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Biomedical subjects

O E Nelson

Publications and source records attributed to O E Nelson.

At least 37 records · Page 2Linked to original sources
PubMed1982

Source-sink relations in maize mutants with starch-deficient endosperms.

Partitioning and translocation of photosynthates were compared between a nonmutant genotype (Oh 43) of corn (Zea mays L.) and two starch-deficient endosperm mutants, shruken-2 (sh2) and brittle-1 (bt1), with similar genetic backgrounds. Steady-state levels of (14)CO(2) were supplied to source leaf blades for 2-hour periods, followed by separation and identification of (14)C-assimilates in the leaf, kernel, and along the translocation path. An average of 14.1% of the total (14)C assimilated was translocated to normal kernels, versus 0.9% in sh2 kernels and 2.6% in btl kernels. Over 98% of the kernel (14)C was in free sugars, and further analysis of nonmutant kernels showed 46% of this label in glucose and fructose. Source leaves of mutant plants exported significantly less total photosynthate (24.0% and 36.3% in sh2 and bt1 compared to 48.0% in the normal plants) and accumulated greater portions of label in the insoluble (starch) fraction. Mutant plants also showed lower percentages of photosynthate in the leaf blade and sheath below the exposed blade area. The starch-deficient endosperm mutants influence the partitioning and translocation of photosynthates and provide a valuable tool for the study of source-sink relations.

Journal Article↗
PubMed1981

Movement of C-compounds from Maternal Tissue into Maize Seeds Grown in Vitro.

Uptake from nutrient media into the cob and translocation of various (14)C-compounds from maternal tissue (cob) into developing maize seeds was examined by using caryopsis cultures. Based on relative (14)C concentrations in the cob and the endosperm, it was concluded that the relative efficiencies of movement of amino acids (leucine, phenylalanine, proline), vitamins (thiamine HCl, nicotinic acid), and nucleic acid bases (adenine, thymine) from the cob to the endosperm were 11 to 250 times lower than that of sucrose. Thiamine was unique in that it was concentrated in the embryo at a level that was almost 10 times higher than in the endosperm. The absence of auxotrophic mutants requiring an organic supplement in higher plants (other than thiamine auxotrophs) may be explained by inadequate translocation of these essential metabolites into the mutant zygotes (embryos) to enable their development to mature seeds.

Journal Article↗
PubMed1979

Heterogeneous flavonoid glucosyltransferases in purple derivatives from a controlling element-suppressed bronze mutant in maize.

The Ds(Dissociation)-suppressed bronze mutant bz-m2(DI), which is extremely stable in the absence of the regulatory element Ac (Activator), becomes both somatically and germinally unstable when Ac is present. Instability in the germ line may result in the generation of stable (Ac-nonresponding), colored Bz' derivatives. The characterization of several properties of the Bz-controlled enzyme UDPglucose: flavonoid 3-O-glucosyltransferase from mature kernel endosperms of 15 such Bz' derivatives has revealed two distinct groups. The enzyme in group I Bz' derivatives does not differ from the normal enzyme in the Bz progenitor allele of bz-m2(DI). Group II Bz' derivatives, on the other hand, make altered, labile UDPglucose:flavonoid glucosytransferases different from the normal enzyme and probably different from each other. The significance of these two classes of derivatives and possible mechanisms involved in their origin are discussed.

Journal Article↗
PubMed1979

Starch-synthesizing Enzymes in the Endosperm and Pollen of Maize.

Two mutations, amylose-extender and waxy, which affect the proportion of amylose and amylopectin of starch synthesized in the endosperm of maize (Zea mays L.) seeds, are also expressed in the pollen. However, most mutations that affect starch synthesis in the maize endosperm are not expressed in the pollen. In an attempt to understand the nonconcordance between the endosperm and pollen, extracts of mature pollen grains were assayed for a number of the enzymes possibly implicated in starch synthesis in the endosperm. Sucrose synthetase (sucrose-UDP glucosyl transferase, EC 2.4.1.13) activity was not detectable in either mature or immature pollen grains of nonmutant maize, but both bound and soluble invertase (EC 3.2.1.26) exhibited much greater specific activity (per milligram protein) in pollen extracts than in 22-day-old endosperm extracts. Phosphorylase (EC 2.4.1.1) activity was also higher in pollen than in endosperm extracts. ADP-Glucose pyrophosphorylase (EC 2.7.7.27) activity was much lower in pollen than endosperm extracts, but mutations that drastically reduced ADP-glucose pyrophosphorylase activity in the endosperm (brittle-2 and shrunken-2) did not markedly affect enzymic activity in the pollen. Specific activities of other enzymes implicated in starch synthesis were similar in endosperm and pollen extracts.

