Biochemical characteristics of the metacyclic forms of Leishmania major and L. mexicana mexicana.

Metacyclic forms of Leishmania major and putative metacyclics of L. mexicana mexicana were found to occur in abundance in stationary phase cultures. These forms have been compared in several ways with promastigotes from mid-log phase cultures and, in the case of L. m. mexicana, amastigotes. Metacyclics are smaller, contain less protein and appear more active than other promastigotes. Both forms of promastigote respire at a high rate in the absence of exogenous substrate. The free amino-acid contents of the various forms of the two species have been analysed. They differ in detail but alanine was the major amino acid in all cases. The isoenzyme content of the different forms differed significantly. That of the putative metacyclics of L. m. mexicana was in several respects more similar to amastigotes than promastigotes, suggesting that the form is pre-adapted for life in a mammal. Metacyclics of L. major apparently did not divide in culture but transformed back over a period of 48 h to mid-log phase cells. The results provide further detail of the molecular differences between mid-log phase and metacyclic promastigotes and confirm that metacyclics are a distinct form in the life-cycle.