P. Grattan Roughan

Some evidence for the reversibility of the cholinephosphotransferasecatalysed reaction in developing linseed cotyledons in vivo

Abstract The relative proportions of3H and32P incorporated into the glycerol and phosphorus moieties of individual phospholipids of linseed cotyledons doubly-labelled with [2-3H]glycerol and32PO4 differed considerably-Whereas [3H]glycerol was incorporated predominantly into PC,32PO4 was not and the ratio of3H/32P in this phospholipid was appreciably greater than in other phospholipids. Neither [methyl-14C]choline nor32PO4was incorporated into the different molecular species of PC in the same relative proportions as [pH]glycerol, and there was more radioactivity in the phosphorylcholine moiety relative to the glycerol moiety in the highly unsaturated molecular species. The specific radioactivities of the phosphorus moieties of most PC species were similar at a particular harvest, whereas the glycerol specific radioactivities of highly unsaturated species were lower than those of the more saturated species. Little [14Ccholine accumulated in CDPcholine in cotyledons supplied with [methyl-14Ccholine and the amount of radioactivity in this compound was less than 1% of that in phosphorylcholine plus PC. The specific radioactivity of phosphorylcholine was appreciably greater than those of the other choline derivatives. Comparisons of the changes with time in the specific radioactivities of the C18 unsaturated fatty acids in PC and 1,2-di acylglycerol in cotyledons labelled with [14Cacetate and of the glycerol moieties of the individual species these two lipids in cotyledons labelled with [3H]glycerol indicated that an exchange of diacylglycerol must occur between PC and diacylglycerol pools in the tissue. It is argued that each of the independent observations described can best be explained as resulting from the reversibility of the choline phosphotransferase reaction in the cotyledons.

Acyl lipid synthesis by chloroplasts isolated from the chilling-sensitive plant Amaranthus lividus L

Abstract Mesophyll chloroplasts isolated from leaves of the chilling-sensitive C4 plant, Amaranthus lividus, exhibited high rates of light-dependent biosynthetic activities. Long-chain fatty acid synthesis from [1-14C]acetate (up to 1.55 μmol of acetate incorporated/h per mg chlorophyll) and phosphatidylglycerol synthesis from phosphatidate were both strongly inhibited by the presence of Mg2+ in incubation media. Phosphatidyl-glycerol synthesis in vitro was dependent upon exogenous. sn-glycerol 3-phosphate and CTP, and could account for up to 12%, whereas non-esterified fatty acids accounted for a minimum of 63% of incorporated acetate. Both phosphatidate and phosphatidylglycerol synthesised by A. lividus chloroplasts contained 84–88% saturated fatty acids and, therefore, were comprised of 68–76% disaturated molecular species. Non-esterified fatty acids, on the other hand, were comprised predominantly (75–80%) of oleate. In the absence of exogenous CTP, phosphatidate was hydrolysed to 1,2-diacylglycerol which was then converted to diacylgalactosylglycerol in the presence of UDPgalactose. A. lividus appears to synthesise highly saturated ‘prokaryotic’ glycerolipids because the chloroplast sn-glycerol-3-phosphate:acyl-acyl carrier protein acyltransferase does not discriminate between palmitoyl- and oleoyl-acyl carrier protein.