Effects of oxygen concentration on the intermediary metabolism of Leishmania major promastigotes.

Leishmania major promastigotes grown in late log phase were incubated with glucose as sole exogenous carbon source in the presence of 5% CO2 and the amounts of glucose consumed and of the major products formed--succinate, pyruvate, alanine, acetate, glycerol, and D-lactate--were measured as a function of pO2. Glucose consumption increased as pO2 was lowered to 6% (a positive Pasteur effect) and then declined to the same level at 95% N2 as at 95% O2. The production of D-lactate and of glycerol increased as pO2 dropped from 95%, reaching a maximum at about 2% O2. Succinate production, however, increased dramatically when pO2 was reduced to 6% and remained at that level with further reduction of pO2. The amount of succinate produced relative to the amount of glucose carbon consumed suggests utilization of an endogenous carbon source. Acetate production did not change between 95% O2 and 6% O2 and then declined with decreasing pO2. These observations suggest the presence of two sensors, one with a high and one with a low affinity for oxygen. When glycerol or alanine were the only exogenous sources of carbon, the primary products released were acetate and succinate. Acetate production from alanine declined slightly as pO2 was reduced to 2%, and then dropped markedly when pO2 was reduced to 0%. Acetate production from glycerol increased over 4-fold when the pO2 was reduced from 95% to 4%, and then declined with further reduction in pO2. No succinate was formed from either substrate until complete anaerobiosis. This pattern of response, while differing from that when glucose was sole exogenous carbon source, is also consistent with the regulation of metabolism by a high and a low affinity O2 sensor. Cells from cultures in early stationary phase, before the appearance of metacyclic forms, consumed glucose at about the same rate as log phase promastigotes, but did not show a Pasteur effect. Stationary cells also consumed glycerol at the same rate as did log phase promastigotes, but consumed alanine at a much lower rate. Reduction of pO2 affected product formation from each of these substrates differently than for log phase promastigotes, demonstrating the sensitivity of several pathways of intermediary metabolism to regulation by pO2 during the transition from log to stationary phase.