SUCROLYTIC ENZYME-ACTIVITIES IN COTYLEDONS OF THE FABA BEAN - DEVELOPMENTAL-CHANGES AND PURIFICATION OF ALKALINE INVERTASE

In terms of maximum extractable catalytic activity, sucrose synthase i s the predominant sucrolytic enzyme in developing cotyledons of faba b ean (Vicia faba L.). Although acid invertase activity is extremely low , there is significant activity of alkaline invertase, the majority of which is extractable only with high concentrations of NaCl. Calculati ons of potential activity in vivo indicate that alkaline invertase is the predominant sucrolytic enzyme from 50 days after anthesis onward. However, at almost all stages of cotyledon development analyzed, the m aximum extractable catalytic activities of both enzymes is. in excess of the actual rate of starch deposition. Two forms of alkaline inverta se were identified in developing cotyledons. The major form has been p urified to homogeneity, and antibodies have been raised against it. Th e native protein has a molecular mass of about 238 +/- 4.5 kD. It is a pparently a homotetramer (subunit molecular mass 53.4 +/- 0.9 kD). The enzyme has a pH optimum of 7.4, an isoelectric point of 5.2, and a K- m[sucrose] of 10 mM and is inhibited by Tris (50% inhibition at 5 mM) and fructose (30% inhibition at 10 mM). Bean alkaline invertase is a b eta-fructofuranosidase with no significant activity against raffinose, stachyose, trehalose, maltose, or lactose.