Gene isolation, analysis of expression and in vitro synthesis of a glutathione S-transferase from orange fruit [Citrus sinensis (L.) Osbeck]

Glutathione S-transferases (GSTs) (EC 2.5.1.18) are ubiquitous enzymes that have a defined role in xenobiotic detoxification, but a deeper knowledge of their function in endogenous metabolism is still lacking. In this work, we isolated the cDNAs as well as the genomic clones of orange GSTs. Having considered gene organization and homology data, we suggest that the isolated GST gene is probably involved in the vacuolar import of anthocyanins. We also found that the blood and blond orange GSTs shared the same nucleotide sequences, but as expected, the GST expression in the nonpigmented orange cultivar [Citrus sinensis L. (Osbeck)] (Navel and Ovale) was strongly reduced as compared to that of the pigmented orange (Tarocco). Interestingly, in the crude extracts of pigmented orange fruits, the GST activity was reproducibly detected by providing either 1-chloro-2,4 dinitrobenzene (CDNB) or cyanidin-3-O-glucoside (C-3-G) as substrates; moreover, we have shown that cyanidin-3-O-glucoside acted as a powerful competitive inhibitor of 1-chloro-2,4 dinitrobenzene conjugation to reduced glutathione (GSH) in the pigmented orange, confirming that this molecule might easily bind to the active site of the enzyme and functions as a putative substrate. In addition, we have reported here the successful in vitro expression of orange GST cDNAs leading to a GST enzyme that is active against cyanidin-3-O-glucoside, thus suggesting the probable involvement of the isolated gene in the tagging of anthocyanins for vacuolar import. This last result will help to study the kinetic and structural properties of orange fruit GST avoiding time-consuming protein purification procedures.