Biosynthesis in vitro of pectic (1->4)-beta-D-galactan

Membrane preparations from mung bean (Vigna radiata) hypocotyls catalysed the incorporation of label from UDP-C-14-galactose (UDP-Gal) into a polymeric product which released only C-14-galactose on acid hydrolysis. The product was characterised as (1-->4)-beta-D-galactan by hydrolysis using two pure enzymes, a novel exo-(1-->4)-beta-D-galactanase from the cotyledons of germinated Lupinus angustifolius seeds [1] and an endo-(1-->4)-beta-D-galactanase from Aspergillus niger, followed by TLC separation and quantitative digital autoradiography of the labelled digestion products. Digital autoradiographic methods were used also to show that other UDP-Gal utilising enzyme activities, including UDP-Gal-4-epimerase and phosphodiesterases, were present in the catalytic membrane preparations, and competed significantly with the galactan synthase for the UDP-Gal substrate. The galactan synthase had an apparent pH optimum at pH 6.5 in the presence of Mg+2. Galactan synthase activity peaked two days after the seeds were set out to germinate, and this was followed by a marked increase in the (1-->4)-beta-D-galactan content of the tissue cell walls.