Ricinoleic acid biosynthesis and triacylglycerol assembly in microsomal preparations from developing castor-bean (Ricinus communis) endosperm.
Microsomal membrane preparations from the developing endosperm of castor bean (Ricinus communis) catalysed the transfer of oleate from [14C]oleoyl-CoA to phosphatidylcholine (PtdCho). In the presence of NADH, radioactive ricinoleate (12-hydroxyoctadec-9-enoate) was synthesized from [14C]oleate, and this was largely recovered in PtdCho and as free fatty acid. The addition of unlabelled ricinoleoyl-CoA to these incubation mixtures did not increase the low [14C]ricinoleate concentration found in the acyl-CoA fraction nor decrease the [14C]ricinoleate concentration in PtdCho and free fatty acid, and thus no evidence was obtained for a hydroxylation with oleoyl-CoA as a substrate. The addition of NADH, necessary for the formation of ricinoleate, caused a decrease of the total radioactivity in PtdCho with a corresponding increase in the amount of label in free ricinoleic acid. This increase was due to the action of a phospholipase A, which released ricinoleic acid but not oleic acid from PtdCho. Such a phospholipase activity, attacking ricinoleoyl-PtdCho but not oleoyl-PtdCho, was also demonstrated in microsomal preparations from developing cotyledons of safflower and oil-seed rape. An analysis of the acyl groups at different positions in microsomal PtdCho of castor bean showed that ricinoleate was almost entirely associated with position sn-2. Likewise the [14C]ricinoleate in [14C]PtdCho formed after incubations with microsomal preparations with NADH and [14C]oleoyl-CoA resided in position sn-2 with none in position sn-1. In contrast, the [14C]linoleate formed by desaturation of [14C]oleoyl-PtdCho was present at both positions. In the presence of ATP, CoA and Mg2+, the ricinoleate acid released from PtdCho was activated to ricinoleoyl-CoA. The ricinoleoyl-CoA was an efficient acyl donor in the acylation of glycerol 3-phosphate (Gro3P) to yield phosphatidic acid and triacylglycerols. In microsomal preparations incubated with an equimolar mixture of [14C]oleoyl-CoA and [14C]ricinoleoyl-CoA in the presence of Gro3P, only a minor amount of [14C]ricinoleate entered PtdCho, and this was believed to be via the exchange of phosphocholine groups between a diacylglycerol pool and the PtdCho. On the basis of our results, a scheme of ricinoleate formation and its incorporation into triacylglycerols in castor-bean endosperm is proposed.
Bafor
M
M
Department of Plant Physiology, Swedish University of Agricultural Sciences, Uppsala.
Smith
M A
MA
Jonsson
L
L
Stobart
K
K
Stymne
S
S
D000255:Adenosine Triphosphate / Q000378:metabolism; D002855:Chromatography, Thin Layer; D003065:Coenzyme A / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D009243:NAD / Q000378:metabolism; D019301:Oleic Acid; D009829:Oleic Acids / Q000378:metabolism; D010713:Phosphatidylcholines / Q000378:metabolism; D010947:Plants, Toxic; D012277:Ricinoleic Acids / Q000378:metabolism*; D012278:Ricinus / Q000196:embryology / Q000378:metabolism*; D014280:Triglycerides / Q000378:metabolism*
D009829:Oleic Acids; D010713:Phosphatidylcholines; D012277:Ricinoleic Acids; D014280:Triglycerides; D009243:NAD; D019301:Oleic Acid; D000255:Adenosine Triphosphate; D006899:Mixed Function Oxygenases; C030521:ricinoleic acid; D008274:Magnesium; D003065:Coenzyme A