The Binding of Phthalein Dyes to a Glutathione S-Transferase from Galleria Mellonella

Abstract

The glutathione S-transferase from Galleria mellonella has the ability to bind the phthalein dyes, bromocresol green, bromosulphophthalein and 3,6-dibromosulphophthalein with apparent dissociation constants in the vicinity of 1 x 10-6M. This represents an affinity similar to that reported for the glutathione S-transferases isolated from rat liver. The effect of glutathione on the affinity of the dyes for the enzyme is of a competitive nature which represents the major difference between this transferase and those isolated from rat liver. The presence of glutathione appears to increase the number of binding sites at the expense of overall affinity and suggests an enzyme with multiple site binding, some of these sites being glutathione dependent. The fluorescent probe, 1-anilino-8-naphthalene sulphonate, does bind to the transferase but the increase in fluorescence observed for this interaction is not as large as that reported in the literature for the interaction of phthalein dyes with the rat liver glutathione S-transferases. This suggests the binding environment of these enzymes is more hydrophobic than that of the glutathione S-transferase isolated in this study. The localisation of glutathione S-transferase activity in the fat body of the Galleria mellonella larvae and the lack of activity in the malpighian tubules discounts a major role of this enzyme in the specific uptake of phthalein dyes into the proximal section of the malpighian tubules.