Takao Murata

Enzymic mechanism of starch synthesis in glutinous rice grains

Although a considerable amount of evidence has been accumulated showing the important role of ADPG in the biosynthesis of starch in various plant organs (De Fekete and Cardini, 1964; Murata etal., 1964 b), the enzymic mechanism of starch formation in the grains of glutinous (waxy) varieties of the cereal grains has remained obscure. Nelson and Rines (1962) reported the absence of UDPG-starch transglucoyslase in starch granules prepared from waxy maize seeds, and proposed a different pathway of amylopectin biosynthesis from that of amylose. Recently, Nelson and Tsai (1964) obtained a piece of evidence that ADPG-starch transglucosylase in waxy maize is localized in the starch granules derived from embryo and maternal tissues, but not in the endosperm, which is the principal site of starch synthesis in the seed. They found that the transglucosylase activity of waxy maize is about one tenth of the magnitude of that of non-waxy maize and essentially all of its activity can be accounted for by the starch granules present in the sporophytic tissues. The lower transglucosylase activity of waxy maize has been observed by Frydman (1963).

Enzymic mechanism of starch synthesis in ripening rice grains: III. Mechanism of the sucrose-starch conversion

Abstract In a coupling system of sucrose-synthetase and starch-synthetase, both isolated from the ripening rice grains, the more efficient transfer of glucose-C 14 from sucrose-C 14 to starch occurred in the presence of ADP as compared to UDP, indicating the predominant role of ADPG in the process. However, UDP was found to inhibit profoundly the ADPG-mediated glucose-C 14 transfer to starch from sucrose-C 14 , which was based on its specific inhibition of the ADPG-sucrose transglucosylation reaction. The Km value of sucrose synthetase was determined to be 1.5 × 10 −3 M (ADP) and 1.1 × 10 −4 M (UDP), respectively. These findings may support a view that the sucrose breakdown proceeds through the reversal of UDPG-sucrose transglucosylation rather than directly by way of the ADPG-sucrose transglucosylation. By feeding the rice grains at the mid-milky stage with sucrose-C 14 , incorporation of the radioactivity into the starch fraction was observed, although most of the radioactivity in the acid-soluble fraction was recovered as sucrose. It was noted also that a relatively high radioactivity was detected in glucose and fructose. The formation of a prominent radioactive compound(s) emerging around the fraction of glucose-6-phosphate was demonstrated in a Dowex-1 anion-exchange column chromatography; neither its chemical nature nor the role in the sucrose-starch conversion has been revealed. Both ADPG and UDPG became labeled to a much lesser extent than the above compounds under the experimental conditions employed. Attempts to distinguish their predominant role in the biosynthetic conversion of sucrose to starch were unsuccessful, although the specific radioactivity of ADPG was about twice as high as that of UDPG. Based on the experimental results obtained, possible enzymic mechanisms underlying the sucrose-starch conversion in developing rice grains are discussed.

Enzymic mechanism of starch synthesis in ripening rice grains. IV. Starch synthesis in glutinous rice grains.

Abstract In contrast to the particulate nature of the ADPG-starch transglucosylase in nonglutinous rice grains, in the glutinous variety the enzyme was found to be present almost exclusively in a soluble form. This was established by the determination of the enzymic activities in various fractions of the glutinous rice grain extract. The mode of action of the soluble ADPG-starch transglucosylase was found to be identical to that of the granular transglucosylase by the following criteria: (a) formation of α(1 → 4) glucosidic bonds, and (b) maltose being the shortest glucose acceptor molecule in the transglucosylation reaction. Starch granules prepared from glutinous and nonglutinous rice, corn starch, amylose, amylopectin, and oyster glycogen were found to be effective glucose acceptors in the reaction catalyzed by the transglucosylase, with glycogen showing the highest activity. In the sucrose-C 14 feeding experiment, both ADPG and UDPG became labelled, showing essentially identical mechanisms operative in sucrose-starch conversion in glutinous rice and in nonglutinous plant. The natural occurrence of ADPG-starch transglucosylase in the granular fraction of glutinous cereals was related to the formation of an amylose-ADPG-starch transglucosylase complex in the starch granules. A reconstituted amylose-enzyme complex was heat stable and sensitive to SH-reagents.

C4 syndrome of the species in theDichotomiflora group of the genusPanicum (Gramineae)

Leaf anatomy, pattern of post-illumination CO2 burst (PIB) and activity of three C4-acid decarboxylating enzymes in C4 photosynthesis were investigated with the leaves of five species in theDichotomiflora group of the genusPanicum. All species had mestome sheaths, exhibited the sharp pattern of PIB in less than 30 sec of darkness and were classified as NAD-malie enzyme species biochemically. However, they clearly fell into two groups according to the difference in chloroplast location in bundle sheath cells (BSC).P. coloratum var.makarikariense, P. lanipes andP. stapfianum had centripetal chloroplasts, whereasP. laevifolium andP. longijubatum had centrifugal chloroplasts, whereas cv. Kabulabula and cv. Solai had centrifugal chlorplasts. The results indicate that theDichotomiflora group had the two leaf anatomical variations of NAD-malic enzyme species. In addition, the results onP. coloratum suggest that this species may be divided into two separate species by chloroplast location in BSC.The ultrastructural features of leaves ofP. dichtomiflorum, NAD-malic enzyme species with centrifugal chloroplasts, were also investigated. Chloroplasts in BSC had well-developed grana, and numerous large mitochondria with extensively developed internal membrane structure were restricted to the area between the chloroplsts and the vacuole in BSC.