The apparent absence of a pathway for synthesis of acetyl coenzyme A in pea chloroplasts: Lack of (14)CO 2 incorporation into lipids by the isolated organelle.

We have examined the extent to which isotopic lable derived from photosynthetically fixed (14)CO2 can be transferred to lipids by aqueously isolated chloroplasts of Pisum sativum. Although photosynthetically active, chloroplast preparations incubated with (14)CO2 showed little or no accumulation of label in lipids under any condition tested. Under identical conditions the chloroplasts were readily able to incorporate [(14)C]acetate into the lipid fraction; a fatty-acid synthesizing system was therefore operative in these chloroplasts.The essential failure of the isolated chloroplasts to incorporate label from fixed (14)CO2 into fatty acids supports the view that the organelle itself does not possess a self-contained pathway for the synthesis of acetyl coenzyme A, and favours the possibility that a shuttle mechanism involving the participation of extra-chloroplastic enzymes may be responsible for supplying the chloroplast with acetyl coenzyme A in vivo.