ENZYMATIC CHARACTERISTICS OF UDP-SULFOQUINOVOSE - DIACYLGLYCEROL SULFOQUINOVOSYLTRANSFERASE FROM CHLOROPLAST ENVELOPES

Envelope membranes from chloroplasts contain UDP-sulfoquinovose:diacylglycerol sulfoquinovosyltransferase which catalyses the final step in sulfolipid assembly. In situ produced diacylglycerol served as radioactive acceptor to measure enzymatic activity. With this assay, several enzymatic parameters were investigated. The enzyme, which has maximal activity at pH 7.5, was stimulated by magnesium ions due to a decrease of the K(m) for uridine 5'-diphospho-sulfoquinovose from 80-mu-M (no magnesium) to 10-mu-M (5 mM magnesium). This stimulation had a K(m) of 0. 7 mM magnesium and may be relevant in light/dark modulation of the enzymatic activity. The lower efficiency of guanosine 5'-diphospho-sulfoquinovose observed before can be ascribed to a higher K(m) of this sugar nucleotide (400-mu-M). Under optimized and linearized conditions the sulfoquinovosyltransferase displayed about 10 % of the activity of the UDP-galactose: diacylglycerol galactosyltransferase which competes in the same membrane system for diacylglycerols. Addition of acidic lipids, such as sulfolipid and phosphatidylglycerol, to envelope membranes resulted in an inhibition of the sulfoquinovosyltransferase, whereas the galactosyltransferase was not affected. In vivo this may contribute to an adjustment of the sulfolipid proportion in plastid membranes. In contrast to the galactosyltransferase the sulfoquinovosyltransferase did not discriminate against the dipalmitoyl molecular species of diacylglycerol when offered together with the oleoyl-palmitoyl species. Under conditions when oleoyl-palmitoyl-and dipalmitoyl-diacylglycerols were synthesized with concurrent conversion to monogalactosyl and sulfoquinovosyl diacylglycerol, the sulfolipid was highly enriched in the fully saturated species. This may explain the occurrence of dipalmitoyl species in sulfolipids, as found in many plants.