Abstract. The ADPglucose pyrophosphorylases of 7 plant-leaf tissues were partially purified and characterized. In all cases the enzymes showed stability to heat treatment at 650 for 5 minutes in the presence of 0.02 M phosphate buffer, pH 7.0. The leaf ADPglucose pyrophosphorylases were activated 5 to 15-fold by 3-phosphoglycerate. Fructose-6-phosphate and fruotose 1, 6-diphosphate stimulated ADPglucose pyrophosphorylase to lesser extents.The AO,5 (conc of activator required to give 50 %o of the observed maximal activation) of 3-phosphoglycerate for the barley enzyme was 7 X 10-6 M while for the sorghum enzyme it was 3.7 X 10-4 M. Inorganic phosphate proved to be an effective inhibitor of ADPglucose synthesis. The 10.5 (conc of inhibitor that gave 50 % inhibition of activity for the various leaf enzymes varied from 2 X 10-M (barley) to 1.9 X 10-4 M (sorghum). This inhibition was reversed or antagonized by the activator 3-phosphoglycerate. These results form the basis for an hypothesis of the regulation of leaf starch biosynthesis.The hiosynithesis of the a-1,4 gluco,side linkage of starch from UDPglucose3 and ADPglucose in many pliawt exltracts have been the subject of many reporits (1, 2, 5, 8-16, 29--36) since the initiall obseirvatilons by Lelloir'is group (7,26,38). In alil extradts the rate of transfer of glucose from ADPglucose (reacltion 1) to the polysaicdharide prime,r was many-fold fa!ster than the rate of transfer from uridine dipho,sphalte gluco,se (UDPglucose) (re,action 2).
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