Marmoset glutathione transferases with ketosteroid isomerase activity

The common marmoset Callithrix jacchus encodes two glutathione transferase (GST) enzymes with ketosteroid double-bond isomerase activity. The most active enzyme is CjaGST A3-3 showing a specific activity with 5-androsten-3,17-dione (Δ⁵-AD) of 62.1 ± 1.8 μmol min^-1 mg^-1, and a k_cat value of 261 ± 49 s^-1. The second ketosteroid isomerase CjaGST A1-1 has a 30-fold lower specific activity with Δ⁵-AD and a 37-fold lower k_cat value. Thus, the marmoset CjaGST A3-3 would be the main contributor to the biosynthesis of the steroid hormones testosterone and progesterone, like the human ortholog HsaGST A3-3. Two residues differ in the H-site of the 91.4% sequence identical CjaGST A1-1 and CjaGST A3-3, and modeling of the structures suggests that the bulky phenyl ring of Phe111 in CjaGST A1-1 causes steric hindrance in the binding of the steroid substrate. Tributyltin acetate (IC₅₀=0.16 ± 0.004 μM) and ethacrynic acid (IC₅₀=3.3 ± 0.2 μM) were found to be potent inhibitors of CjaGST A3-3, as previously demonstrated with the human and equine orthologs.