S. L. Desborough

Potato tuber proteins and their nutritional quality

SummaryProteins in Red Pontiac potato tubers were fractionated by two methods. Tuberin was the main protein (71%) obtained by Method I (though perhaps by this method up to 40% of the ‘tuberin’ was albumin), while albumin was about half of the total protein obtained by Method II. The amino acid composition of protein fractions obtained by either method was quite similar. All the protein fractions except prolamine were well balanced in essential amino acids and comparable to FAO reference protein. Methionine was the limiting amino acid of the potato fractions. The chemical score, essential amino acid indices and biological value of albumin, globulin, glutelin and residual protein did not vary significantly. Since all the fractions except prolamine, which is a negligible portion of total protein, are of high nutritional quality, Red Pontiac has high-quality protein.ZusammenfassungKartoffelknollenproteine wurden mittels zweier Methoden fraktioniert. Bei Methode I (Tabelle 1) stellte Tuberin mit 71% des Gesamtproteins die Hauptfraktion dar, Bei Methode II dagegen machte das wasserlösliche Albumin ungefähr die Häfte des Gesamtproteins aus. In beiden Methoden war Prolamin die kleinste Fraktion. Die Extraktionsausbeute beider Methoden war sehr hoch. Ungefähr 9% des Gesamtstickstoffs konnte weder mit der einen noch mit der andern Methode extrahiert werden. Es zeigte sich, dass bei Methode I ein Teil des Albumins durch Sättigung mit 40%igem Ammonsulfat ausgefällt wurde. Dies wurde auch durch Elektrophorese dieser Fraktionen bestätigt (Abb. 1). Die Aminosäure-Zusammensetzung der Proteinfraktionen war bei beiden Methode ähnlich (Tabellen 2 und 3). In den essentiellen Aminosäuren waren Albumin, Globulin, Glutelin und Restanteile gut ausgeglichen. Sie waren mit dem FAO-Referenz-Protein vergleichbar, aber weniger wertvoll als das Eiprotein (Tabelle 4). Alle essentiellen Aminosäuren waren in Prolamin schwach vertreten. Die Aminosäure Methionin war in diesen Fraktionen der beschränkende Faktor (Tabelle 5). Der chemische Wertmass-stab (chemical score), der essential amino acid index (EAA-Index), der Bedarfs-Index und die biologische Wertigkeit von Albumin, Globulin, Glutelin und Restanteil wichen nicht stark voneinander ab. Prolamin war in diesen charakterisierenden Eigenschaften schwach vertreten. Da die Prolamine nur einen sehr kleinen Teil des Gesamtproteins ausmachen und alle andern Fraktionen eine sehr hohe Nährstoffqualität aufweisen, ist das Kartoffeleiweiss im ganzen von hoher Wertigkeit.RésuméLes protéines du tubercule de pomme de terre sont fractionnées suivant deux méthodes. Suivant la méthode I (tableu 1), la tubérine est la fraction la plus importante, formant 71% des protéines totales. Au contraire suivant la méthode II, l’albumine soluble dans l’eau constitue la fraction la plus basse. L’efficacité de l’extraction des deux méthodes est vraiment élevée. Il se pourrait que quelque 9% de l’azote total ne soit pas extrait par l’une et l’autre méthode. II apparait que, dans la méthode I, un peu d’albumine est précipité par du sulfate d’ammonium saturé à 40%. Ceci est également attesté par l’électrophorèse des fractions (fig. 1). La composition en acides aminés des fractions de protéine obtenues par les deux méthodes est semblable (tableaux 2 et 3). Albumine, globuline, gluteline et résidu sont bien équilibrés en acides aminés essentiels (tableau 4). Les teneurs de tous les acides aminés essentiels sont basses dans la prolamine. La méthionine est l’acide aminé limitant dans ces fractions (tableau 5). La composition chimique, l’index des acides aminés essentiels, l’index exigé et la valeur biologique des albumine, gluteline et résidu ne diffèrent pas beaucoup. A cet égard la prolamine est inférieure. Puisque la prolamine ne constitue qu’une très petite partie de la protéine totale et que toutes les autres fractions ont une très haute valeur nutritive, la protéine de pomme de terre dans son ensemble est de haute qualité.

Pyrophosphorylases in Solanum tuberosum (IV. Purification, Tissue Localization, and Physicochemical Properties of UDP-Glucose Pyrophosphorylase)

