2. Starchy endosperm. Proteinases effective in breaking down the major protein reserves within the starchy endosperm may come from two sources: the aleurone layer or the starchy endosperm itself. In barley, for example, endopep- tidases released from the aleurone layer may play a central role in endosperm protein hydrolysis. But there are also preformed proteinases within the en¬dosperm of dry grains and these also may be involved in protein mobilization. Besides their action in degrading reserve proteins, proteinases in the starchy endosperm also serve to release and activate bound enzymes (e.g., p-amylase), and to aid in dissolution of the cell walls by hydrolyzing links between glucan and protein components.