Some observations on the biosynthesis of the plant sulpholipid by Euglena gracilis.

1. dl-Cysteine decreases the uptake of (35)SO(4) (2-) by Euglena gracilis but does not decrease the relative incorporation of the isotope into sulpholipid; cysteic acid, on the other hand, does not affect the uptake of (35)SO(4) (2-) but does dilute out its incorporation into the sulpholipid. 2. Both l-[(35)S]cysteic acid and dl-+meso-[3-(14)C]cysteic acid appear almost exclusively in 6-sulphoquinovose. 3. Molybdate inhibits the incorporation of (35)SO(4) (2-) into sulpholipid but not its uptake into the cells; this suggests that adenosine 3'-phosphate 5'-sulphatophosphate may be concerned with the biosynthesis of sulpholipid, and it was shown to be formed by chloroplast fragments. 4. An outline scheme for sulpholipid biosynthesis based on these observations is discussed.