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. 1992 Apr;98(4):1396–1402. doi: 10.1104/pp.98.4.1396

Mannitol Synthesis in Higher Plants 1

Evidence for the Role and Characterization of a NADPH-Dependent Mannose 6-Phosphate Reductase

Wayne H Loescher 1,2,3, R Huw Tyson 1,2,3, John D Everard 1,2,3, Robert J Redgwell 1,2,3, Roderick L Bieleski 1,2,3
PMCID: PMC1080363  PMID: 16668806

Abstract

Mannitol is a major photosynthetic product in many algae and higher plants. Photosynthetic pulse and pulse-chase 14C-radiolabeling studies with the mannitol-synthesizing species, celery (Apium graveolens L.) and privet (Ligustrum vulgare L.), showed that mannose 6-phosphate (M6P) and mannitol 1-phosphate were among the early photosynthetic products. A NADPH-dependent M6P reductase was detected in these species (representing two different higher plant families), and the enzyme was purified to apparent homogeneity (68-fold with a 22% yield) and characterized from celery leaf extracts. The celery enzyme had a monomeric molecular mass, estimated from mobilities on sodium dodecyl sulfate-polyacrylamide gels, of 35 kilodaltons. The isoelectric point was pH 4.9; the apparent Km (M6P) was 15.8 millimolar, but the apparent Km (mannitol 1-phosphate) averaged threefold higher; pH optima were 7.5 with M6P/NADPH and 8.5 with mannitol 1-phosphate/NADP as substrates. Substrate and cofactor requirements were quite specific. NADH did not substitute for NADPH, and there was no detectable activity with fructose 6-phosphate, glucose 6-phosphate, fructose 1-phosphate, mannose 1-phosphate, mannose, or mannitol. NAD only partially substituted for NADP. Mg2+, Ca2+, Zn2+, and fructose-2,6-bisphosphate had no apparent effects on the purified enzyme's activity. In vivo radiolabeling results and the enzyme's kinetics, specificity, and distribution (in two-plant families) all suggest that NADPH-dependent M6P reductase plays an important role in mannitol biosynthesis in higher plants.

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Selected References

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