Journal Article↗
PubMed1978

Nucleoside Diphosphate Sugar-Starch Glucosyl Transferase Activity of wx Starch Granules.

Starch granule preparations from the endosperm tissue of all waxy maize (Zea mays L.) mutants tested have low and approximately equal capability to incorporate glucose from adenosine diphosphate glucose into starch. As the substrate concentration is reduced, however, the activity of waxy preparations relative to nonmutant increases until, at the lowest substrate concentration utilized (0.1 muM), the activity of the waxy preparations is nearly equal to that of the nonmutant preparation. The apparent K(m) (adenosine diphosphate glucose) for starch granule preparations from wx-C/wx-C/wx-C endosperms was 7.1 x 10(-5) M, which is compared to 3 x 10(-3) M for preparations from nonwaxy endosperms. Starch granule preparations from three other waxy mutants of independent mutational origin have levels of enzymic activity approximately equal to wx-C at a given substrate concentration giving rise to similar apparent K(m) estimates. We conclude that there is in maize endosperm starch granules a second starch granule-bound glycosyl transferase, whose presence is revealed when mutation eliminates activity of the more active glucosyl transferase catalyzing the same reaction.

Journal Article↗
PubMed1977

Genetic control of UDPglucose:flavonol 3-O-glucosyltransferase in the endosperm of maize.

The enzyme UDPglucose:flavonol 3-O-glucosyltransferase is shown to be under the coordinate control of three genes involved in anthocyanin biosynthesis in the aleurone of maize: C, R, and BZ. Of the three, BZ appears to be the structural gene. Data presented here (dosage comparisons, induction in the mutant c-p, and effect of paramutation at R) indicate that the enzyme is inducible by substances resulting from the action of the C and R genes and that active forms of C and R are required for this induction. Mechanisms of regulation of the BZ gene by C and R are discussed.

Flavonoids↗
PubMed1977

Controlling element-induced alterations in UDPglucose:flavonoid glucosyltransferase, the enzyme specified by the bronze locus in maize.

The bz locus in maize, which directs synthesis of the enzyme UDPglucose:flavonoid glucosyltransferase (UFGT) in the endosperm, is one of many loci at which controlling element-induced phenotypic changes are known. To characterize the nature of such modifications, several bz mutants derived by transposition of the controlling element Ds to the Bz gene were analyzed. Three mutants were found to lack UFGT at all stages of endosperm development. Two others appeared to make an altered UFGT, and in one of these the enzyme showed an altered developmental profile.

Journal Article↗
PubMed1976

The enzymatic deficiency conditioned by the shrunken-1 mutations in maize.

Evidence is presented to show that the Sh locus specifies sucrose synthetase in the developing endosperm of maize. The sh/sh/sh endosperm possesses less than 10% sucrose synthetase activity as compared to the normal Sh/sh/sh endosperm. The residual enzyme activity in five independently derived mutant genotypes is attributable to a protein molecule of different electrophoretic and immunochemical specificities that is presumably independent of the sh locus. Sucrose synthetase activity in the embryo in both the genotypes is electrophoretically indistinguishable from the one present in the mutant endosperm. Mutant endosperm has a reduced starch content as compared to the normal. This observation constitutes genetic evidence supporting a critical role for sucrose synthetase in starch biosynthesis.

Alleles↗
PubMed1976

Characterization of ADP-glucose pyrophosphorylase from shrunken-2 and brittle-2 mutants of maize.

Electrophoretic characterization of adenosine diphosphate glucose pyrophosphorylase from the developing endosperms of nine shrunken-2 and four brittle-2 mutants revealed that (1) all mutants had low but detectable levels of activity, (2) mutation at either locus decreased activity of pyrophosphorylases A and B, and (3) differences in mobility were not found. However, pyrophosphorylase B extracted from several shrunken-2 and brittle-2 mutants differed from normal in extent of urea denaturation, Km (glucose-1-phosphate) or type of glucose-1-phosphaociation with the sh2 locus) appears to differ from normal in Km (glucose-1-phosphate).

