Distribution of radioactive lipids between envelopes and thylakoids from chloroplasts labelled in vivo.

Spinach leaves were labelled with 14CO2 for subsequent isolation of radioactive chloroplasts, which were separated into envelopes and thylakoids. The analyses carried out with the lipid extracts from these membranes were concerned with the following questions: are chloroplast envelopes a functional interface between endoplasmic reticulum and thylakoids, do they also play a predominant role in galactolipid biosynthesis in vivo, and is it possible to demonstrate galactolipid export from envelopes into thylakoids? Taken together the results show that lipid export is apparently too fast in vivo to be followed by the labelling and separation technique used, since thylakoid lipids contained always far more total label than envelope counterparts, whereas the specific activity of envelope lipids was higher. Phosphatidylcholine, which has been suggested to function as acyl carrier between endoplasmic reticulum and chloroplasts, was never found labelled to any extraordinary extent in envelopes. Envelopes may be regarded as small, but rapidly turned over lipid pools.