PLANT PHYSIOLOGY , Vol 111, Issue 1 329-338, Copyright © 1996 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Sucrolytic Enzyme Activities in Cotyledons of the Faba Bean (Developmental Changes and Purification of Alkaline Invertase)
H. A. Ross, D. McRae and H. V. Davies
Department of Cellular and Environmental Physiology, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom
In terms of maximum extractable catalytic activity, sucrose synthase is the
predominant sucrolytic enzyme in developing cotyledons of faba bean (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. Calculations 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 maximum
extractable catalytic activities of both enzymes is in excess of the actual
rate of starch deposition. Two forms of alkaline invertase were identified
in developing cotyledons. The major form has been purified to homogeneity,
and antibodies have been raised against it. The native protein has a
molecular mass of about 238 [plus or minus] 4.5 kD. It is apparently a
homotetramer (subunit molecular mass 53.4 [plus or minus] 0.9 kD). The
enzyme has a pH optimum of 7.4, an isoelectric point of 5.2, and a
Km[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
[beta]-fructofuranosidase with no significant activity against raffinose,
stachyose, trehalose, maltose, or lactose.
