Pyrophosphate dependent phosphofructokinsase (PFP) activity and other aspects of sucrose metabolism in sugarcane internodal tissues.
The biochemical basis for the regulation of sucrose accumulation is not fully understood. The present study was thus aimed at investigating aspects of 'coarse' (enzyme activity) and 'fine' (metabolite) control of glycolytic enzyme activity in relation to carbon partitioning in the developing stalk (internodes 3 to 10), and between varieties with significant differences in sucrose content. Particular emphasis was placed on studying pyrophosphate: D-fructose-6-phosphate 1-phosphotransferase (PFP, EC 188.8.131.52), since this enzyme has been implicated in sucrose metabolism in other plant species. Within the developing stalk, internodal maturation was associated with a redirection carbon from the insoluble matter and total respiration (C02 production and biosynthesis) to sucrose storage. Between varieties, with significant variation in sucrose content, there was an inverse relationship between hexose monophosphate partitioning into respiration and sucrose. The reduction in carbon flux to respiration was not associated with a decline in the extractable specific activity of PK, PFK and PFP. There was also no alteration in the regulation of PK, PFK and FBPase by change in the mass action ratios. Hexose monophosphate concentration declined approximately two to three-fold from internodes 3 to 9 and Fru-6-P concentration was within the lower Km or 80.5 range (Fru-6-P) of PFP and PFK, respectively (as reported from the literature) . Within the developing stalk, substrate limitation might have contributed to the decline in carbon partitioning to respiration. In sugarcane, the levels of PFP activity were controlled in part by PFP protein expression. 8ugarcane PFP polypeptide(s) are resolved as a single protein with a molecular mass of approximately 72 kO. PFP catalysed a reaction close to equilibrium in all intemodes investigated, and the concentration of Fru-2,6-P2 was shown to be in excess of the requirement to stimulate PFP activity. Carbon flux from the triose-P to hexose monophosphate pool was apparent in sugarcane, suggesting that PFP activity was functional in vivo. The developmental profile of specific PFP activity was not positively correlated to the increasing rate of sucrose accumulation in the top ten internodes of the developing stalk. Between different sugarcane varieties, specific PFP activity was shown to be inversely correlated to sucrose content.