AROMATIC AMINO-ACID TRANSAMINATION AND METHIONINE RECYCLING IN TRYPANOSOMATIDS

Although trypanosomatids are known to rapidly transaminate exogenous a romatic amino acids ill vitro and in vivo, the physiological significa nce of this reaction is not understood. In postmitochondrial supernata nts prepared from Trypanosoma brucei brucei and Crithidia fasciculata, we have found that aromatic amino acids were the preferred amino dono rs for the transamination of alpha-ketomethiobutyrate to methionine. I ntact C. fasciculata groan in the presence of [N-15]tyrosine were foun d to contain detectable [N-15]methionine, demonstrating that this reac tion occurs in situ in viable cells. This process is the final step in the recycling of methionine from methylthioadenosine, a product of de carboxylated S-adenosylmethionine from the polyamine synthetic pathway . Mammalian liver, in contrast, preferentially used glutamine for this reaction and utilized a narrower range of amino donors than seen with the trypanosomalids. Studies with methylthioadenosine showed that thi s compound was readily converted to methionine, demonstrating a fully functional methionine-recycling pathway in trypanosomatids.