The enzyme UDP-glucose pyrophosphorylase (UGPase) from potato (Solanum tuberosum L. cv Norchip) tubers was purified 177-fold to near homogeneity and to a specific activity of 1099 international units/mg of protein. The molecular mass of the purified enzyme was 53 kD as determined by SDS-PAGE and gel filtration. Immunological and activity assays detected UGPase at similar levels in potato stems, stolons, and tubers. Leaves and roots contained lower levels of UGPase activity and protein. Lineweaver-Burk plots for substrates inorganic pyrophosphate and UDP-glucose were linear in the pyrophosphorolytic direction, yielding Km values of 0.13 and 0.14 mM, respectively. However, Lineweaver-Burk plots for the substrates glucose-1-P and UTP were biphasic in nature when UGPase was assayed in the direction of UDP-glucose synthesis. At physiological substrate concentrations (i.e. from 0.05–0.20 mM), Km values of 0.08 mM (glucose-1-P) and 0.12mM (UTP) were obtained. When substrate concentrations increased above 0.20 mM, Km values increased to 0.68 mM (glucose-1-P) and 0.53 mM (UTP). These kinetic patterns of potato UGPase suggest a “negative cooperative effect” (A. Conway, D.E. Koshland, Jr. [1968] Biochemistry 7: 4011–4022) with respect to the substrates glucose-1-P and UTP. The biphasic substrate saturation curves were similar to the kinetics of the dimeric form of UGPase purified from Salmonella typhimurium (T. Nakae [1971] J Biol Chem 246: 4404–4411). The in vivo significance of the enzymes “negative cooperativity” in the direction of UDP-glucose synthesis and potato sweetening is discussed.

Improving potato protein I. Evaluation of selection techniques

Nitrogen, amino acids and protein analyses were compared for determination of tuber protein. Micro-Kjeldahl nitrogen provided a poor estimate of both protein and amino acids (r = 0.55 and 0.45 respectively). Potato protein contains about 13.5% nitrogen, making 7.5 an appropriate conversion factor. Autoanalyzer determinations of amino acids were correlated with protein analyzed by a modified Potty technique (r = .82). Amounts of six specific protein-bound amino acids were also correlated. with protein. Free amino acids accounted for about 10% of the total and non-essential amino acids constituted over 90% of this free pool. Only 23% of total nitrogen was accounted for in the free pool. Protein content and tuber size were not correlatel. Protein content was similar in the cortical, medullary and pith region. Loss in tuber protein during six months of storage at about 3 C (38-40 F) was 3%.

Protein comparisons in selected Phureja-haploid Tuberosum families

Quantitative measurements were made of soluble and total proteins in a study of tuber protein inheritance in selected Phureja-haploid Tuberosum families. The effects of ploidy and location were also examined. A preliminary study comparing methods for measuring tuber proteins revealed that the technique described by Gusta et al., (1), was most suitable for total protein; while the TCA technique was satisfactory for analyzing soluble proteins. The method described by Potty (2) was used in both classes for final protein determination. Both protein classes appeared to be directly influenced by ploidy level and growing location. More important, the relative amounts of tuber protein were substantially increased in the first generation of selection.

HPLC isolation and nutritional value of a major tuber protein

A predominant protein complex from the albumin fraction of potato (Solanum tuberosum L.) tuber has been isolated by size exclusion high performance liquid chromatography. The isolate has an estimated native molecular weight of 60 kD. Anionic gel electrophoresis of the isolate separated it into several polypeptides, all of which were glycosylated and had non-specific esterase activity. Two polypeptides between 35 and 42 kD were identified with SDS electrophoresis, thus the native form is postulated to be a dimer. The proteins in the isolate are antigenic to an antibody developed against ‘patatin,’ a tuber glycoprotein. Nutritional values for the isolate were extremely high, which suggests that it is at least partly responsible for the excellent nutritional value of potato protein.CompendioMediante cromatografía líquida de exclusión de alta precisión (HPLC), se aisló de la fracción albuminoidea de tubérculos de papa (Solanum tuberosum L.) un complejo proteico predominante. El aislamiento tiene un peso molecular natural estimado de 60 kD. La electroforesis separó al aislamiento en varios polipectidos, todos ellos glicosilatados y con actividad de esterasa no específica. Por medio de electroforesis (SDS) se identificaron dos polipectidos, entre 35 y 42 kD, por lo que se considera que la forma natural del aislamiento es un dímero. Las proteínas del aislamiento son antigénicas a un anticuerpo desarrollado contra la’patatina,’ una glicoproteina del tubérculo. Los valores nutricionales del aislamiento fueron extremadamente altos, lo que sugiere que este complejo es por lo menos parcialmente responsable del excelente valor nutricional de la proteína de la papa.

Accumulation of protein, non-protein nitrogen and starch during tuber growth of three potato cultivars

The proportion of protein and non-protein nitrogen (NPN) in tuber dry matter decreases rapidly during the initial stages of tuber growth. Protein then stabilizes but NPN increases during the final stages of tuber growth. The percentage of starch in the dry matter increases rapidly during early tuber growth and then either stabilizes or increases at a slower rate. Protein and starch are accumulated at constant rates with respect to tuber growth and the relative amount of protein or starch in the tuber is logarithmically related to tuber size. During tuber growth some protein molecular weight classes increase in concentration while others decrease. We propose a model, based on the growth of storage parenchyma, to explain the data.ResumenLa proporción de nitrogeno proteico y no-proteico (NPN) en la materia seca del tubérculo disminuyó rápidamente durante los estados iniciales de su crecimiento. Luego la proteina se estabiliza pero el NPN aumenta durante los estados finales del crecimiento del tubérculo. El porcentaje de almidón en la materia seca aumenta rapidamente durante la fase inicial de crecimiento del tubérculo y luego se estabiliza ó aumenta a una tasa lenta. Proteina y almidón se acumulan a tasas constantes con respecto al crecimiento del tubérculo y la cantidad relativa de proteina o almidónen en el tubérculo se relaciona logarítmicamente con el tamaño del tubérculo. Durante el crecimiento del tubérculo algunas clases de peso moleculares de proteinas aumentan en concentración, mientras que otras disminuyen. Nosotros proponemos un modelo, basado en el crecimiento del parénquima de reserva, para explicar los resultados.

