Studies on multiple forms of alpha-galactosidase from Vicia faba

Abstract

The various molecular forms of alpha-galactosidase from dormant and germinated Vicia faba seeds have been studied. The enzymes have been purified by multistage procedure and have been examined by ultracentrifugation and gel-filtration. The molecular weights of the enzymes, as determined by ultracentrifugation, were160,000 +/- 2850 for enzyme I from dormant seeds, 54,340 +/- 5225 forenzyme II from dormant seeds, and 47,060 +/- 2941 for enzyme II from germinated seeds. There was some discrepancy between these values and those obtained by gel-filtration and this is discussed.Enzyme II from dormant seeds can be resolved on Cm-eellulose into two active fractions (II1 and II2) together with four other inactiveprotein fractions. Enzyme II and II have molecular weights of45,730 +/- 3073 and 43,390 +/- 1409, respectively, as shown by ultra-centrifuge measurements. Gel filtration measurements give similar values. Enzyme II from germinated seed can also be resolved into similar fractions by ion-exchange chromatography.

Amino acid analysis of all the molecular forms of the enzyme together with the molecular weight data, suggest that enzyme I is composed of subunits of II; the latter enzyme may be composed of a mixture of II1 and II2 together with other inactive proteins. This theory is to some extent supported by the fact that a preparation of enzyme II was observed to convert in vitro to an enzyme with very similar and physical properties to enzyme I from dormant seeds. Some factors affecting this conversion have been investigated.

The kinetic properties of the various forms of alpha-galactosidase have been examined including substrate specificity, effect of pH, Vmax and Km determinations with p-Nitrophenyl alpha-D-galactopyranoside as substrate and activation by K+ and other alkali metal ions.

The physiological role of alpha-galactosidases in seeds has been considered.