Adenosine Diphosphate Glucose↗
PubMed1975

Maize alpha-glucan phosphorylase.

The major isozyme of alpha-glucan phosphorylase from developing maize seeds has been purified to homogeneity as verified by gel electrophoresis, ultracentrifugation and immunoprecipitation. The enzyme appears to be dimeric and has an estimated molecular weight of 223000 +/- 10000 based on ultracentrifugation, dodecylsulfate gel electrophoresis, and pyridoxal phosphate content. Adenosine diphosphoglucose appears to be a physiologically important inhibitor and interacts with the enzyme to give sigmoid kinetics when glucose 1-phosphate is the variable substrate. There are no properties of the enzyme which distinguish it from other phosphorylases as having a primarily synthetic role.

Adenosine Diphosphate Glucose↗
PubMed1975

Characterization of adenosine diphosphate glucose pyrophosphorylases from developing maize seeds.

Electrophoretic examination of 22-day-old, normal maize (Zea mays L.) endosperm extracts revealed two zones of adenosine diphosphate glucose pyrophosphorylase activity. The enzymes are identical in terms of Km for glucose 1-phosphate and the effect of 3-phosphoglyceric acid on apparent Km for glucose 1-phosphate. Both enzymatic activities increase with increasing doses of the functional alleles at the shrunken-2 and brittle-2 loci. Molecular weight differences between the two electrophoretic species were inferred from sucrose gradient centrifugation. It is suggested that the two bands of activity represent different aggregation states of the same enzyme because under different extraction conditions, only one enzyme is found. Molecular weight estimates of 237,000 and 253,000 were obtained for the smaller enzyme. It is suggested that this enzyme is an aggregate of several subunits. Comparison of the embryo and endosperm pyrophosphorylases showed the embryo activity to be more heat stable and probably independent of direct shrunken-2 or brittle-2 control.

Journal Article↗
PubMed1972

Multiple forms of glucosephosphate isomerase in maize.

Three apparently different glucosephosphate isomerases are found in the developing seeds of maize (Zea mays L.). Glucosephosphate isomerase I is found in both the endosperm and embryo. It is separable by column chromatography from glucosephosphate isomerase II of the developing endosperm and glucosephosphate isomerase III of the developing embryo and is further distinguished from them by heat stability, temperature activation, and relative insensitivity to the presence of zinc ions in the reaction mixture. Glucosephosphate isomerases II and III elute in the same fractions from diethylaminoethyl cellulose columns but are distinguished by electrophoretic mobility and reaction to the presence of adenosine 5'-triphosphate in the reaction mixture. All three isomerases give multiple banding patterns on electrophoresis. An extensive investigation of the conditions generating additional electrophoretic species and chromatographically separable minor activity peak (Ia) from glucosephosphate isomerase I has shown that these transformations are enhanced by dialysis, column chromatography, ammonium sulfate fractionation, and treatment with urea. The transformations are retarded by the presence of mercaptoethanol during these operations. We concluded that the multiple banding pattern seen on electrophoresis of glucosephosphate isomerase I prepared by certain procedures is artifactual. In germinating seeds of maize, glucosephosphate isomerases I and III are detectable, but II is not. It is possible that glucosephosphate isomerase II specifically catalyzes a step in starch biosynthesis.

Journal Article↗
PubMed1971

Invertase Activity in Normal and Mutant Maize Endosperms during Development.

A bound invertase and two soluble invertases are found in the developing endosperm of maize (Zea mays L.). The two soluble invertases can be separated on diethylaminoethyl-cellulose and Sephadex columns and distinguished by their kinetic constants. One soluble invertase, invertase I, is present from the 10- to 28-day stages of endosperm development with maximal activity per normal endosperm at the 12-day stage. In two endosperm mutant lines, shrunken-1 and shrunken-2, there is a second increase in invertase I activity later in development which could be a secondary effect caused by the abnormal metabolism in these lines. Another soluble invertase, invertase II, is present in the embryo upon germination and is also found in the very young developing endosperm (6-day stage). The third form of invertase, bound invertase, is present in the endosperm by the 6-day stage, and its activity remains approximately constant during development.

