Hock-Hin Yeoh

Protein contents, amino acid compositions and nitrogen-to-protein conversion factors for cassava roots

Root protein contents of 15 cassava varieties (Manihot esculenta Crantz) ranged from 5 to 19 g kg -1 dry matter. Intervarietal differences in amino acid profiles of cassava roots were evident. Differences in the levels of aspartic acid, glutamic acid and arginine were most notable. The nitrogen-to-protein conversion factors (k AA ) based on nitrogen recovered from total amino acid analyses including ammonia ranged from 4.75 to 5.87, showing that the traditional conversion factor of 6.25 was not valid for cassava root proteins. Conversion factors (k p ) for 15 cassava varieties based on Kjeldahl nitrogen ranged from 2.49 to 3.67. Therefore an average k p value of 3.24 ± 0.31 may provide a better estimate of the protein content in cassava roots.

Kinetic properties of β-glucosidase from cassava

Abstract β-Glucosidases from the leaf, peel and tuber cortex of cassava cv. Merah Jambu exhibited linamarase activity and had in common many kinetic properties. They were also capable of hydrolysing p -nitrophenyl-β- d -monoglycosides and cyanogenic β-monoglucosides but lacked activity towards the p -nitrophenyl-β- d -diglycosides, a cyanogenic diglucoside, and other β- or α-linked disaccharides. The K m values for p -nitrophenyl-β- d -monoglycosides were generally lower than those for linamarin, prunasin and salicin. Ag 2+ inhibited both β-glucosidase and linamarase activities. Glucono-1,5-lactone inhibited the enzyme competitively, irrespective of the substrates used, while imidazole showed competitive inhibition with linamarin but non-competitive (mixed) inhibition with p -nitrophenyl-β- d -glucoside. The enzyme was unaffected by glucose.

Quantitative analysis of linamarin in cassava using a cassava β-glucosidase electrode

Abstract A simple protocol was developed for the quantitative analysis of linamarin in cassava roots using a cassava β-glucosidase (linamarase) electrode. A steady state value could be obtained within 15–20 min and the limit of detection was 0.1 m m linamarin. The linamarin content of 18 cassava varieties determined by this procedure ranged from 24 to 395 mg per 100 g fr. wt. root. These values were comparable to those obtained using a spectrophotometric method. The enzyme electrode method is reliable and simple to perform. Furthermore, it does not require the use of hazardous chemicals like pyridine which is needed by the spectrophotometric procedure.

Properties of β-glucosidase purified from Aspergillus niger mutants USDB 0827 and USDB 0828

SummaryTwo extracellular β-glucosidases (EC 3.2.1.21) were isolated from Aspergillus niger USDB 0827 and A. niger USDB 0828, and their physical and kinetic properties studied. Both enzymes were very similar in terms of molecular size (230000 Da), pH optimum (pH 4.6), temperature optimum (65° C), stability at high temperatures and substrate preferences. They were capable of hydrolysing β-linked disaccharides, phenyl β-d-glucoside, p-nitrophenyl β-d-glucoside (PNPG), o-nitrophenyl β-d-glucoside, salicin and methyl β-d-glucoside but lacked activity towards α-linked disaccharides, a range of p-nitrophenyl monoglycosides and p-nitrophenyl diglycosides. Both β-glucosidases were better at hydrolysing cellobiose than cellotriose, cellotetraose or cellopentaose. For both enzymes, glucose showed competitive inhibition with PNPG as substrate but had no effect with cellobiose. However, the two β-glucosidases differed in inhibition by glucono-1,5-lactone and affinity for cellobiose. β-Glucosidase from A. niger USDB 0827 also gave lower specific activity, and was more susceptible to metal ions (Ag+, Fe2+ and Fe3+) inhibition than that of A. niger USDB 0828.

Monitoring the cyanogenic potential of cassava: the trend towards biosensor development

Abstract Cassava ( Manihot esculenta Crantz) is an important staple crop for more than 500 million people in the developing countries. It contains linamarin as the principal cyanogenic glucoside, which upon hydrolysis liberates hydrogen cyanide. Toxicity resulting from the eating of inadequately processed cassava has been reported and there are world-wide efforts to make cassava consumption safer. An important contribution toward this is the development of methods to determine the cyanogenic potential of cassava and its products. Hence, procedures – new or modified ones – are being developed in response to the needs of the end-users. This article describes the rationale behind this development and reviews some of the more recent findings.

