Until recently, b-oxidation was believed to be exclusively located in the peroxisomes of all higher plants. Whilst this is true for germinating oilseeds undergoing gluconeogenesis, evidence demonstrating mitochondrial b-oxidation in other plant systems has refuted this central dogma of plant lipid metabolism. This report describes a comparative study of the dual mitochondrial and peroxisomal b-oxidation capacities of plant organs. Oxidation of [1-14 C]palmitate was measured in the cotyledons, plumules and radicles of Pisum sativum L., which is a starchy seed, over a 14 day period from the commencement of imbibition. Respiratory chain inhibitors were used for di¡erentiating between mitochondrial and peroxisomal b-oxidation. Peroxisomal b-oxidation gave a steady, baseline rate and, in the early stages of seedling development, accounted for 70^100% of the b-oxidation observed. Mitochondrial b-oxidation gave peaks of activity at days 7 and 10^11, accounting for up to 82% of the total b-oxidation activity at these times. These peaks coincide with key stages of seedling development and were not observed when normal development was disrupted by growth in the dark. Peroxisomal b-oxidation was una¡ected by etiolation. Since mitochondrial b-oxidation was overt only during times of intense biosynthetic activity it might be switched on or o¡ during seedling development. In contrast, peroxisomes maintained a continuous, low b-oxidation activity that could be essential in removing harmful free fatty acids, e.g. those produced by protein and lipid turnover.
H'''CO, was not incorporated into fatty acids by isolated pea leaf chloroplasts, which, therefore, do not possess a self-contained pathway for the synthesis of fatty acids from early intermediates of the Calvin cycle. Cittate, pyruvate, acetate and L-acctylcarnitine were all shown to act as sources of acetyl groups for fatty acid synthesis by pea leaf chloroplasts. L-acetylearniline was the best substrate, being incorporated into fatty acids at rates that were at least live-fold higher than those achieved with the other substrates. Citrate was incotporated into fatty acids at the lowest rate, followed by pyruvate, with acetate being incorporated al the second highest rate of all. When the isolated ehloroplasls wctc ruptured, an inhibition of L-acctylcarnitinc incorpotation into fatty acids was noted, whilst acetate incorpotation remained unaffected. L-acctylcarnitine also increased the ratio of monoenoic: saturated fatty acids synthesized, compated wilh a 1 : 1 ratio observed when citrate, pyruvale and acetate were supplied as substrates. It is suggested that L-earnitine and carnitine acyllransfcrascs play a central role in plant acyl CoA metabolistn by facilitating the transfer of activated acyl gtoups across membranes (acyl CoA barriers).
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