Class pi glutathione S-transferase: Meisenheimer complex formation

The enzyme-catalysed formation of the dead-end Meisenheimer complex, 1-(S-glutathionyl)-2,4,6-trinitrocyclohexadienate, between glutathione and 1,3,5-trinitrobenzene by two class pi glutathione S-transferases was studied under equilibrium conditions. The apparent formation constant of the complex at pH6.5, is 1.21 x 10(3) M-1 and 1.47 x 10(3) M-1 for isoenzyme pGSTP1-1 from porcine lung and hGSTP1-1 the human recombinant orthologue, respectively. These values are about 40- to 50-times larger than that determined for the nonenzymatic reaction in solution. Competitive inhibitors in the form of glutathione analogues that bind the G-site (glutathione sulphonate) or both the G-site and the H-site (S-hexylglutathione) regions of the active site markedly diminish complex formation. Comparison of kinetic data for glutathione S-transferase isoenzymes from the pi and mu gene classes suggests that the catalytic efficiencies for nucleophilic aromatic substitution reactions correspond with the ability of the enzyme's active site to stabilise the Meisenheimer complex. Formation of the red-coloured complex in orthorhombic crystals of pGSTP1-1 demonstrated that the crystallized protein retains its catalytically functional conformation in the crystal lattice.     

Combined effects of chemical dip and/or carrageenan coating and/or controlled atmosphere on quality of fresh-cut banana

The combined effect of chemical dip and/or edible coating and/or controlled atmosphere (CA) on quality of fresh-cut banana was investigated. Banana slices were subject to a 3-min dip into a solution containing 1% (w/v) calcium chloride, 0.75% (w/v) ascorbic acid and 0.75% (w/v) cysteine and/or combined with a carrageenan coating and/or combined with controlled atmosphere (3% O₂ + 10% CO₂). Physico-chemical and microbiological qualities were evaluated during 5 days of storage at 5 °C. Dip combined with CA treatment prevented product weight loss and increase of polyphenol oxidase activity during the 5 days of storage. Colour, firmness, pH, tritatable acidity and total soluble solids values and total phenolic content presented the smallest changes. Microbial analysis showed that minimally processed bananas were within the acceptable limits during 5 days of storage at 5 °C.