Biosynthesis of galactan by a particulate enzyme preparation from Phaseolus aureus seedlings

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Abstract
A particulate cell-free enzyme system was prepared from Phaseolus aureus shoots. This preparation was able to incorporate [14C]galactose from UDP-[14C]galactose into a water-soluble polysaccharide, which has a probable molecular weight of at least 4600. The only labelled component detectable in the polymer was shown to be [14C]galactose; two labelled oligosaccharides containing only [14C]galactose were isolated by partial hydrolysis. The galactan-synthesizing activity of this particulate preparation is maximal at 30° and pH7·1 in the presence of 5·0mm-magnesium chloride and 0·2m-sucrose. Although 3-day-old seedlings were used as a source of enzyme, it appears that 4- or 5-day-old beans contain greater synthetase activity. The enzyme system has an apparent Michaelis constant of 5·8×10−6m, and will catalyse the polymerization of galactose residues at the rate of 7·5mμmoles/mg. of protein/min. at a substrate concentration of 9·6mm.