Trypanosoma brucei has two phosphoglycerate kinase (PGK) isoenzymes, one is particle-bound and localized in glycosomes while the other is present in the cytosol. The cytosolic isoenzyme (cPGK) was 900-fold purified from cultured procyclic trypanosomes by hydrophobic interaction chromatography on phenyl-Sepharose followed by affinity chromatography on 2',3'-ATP-Sepharose and had a specific activity of 275 units/mg protein. cPGK was compared with the purified glycosomal isoenzyme (gPGK) from bloodstream-form trypanosomes as well as with the commercially available PGKs from yeast, rabbit muscle and Spirulina platensis, a blue-green alga. Like all other PGKs, cPGK was a monomeric protein with a molecular mass of approximately 45 kDa similar to that of the PGKs from other organisms but 2 kDa smaller than that of gPGK. Despite this difference in length and a great difference in isoelectric point, the two trypanosome isoenzymes strongly resembled each other in several respects. The kinetic parameters did not differ significantly from each other or from the PGKs of other organisms. Both trypanosome enzymes resembled the enzyme from S. platensis in that they had an almost absolute requirement for ATP, contrary to the enzymes from yeast and rabbit muscle, which were capable of utilizing GTP and ITP also. This difference in substrate specificity may be related to the amino acid substitutions, Trp 308----His and Ala 306----Glu in the adenine-binding site, which are only found in the two Trypanosoma isoenzymes. Kinetic analysis showed that these substitutions do not prevent binding of the ATP analogues, but probably prevent phosphoryl-group transfer. Both isoenzymes displayed an activity optimum at pH 6.0-9.0 similar to that for the enzyme of yeast. Both gPGK and cPGK were inhibited by the trypanocidal drug Suramin. This inhibition could be described as competitive both with ATP and 3-phosphoglycerate with two inhibitor molecules binding to one molecule of enzyme. The gPGK, however, was much more sensitive (Ki app. = 8.0 microM) to Suramin than either the cPGK (Ki app. = 20 microM) or the enzymes from rabbit muscle (Ki app. = 55 microM), yeast (Ki app. = 167 microM) or S. platensis (Ki app. = 250 microM). It is suggested that positive charges on the enzyme's surface may play an important role in the potentiation of the binding of the negatively charged Suramin molecule.