grain germinates the reserve proteins are hydrolysed to amino acids and transported to the growing tissues of the seedling. In the resting grain most of the reserve proteins are 'packed' into the non-living storage tissue, the starchy endosperm. During germination the internal pH of the starchy endosperm is about 5, and it contains high activities of proteinases (secreted by the living aleurone cells) and carboxypeptidases, all with pH optima between 4 and 6. As a whole the starchy endosperm of a germinating grain resembles a giant secondary lysosome. Adjacent to the starchy endosperm is a specialized absorptive and processing tissue, the scutellum. This organ contains very high activities of the 'acid carboxypeptidases' and also two 'alkaline peptidase': a leucine aminopeptidase and a dipeptidase, both pH optima at 8 to 10. The high peptidase activities in the scutellum suggest that the hydrolysis products of the reserve proteins are absorbed from the starchy endosperm as a mixture of amino acids and small peptides, which are hydrolysed to amino acids in the scutellum before transport to the growing seedling tissues.