A GLUTATHIONE-S-TRANSFERASE (GST) ISOZYME FROM BROCCOLI WITH SIGNIFICANT SEQUENCE HOMOLOGY TO THE MAMMALIAN THETA-CLASS OF GSTS

A novel glutathione S-transferase (GST) was purified from broccoli (Brassica oleracea var. italica). Partial amino-acid sequencing indicated that the protein shared significant homology with several different plant GSTs from maize, silene, Dianthus, Nicotiana and Triticum, but little homology to yeast (Issatchenkia) [6] GST. One region of the polypeptide near the N-terminal also shared significant homology to a region of rat 5-5, rat 12-12 and human theta-GST (collectively referred to as the theta-GST-class) [7,8] but little structural homology to the common mammalian cytosolic GSTs (alpha-, mu- or pi-classes) [9]. The broccoli GST was retained on a novel membrane based glutathione affinity matrix and displayed activity towards 1-chloro-2,4-dinitro-benzene (CDNB), a general GST substrate, as well as 4-nitrophenethyl bromide, a marker substrate for the theta-class of GSTs. The characteristics of the broccoli GST potentially define it as a member of the theta-class. This is consistent with the view that the theta-class may have arisen prior to the divergence of animals and plants while the mammalian mu-, pi- and alpha-classes evolved after the two kingdoms were established.