Control of storage protein metabolism in the cotyledons of germinating mung beans: role of endopeptidase

Abstract

The autodigestive proteolytic activity of extracts of cotyledons of mung beans (Phaseolus aureus Roxb.) increased 4- to 5-fold during germination. A similar increase was found in the ability of these extracts to digest added casein or mung bean globulins. The increase occurred after a 2-day lag during the next 2 to 3 days of germination and coincided with the period of rapid storage protein breakdown. To understand which enzyme(s) may be responsible for this increase in proteolytic activity, the hydrolytic activity of cotyledon extracts toward a number of synthetic substrates and proteins was measured. Germination was accompanied by a marked decline in leucine aminopeptidase, while carboxypeptidase increased about 50%. There were no dramatic changes in either alpha-mannosidase or N-acetyl-beta-glucosaminidase, enzymes which may be involved in the metabolism of the carbohydrate moieties of the reserve glycoproteins. The increase in general proteolytic activity was closely paralleled by a 10-fold increase in endopeptidase activity. This activity was inhibited by sulfhydryl reagents such as N-ethylmaleimide. Studies with inhibitors of proteolytic enzymes showed that reagents which blocked sulfhydryl groups also inhibited the rise in general proteolytic activity. Our results suggest that the appearance of a sulfhydryl-type endopeptidase activity is a necessary prerequisite for the rapid metabolism of the reserve proteins which accompanies germination.