Evolution of two alanine glyoxylate aminotransferases in mosquito.

In the mosquito, transamination of 3-HK (3-hydroxykynurenine) to XA (xanthurenic acid) is catalysed by an AGT (alanine glyoxylate aminotransferase) and is the major branch pathway of tryptophan metabolism. Interestingly, malaria parasites hijack this pathway to use XA as a chemical signal for development in the mosquito. Here, we report that the mosquito has two AGT isoenzymes. One is the previously cloned AeHKT [Aedes aegypti HKT (3-HK transaminase)] [Han, Fang and Li (2002) J. Biol. Chem. 277, 15781-15787], similar to hAGT (human AGT), which primarily catalyses 3-HK to XA in mosquitoes, and the other is a typical dipteran insect AGT. We cloned the second AGT from Ae. aegypti mosquitoes [AeAGT (Ae. aegypti AGT)], overexpressed the enzyme in baculovirus/insect cells and determined its biochemical characteristics. We also expressed hAGT for a comparative study. The new cloned AeAGT is highly substrate-specific when compared with hAGT and the previously reported AeHKT and Drosophila AGT, and is translated mainly in pupae and adults, which contrasts with AeHKT that is expressed primarily in larvae. Our results suggest that the physiological requirements of mosquitoes and the interaction between the mosquito and its host appear to be the driving force in mosquito AGT evolution.