Metabolism ofd-Glycero-d-Manno-Heptitol, Volemitol, in Polyanthus. Discovery of a Novel Ketose Reductase1

Abstract

Volemitol (D-glycero-D-manno-heptitol, ␣-sedoheptitol)is an unusual seven-carbon sugar alcohol that fulfills several important physiological functions in certain species of the genus Primula. Using the horticultural hybrid polyanthus (Primula ؋ polyantha) as our model plant, we found that volemitol is the major nonstructural carbohydrate in leaves of all stages of development, with concentrations of up to 50 mg/g fresh weight in source leaves (about 25% of the dry weight), followed by sedoheptulose (D-altro-2-heptulose, 36 mg/g fresh weight), and sucrose (4 mg/g fresh weight). Volemitol was shown by the ethylenediaminetetraacetate-exudation technique to be a prominent phloem-mobile carbohydrate. It accounted for about 24% (mol/mol) of the phloem sap carbohydrates, surpassed only by sucrose (63%). Preliminary 14 CO 2 pulse-chase radiolabeling experiments showed that volemitol was a major photosynthetic product, preceded by the structurally related ketose sedoheptulose. Finally, we present evidence for a novel NADPHdependent ketose reductase, tentatively called sedoheptulose reductase, in volemitol-containing Primula species, and propose it as responsible for the biosynthesis of volemitol in planta. Using enzyme extracts from polyanthus leaves, we determined that sedoheptulose reductase has a pH optimum between 7.0 and 8.0, a very high substrate specificity, and displays saturable concentration dependence for both sedoheptulose (apparent K m ‫؍‬ 21 mM) and NADPH (apparent K m ‫؍‬ 0.4 mM). Our results suggest that volemitol is important in certain Primula species as a photosynthetic product, phloem translocate, and storage carbohydrate.

Postulated Physiological Roles of the Seven-carbon Sugars, Mannoheptulose, and Perseitol in Avocado

“…This is in contrast to what has been previously reported in hybrid polyanthus (Primula ×polyantha Hort.) for volemitol, another phloem-mobile C7 sugar, (Häfliger et al, 1999) whose levels in phloem exudates were substantially lower (only about one-quarter the concentration) than those of sucrose. Interestingly though, the reducing form of C7 sugar, sedoheptulose, corresponding to the C7 polyol volemitol was also found in phloem exudates, as we have found in the present study for mannoheptulose.…”

Sedoheptulose accumulation under CO2 enrichment in leaves of Kalanchoë pinnata: a novel mechanism to enhance C and P homeostasis?

“…However, its presence, albeit at lower levels, in a range of other families including Apiaceae, Aquifoliaceae, Euphorbiaceae, Lamiaceae, Primulaceae, and Saxifragaceae suggests a broader range of occurrence (Tolbert et al ., 1957: Okuda and Mori, 1974; Häflinger et al ., 1999; Soria et al ., 2009). The only other free C7 sugar described in plants is mannoheptulose (Tesfay et al ., 2012).…”

“…Although the basic features of the oxPPP are well established (Kruger and von Schaewen, 2003), details on how exactly the pathway influences other processes are subject to further investigation. In addition to its putative origin, the role of free sedoheptulose in plants remains a matter of debate, except for serving as a precursor for the polyol volemitol in the horticultural hybrid polyanthus (Häflinger et al ., 1999). A possible function as a carbohydrate reserve in CAM (such as starch or soluble sugars), essential to catalyse nocturnal CO 2 uptake, was ruled out earlier by Kull (1965, 1967) based on the absence of clear changing diel patterns of sedoheptulose.…”

Detection of transaldolase deficiency by quantification of novel seven‐carbon chain carbohydrate biomarkers in urine

“…Mannoheptulose (D-manno-2-heptulose), sedoheptulose (D-altro-2-heptulose), perseitol (D-glycero-D-galactoheptitol) and sedoheptitol (volemitol; D-glycero-D-mannoheptitol) ( Fig. 2) have been detected in various plants (Hä fliger et al 1999;La Forge and Hudson 1917;Okuda and Mori 1974) and fruits (Liu et al 2002). Pitkä nen and Sahlstrom (1968) employed paper chromatography in conjunction with GC to demonstrate that sedoheptulose, mannoheptulose and sedoheptitol were present in trace quantities in human urine, speculating that these species derived from tissue metabolism likely associated with the pentose phosphate cycle pool.…”