Sugar Content and Activity of Sucrose Metabolism Enzymes in Milled Rice Grain

Abstract

Most rice (Oryza sativa L.) culfivars grown in the United States were selected for endosperm starch properties and not soluble sugar content. The minor pool of soluble sugar may affect the qualities of rice as a food. Some cultivar variation in soluble sugar content was detected in milled grain, essentially the starchy endosperm, of long grain varieties. Milled grain of cultivars Lemont and Texmati had a soluble sugar content of 0.21 and 0.35% (w/w), respectively, on a fresh weight basis. The dorsal portion of the milled grain contained the greatest amount of soluble sugar, approximately tenfold the amount found in the central core of the grain. Extracts of the milled grain contained sucrosephosphate synthase (EC 2.4.1.14) and sucrose synthase (EC 2.4.1.13) activities, which were separated by anion exchange chromatography. The presence of sucrose-phosphate synthase in the rice endosperm suggested a mechanism for sucrose accumulation which might be involved in carbon partitioning during grain development.Mature storage tissues such as cereal endosperm consist primarily of starch and a minor pool of soluble sugar (9,14,16). Consequently, the major fate of translocated photosynthate entering the developing endosperm is to be metabolized by invertase or sucrose synthase into precursors for starch biosynthesis. The endosperm sucrose pool represents an alternative storage form for incoming photosynthate. It is unclear whether endosperm sucrose accumulation depends on transport of unmetabolized sucrose into the endosperm cell, or whether resynthesis of sucrose occurs from a pool of hexoses and nucleotide sugars common to the starch biosynthesis pathway (7).Resynthesis of sucrose might be catalyzed by sucrose synthase, or by a combination of sucrose-phosphate synthase and sucrose phosphatase activities (7). The presence of sucrosephosphate synthase in many storage organs (4,6,10,12,16) suggests that some sucrose may be resynthesized using this nonreversible pathway.Endosperm mutations are known which increase soluble sugar content of cereal grain (13). Samples of rough rice grown in Beaumont, TX, were dehulled, and the whole caryopsis used for microscopy. Sugar Content of Milled GrainSamples (100 mg) of milled grain flour were extracted with 2.5 mL of 70% (v/v) ethanol as described previously (16). Triplicate aliquots of the extracts were analyzed for sucrose using a resorcinol reagent (8,16), and for soluble reducing sugar using the neocuproine technique relative to standards of glucose (2).Milled grain of cultivar Starbonnet was sliced using a scalpel. The top portion and bottom portion averaged 31 and 37% (w/w), respectively, of the total milled grain weight. The middle portion of the grain was sliced into dorsal, lateral, ventral, and core sections. Portions of the milled grain were ground into flour using a mortar and pestle. Flour samples (25 to 35 mg) were extracted twice at 80°C for 10 min with a total volume of 1.2 mL of 70% (v/v) ethanol. Sucrose and soluble reducing sugar content were measured as indi...

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