Starch synthesis in amyloplasts purified from developing potato tubers

Amyloplasts have been purified from potato tubers by mechanical homogenization and gravity sedimentation through Nycodenz. Based on the recovery and latency of organelle-specific marker enzymes, the recovery of amyloplasts is approximately 13%, exhibiting 65% intactness, with less than 1% contamination by other subcellular fractions. These preparations were able to synthesize starch from glucose-1-phosphate plus ATP, or ADP-glucose but not from glucose-6-phosphate. Rates of starch synthesis from glucose-l-phosphate plus ATP were linear for up to 1 h and sensitive to the inhibitor 4,4-diisothiocyanato-stilbene 2,2-disulphonic acid (DIDS). Starch synthesis was optimal at pH 7.0 and was saturated by 5-10 mM glucose-1-phosphate and by 1 mM ADP-glucose. The results are discussed in the context of the pathway of starch synthesis and the transport of metabolites across the amyloplast envelope.