Abstract
Lactate dehydrogenase (LDH, E.C.1.1.1.27) was found in supernatant (cytoplasmic enzyme) fractions of the trypanosomatid flagellates Trypanosoma conorhini and Crithidia fasciculata if 10 mm cysteine was present in the homogenizing medium. The T. conorhini LDH activity with pyruvate as substrate was increased 35% if 5 mm cysteine was also included in reaction mixtures. K(m) values for the T. conorhini enzyme were 3.3 x 10(-4)m with pyruvate, and 1.6 x 10(-4)m with alpha-ketobutyrate. Cysteine inhibited alpha-ketobutyrate reduction. Comparison of trypanosomatid and human serum LDH enzymes with respect to K(m), substrate activity and inhibition, pH optima, and K(i) values for oxalate and oxamate indicated that the trypanosomatid isoenzymes differed significantly from serum LDH. C. fasciculata LDH was extremely labile, since 59% of the activity was lost 90 min after isolation. The role of LDH enzymes in trypanosomatid metabolism is discussed, and the results are related to other trypanosomatid LDH enzymes. The comparison of homologous enzymes in host and parasite is discussed with regard to metabolic function and a possible model system for chemotherapy.