The Uptake and Metabolism of Cysteine by Giardia lamblia Trophozoites

. The cysteine, cystine, methionine and sulfate uptake and cysteine metabolism of Giardia lamblia was studied. Initial experiments indicated that bathocuproine sulphonate (20 μM) added to Keister's modified TYI‐S‐33 medium supported the growth of G. lamblia at low L‐cysteine concentration. This allowed the use of high specific activity radiolabeled L‐cysteine for further studies. The analyses of L‐cysteine uptake by G. lamblia indicate the presence of at least two different transport systems. The total cysteine uptake was non saturable, with a capacity of 3.7 pmoles per 10 6 cells per min per μM of cysteine, and probably represent passive diffusion. However, cysteine transport was partially inhibited by L‐methionine, D‐cysteine and DL‐homocysteine. indicating that another system specific for SH‐containing amino acids is also present. Cysteine uptake was markedly decreased in medium without serum. In contrast to cysteine, the uptake of L‐methionine and sulfate were carried out by saiurable systems with apparent K m , of 71 and 72 μM, respectively, but the Vmax of the uptake of sulfate was six orders of magnitude lower than the Vmax of methionine uptake. Cystine was not incorporated into trophozoites. [ 35 S]‐labeled L‐cysteine and L‐methionine, but not [ 35 S]sulfate, were incorporated into Giardia proteins, indicating that the parasite lacks the capacity to synthesize cysteine or methionine from sulfate. Neither cystathionine γ lyase nor crystathionine γ synthase activities was detected in homogenates of Giardia lamblia , suggesting that the transsulfuration pathway is not active and there is no conversion of methionine to cysteine. Our data indicate that cysteine is essential for Giardia because the parasite: a) cannot take up cystine, and b) cannot synthesize cysteine de novo.