Subcellular distribution and characterization of glucosephosphate isomerase in Leishmania mexicana mexicana.

The glycolytic enzyme glucosephosphate isomerase (PGI) is present in two different cell compartments of Leishmania mexicana promastigotes; more than 90% of the activity was detected in the cytosol, the remainder in glycosomes. This subcellular distribution contrasts with that in Trypanosoma brucei, in which the enzyme activity has been mainly located in the glycosomes. PGI was partially purified from L. mexicana cell extracts. Throughout the purification procedure only one single PGI activity could be detected. The partially purified protein had the same subunit molecular mass (65 kDa) as the previously characterized glycosomal protein of T. brucei. Both proteins were also very similar with respect to their kinetic and antigenic properties. Using the T. brucei glycosomal PGI gene as a hybridization probe, we cloned the corresponding gene of L. mexicana. Only a single PGI locus could be detected in the L. mexicana genome. Characterization of the cloned gene showed that it codes for a polypeptide of 604 amino acids, with a molecular mass of 67,113. The sequences of the Leishmania and Trypanosoma polypeptides are 69% identical. They differ in calculated net charge (-8 versus -2, respectively) and isoelectric point (6.65 versus 7.35). Our data strongly suggest that the PGI activity in the two cell compartments of L. mexicana and T. brucei is not attributable to different isoenzymes. We discuss the possible metabolic function of the highly different enzyme distribution in the two organisms, and the molecular mechanism that could be responsible for it.