S-adenosylmethionine and Pneumocystis carinii

We previously reported that S-adenosylmethionine (AdoMet), a key molecule i n methylation reactions and polyamine biosynthesis, enhances axenic culture of the AIDS-associated opportunistic fungal pathogen Pneumocystis carinii. Here we report that AdoMet is absolutely required for continuous growth. T wo transporters are present, one high affinity, K-m = 4.5 mu M, and one low affinity, K-m = 333 mu M. The physiologically relevant high affinity trans porter has a pH optimum of 7.5 and no related natural compounds compete for uptake. Transport is 98% inhibited at 4 degrees C, 24% inhibited by 20 mM sodium azide, and 95% inhibited by the combination of 20 mM sodium azide an d 1 mM salicylhydroxamic acid; thus transport is active and dependent on bo th a cytochrome chain and an alternative oxidase. In vitro, AdoMet is used at a rate of 1.40 x 10(7) molecules cell(-1) min(-1). AdoMet synthetase act ivity was not detected by a sensitive radiolabel incorporation assay capabl e of detecting 0.1% of the activity in rat liver. In addition, the AdoMet p lasma concentration of rats is inversely correlated with the number of P. c arinii in the lungs. These findings demonstrate that P, carinii is an AdoMe t auxotroph, The uptake and metabolism of this compound are rational chemot herapeutic targets.