Improvement of potato protein II. Selection for protein and yield

The improvement of potato protein appears feasible by selecting high protein parents and crossing them with adapted Group Tuberosum parents. Initial hybrids were obtained with about 10% protein which is double that of Red Pontiac. This protein is of high nutritional value when compared to whole egg protein. Some of the hybrids appear to have yields approaching standard varieties. Specific gravity values are also comparable.ResumenEl mejoramiento del contenido protéico de la papa parece factible cruzando progenitores seleccionados por alto contenido protéico con progenitores del Grupo Tuberosum. Los híbridos obtenidos inicialmente tuvieron alrededor de 10% de proteína, contenido que representa el doble de aquél del cultivar Red Pontiac. E sta proteína es de un alto valor nutritivo comparada a la del huevo. Algunos híbridos tuvieron rendimientos próximos a los de los cultivares standard. Los valores de gravedad específica fueron también comparables.

The nutritional quality of potato protein from intraspecific hybrids

The nutritional value of potato hybrids with Groups Andigena, Phureja, and Tuberosum parentage was measured by protein efficiency ratio (PER). Three diets at 6.0%, 7.75%, and 9.0% crude protein in potato flakes were fed to rats; 18 genotypes were analyzed and the hybrids had higher PER values than Group Tuberosum varieties, but increased tuber protein did not influence the PER values. The overall mean PER was 2.3, and 12 of the potato-flake diets were not different from the casein controls. Fortification with isoleucine did not improve the nutritional quality of the potato flakes. One diet was improved with the addition of both isoleucine and methionine. The influence of sulfur-containing amino acids in potato protein quality among these hybrids is not clear. It is suggested there may be two types of hybrids, one with dependence on these amino acids and a second type with a different protein factor(s) affecting the nutritional quality of the protein. Protein from these potato hybrids, as measured by PER, is among the highest in biological quality when compared with other proteins in plant foods.

Protein, starch, and non-protein nitrogen accumulation in high protein hybrids and low protein cultivars

Tuber protein, starch, non-protein nitrogen and percent dry matter content were measured during tuber development of four Phureja-Tuberosum-Andigena hybrids and two Tuberosum cultivars. Percentages of starch and protein in the tuber dry matter were curvilinearly related to tuber size in all six clones. The amounts of starch and protein in a tuber were linear functions of tuber size. Rates of protein accumulation in some of the hybrids were higher than those of either cultivar. Starch accumulation rates were similar among all six clones. The high protein character of at least one of the hybrids was associated with starch containing storage tissue. Differences between high and low protein clones were mainly related to quantitative differences in the subunit composition of the protein. The ratio of non-protein nitrogen to protein N was similar among the six clones. Relationships between percent dry matter and starch content were not consistent among the six clones.ResumenSe midió la proteína de tubérculo, almidón, nitrógeno no proteíco y porcentaje de materia seca durante el desarrollo de cuatro hibridosPhureja-Tuberosum-Andigena y de dos cultivaresTuberosum. Los porcentajes de almidón y proteína en la materia seca del tubérculo fueron unilínealmente relacionadas al tamaño del tubérculo en las seis clases. Las cantidades de almidón y proteína en un tubérculo fueron funciones líneales del tamaño del tubérculo. Las tazas de acumulación de proteína fueron más altas en algunos híbridos que en los dos cultivares. Las tazas de acumulación de almidón fueron similares entre las seis clases. El carácter de proteína alta de por lo menos uno de los híbridos estuvo asociado con el tejido de almacenamiento de almidón. Las diferencias entre las clases con alta y baja proteína se relacionan principalmente con diferencias cuantitativas a la compositión de las unidades de la proteína. La proportión de nitrogeno no proteíco a nitrógeno proteico fue similar en las seis clases. Las relaciones entre el porcentaje de materia seca y el contenido de almidón no fueron constitentes entre las seis clases

Rapid extraction and solubilization of potato starch with Dimethylsulfoxide

Dimethyl sulfoxide (DMSO) solubilization of starch from lyophilized potato tissue is equally quantitative and more convenient than solubilization in dilute alkali, especially when analysis of a large number of samples is necessary. Starch extraction with DMSO decreases the handling time of each sample by eliminating sonication, filtration, and centrifugation steps required by the NaOH solubilization technique.ResumenLa solubilización de almidón en tejido liofilizado de papa con Dimetilo sulfoxido (DMSO) es igualmente cuantitativo y más conveniente que la solubilización en álcali diluído, especialmente cuando es necesario el análisis de un gran número de muestras. La extractión de almidón con DMSO disminuye el tiempo de manejo de cada muestra eliminando las fases de sanicación, filtración y centrifugación requeridos por la técnica de solubilización con NaOH.