Sucrose starch interconversions in the potato tuber

Abstract

The storage carbohydrate in the potato tuber is starch and it is biosynthesised from translocated sucrose and deposited as starch grains within tuber amyloplasts. The biosynthetic pathway from sucrose to starch in thepotato tuber has been investigated. Young developing tubers havebeen injected with both [u-14c]sucrose and [u-14c] fructose and themetabolites arising from these labelled compounds have been examined: differences in the distribution of label between the various metabolites were recorded. These results support the theory that sucrose synthetase, and not invertase, is involved in the initial reaction leading to the conversion of sucrose to starch and that ADP-glucose is synthesised preferentially from fructose via hexose phosphate intermediates.The activities and properties of some of the enzymes believed to be involved in the synthesis of starch from sucrose have been examined. These include sucrose synthetase, ADP-glucose-and UDP-glucose-pyrophosphorylase. The latter enzyme has been shown to occur in multiple forms in the tuber. Starch synthetase activities, associated with starch grains, have also been studied and in particular, the inhibitory effect of nucleotides on starch synthesis. Activation of a-gluean. synthesis from ADP-glucose was demonstrated in the presence of both pyrophosphate and inorganic phosphate and the reverse was shown to be true in the case of glucan synthesis from UDP-glucose. Preliminary experiments were carried out in an attempt tosynthesise starch from [14c] fructose and [14c]sucrose using coupled enzyme systems derived from the potato tuber. The regulation of the metabolic pathway from sucrose to starch which is believed to operate in the potato tuber is discussed.