Purification of UDP-galactose: diacylglycerol galactosyltransferase from chloroplast envelopes of spinach (Spinacia oleracea L.).

Uridine 5'-diphosphate(UDP)-galactose: 1,2-diacylglycerol 3-O-β-D-galactopyranosyltransferase (EC 2.4.1.46) is an integral protein of chloroplast envelope membranes from which it has been partially purified (Covès et al., 1986, FEBS Lett. 208, 401-406). We have worked out a purification procedure which after removal of peripheral membrane proteins, solubilization and two chromotographic steps allowed us to identify a 22-kDa protein as the galactosyltransferase. Enrichment of enzymatic activity was paralleled by an enrichment of this protein and its radioactive derivative obtained by photoaffinity labelling with [α-(-32)P]UDP which is a potent inhibitor of the enzyme. The purification factor of about 350 is substantially higher than achieved previously and indicates that the enzyme represents less than 0.3% of the envelope proteins. The purified enzyme has a Km of 87 μM for UDP-galactose with dioleoylglycerol as acceptor and could not be activated by addition of other lipids.