An enzyme-based dip-stick for the estimation of cyanogenic potential of cassava flour

Abstract An enzyme-based dip-stick for cyanogenic potential determination was constructed by gluing three pieces of chromatography paper impregnated with cassava leaf linamarase, phosphate buffer pH 8 and alkaline picrate solution, respectively, onto a plastic strip measuring 10 mm × 45 mm. Using different concentrations of linamarin as reference (equivalent to 0.5–40 μg HCN), the change in colour of the picrate paper was readily distinguishable. Beyond 40–80 μg HCN, it was difficult to differentiate the colour of the picrate paper with accuracy. For quantitative estimation, the colour from the picrate paper was eluted with water and its absorbance measured at 510 nm. Using the dip-stick in conjunction with different types of reaction vessels, it was possible to determine the cyanogenic potential (expressed as mg HCN kg −1 ) of cassava flour. The method was simple to use. It could easily handle large numbers of samples and it seems suitable for field work.

Taxonomic variation in total leaf protein amino acid compositions of monocotyledonous plants

Abstract Protein contents and protein amino acid compositions of different regions of leaf blades and of leaves representing different physiological ages, taken from Aranda Christine (Liliiflorae) and Languas galanga (Zingiberiflorae) showed little intraspecific variation. However, leaf amino acid compositions of 25 plant species covering the major taxonomic groups of the Monocotyledonae showed variation and group by group comparisons revealed taxonomic patterns.

Kinetic properties of β-glucosidase from Aspergillus ornatus

SummaryKinetic properties of extracellular β-glucosidase from Aspergillus ornatus were determined. The pH and temperature optima for the enzyme were found to be 4.6 and 60°C, respectively. Under these conditions, the enzyme exhibited a Km (p-nitrophenyl-β-glucoside) value of 0.76±0.11 mM. The activation energy for the enzyme was 11.8 kcal/mol. Several divalent metal ions inhibited β-glucosidase activity, some of which showed inhibition of enzyme activity only at higher concentrations. Ag2+ was the most potent inhibitor. A metal chelating agent, EDTA, also inhibited β-glucosidase activity. Except for trehalose, glucose, glucono-δ-lactone, cellobiose, gentiobiose, laminaribiose, maltose and isomaltose inhibited β-glucosidase activity. Glucose was found to be a competitive inhibitor, whereas glucono-δ-lactone and other β-linked disaccharides were noncompetitive (mixed) inhibitors of the enzyme.

Systematic variation in leaf amino acid compositions of leguminous plants

Abstract The leaf protein content for 17 species of legumes ranges from 2.8 to 9.4 g% fr. wt, with an average of 5.3 g % fr. wt. Taxonomic pattern is detectable in leaf amino acid patterns, those of the Mimosoideae being distinguishable from those of the Papilionoideae and Caesalpinioideae.

Linamarin sensors: amperometric sensing of linamarin using linamarase and glucose oxidase

Abstract Amperometric sensors for linamarin, a cyanogenic glycoside, were fabricated. They were based on the detection of glucose liberated from the hydrolysis of linamarin by linamarase from cassava leaf. A linamarase-glucose oxidase bienzyme-modified SnO2 electrode was observed to function at +900 mV vs. Ag | AgCl in a hydrogen peroxide detection mode, and could determine linamarin down to 10 μM. In contrast, a linamarase-glucose oxidase bienzyme-modified graphite electrode mediated by poly(N-isopropylacrylamide-co-vinylferrocene) responded to greater than 0.2 mM linamarin at +500 mV vs. Ag | AgCl. A linamarase-glucose oxidase-peroxidase trienzyme electrode was also fabricated by coating a peroxidase-incorporated polypyrrole-modified SnO2 electrode with a linamarase-glucose oxidase cross-linked film. This trienzyme electrode, which is the most practical linamarin sensor, was able to function at a mild potential of +150 mV vs. Ag | AgCl, and has a sensitivity of 5 μM linamarin.