Journal Article↗
PubMed1971

An invertase inactivator in maize endosperm and factors affecting inactivation.

A protein present in the developing endosperm of maize (Zea mays L.) causes a loss of invertase activity under certain conditions of incubation. This protein, designated an inactivator, inactivates invertase I of maize even in the presence of other proteins. No inactivation of invertase II of maize or yeast invertase has been observed. The inactivator and invertase I are found only in the endosperm. The quantity of inactivator increases in the normal endosperm during development while invertase I activity decreases. However, the altered levels of invertase I activity in several endosperm mutant lines do not result from different quantities of inactivator. The inactivator can decrease invertase I activity during a preincubation period before addition of sucrose; inactivation is noncompetitive. Invertase I activity decreases curvilinearly with an increase in inactivator concentration. At high buffer concentrations or low inactivator concentrations in the reaction mixture, a latent period is observed when invertase I is not inactivated. Inactivation increases with an increase in temperature and a decrease in pH.

Journal Article↗
PubMed1970

Enzymes of carbohydrate metabolism in the developing endosperm of maize.

A number of enzymes presumably implicated in starch synthesis were assayed at various stages of endosperm development ranging from 8 days to 28 days after pollination. Activity for invertase, hexokinase, the glucose phosphate isomerases, the phosphoglucomutases, phosphorylase I, uridine diphosphate glucose pyrophosphorylase, and the starch granule-bound nucleoside diphosphate glucose-starch glucosyltransferase was present at the earliest stage of development (8 days) studied. Activity was detectable for phosphorylase III, the soluble adenosine diphosphate glucose-starch glucosyltransferase, adenosine diphosphate glucose pyrophosphorylase, and sucrose-uridine diphosphate glucosyltransferase at 12 days. For phosphorylase II and cytidine diphosphate glucose pyrophosphorylase, activity was first detectable at the 14- and 16-day stages, respectively. Rapid increases in starch content are observed prior to detectable activity for adenosine diphosphate glucose pyrophosphorylase, the soluble adenosine diphosphate glucose-starch glucosyltransferase and phosphorylases II and III. For all enzymes, except invertase, activity per endosperm rises to a peak at 22 or 28 days. Greatest activity for invertase is found at 12 days with a steady decline thereafter. The pattern of invertase activity in comparison with that of sucrose-uridine diphosphate glucosyltransferase supports previous suggestions, that the latter plays a key role in the conversion of sucrose to starch. In addition to phosphorylases I, II, and III, multiple forms of glucosephosphate isomerase and phosphoglucomutase were detected.

Journal Article↗
PubMed1969

Two additional phosphorylases in developing maize seeds.

Two additional phosphorylases (III and IV) have been detected in developing seeds of maize. Phosphorylase IV is found only in the embryo (with scutellum). It is also present in the embryo of the germinating seed where its activity is 90-fold greater than the activity in the developing embryo 22 days after pollination. Phosphorylase IV is eluted from a DEAE-cellulose column in the same fraction as phosphorylase I of the endosperm, and the 2 enzymes are similar in many respects. Phosphorylase IV is distinguished from phosphorylase I by electrophoretic mobility, by pH optimum, and because its properties are not affected by the shrunken-4 mutation. Phosphorylase III is found both in the endosperms and embryos of developing seeds. Activity for this enzyme is not detected in crude homogenates nor eluates from a DEAE-cellulose column apparently because it complexes with a non-dialyzable, heat-labile inhibitor. High activity is found after protamine sulfate fractionation. Phosphorylase III is bound to protamine sulfate and is then removed by washing with 0.3 m phosphate buffer. Phosphorylase III activity in the endosperm is not detectable 8 days after pollination but is present 12 days after pollination. Phosphorylase III differs from phosphorylases I, II, and IV in several respects-pH optimum, pH-independent ATP inhibition, time of appearance in the endosperm, and because purine and pyrimidine nucleotides are equally inhibitory. In common with phosphorylase II, phosphorylase III apparently does not require a primer to initiate the synthesis of an amylose-like polymer.

Glucosyltransferases↗