Intracellular protein degradation in Leishmania tropica promastigotes

Leishmania amazonensis: early proteinase activities during promastigote-amastigote differentiation in vitro

Leishmania proteinase activity is known as parasite differentiation marker, and has been considered relevant for leishmanial survival and virulence. These properties suggest that Leishmania proteinases can be promising targets for development of anti-leishmania drugs. Here, we analyze the activities of four proteinases during the early phase of the Leishmania amazonensis promastigotes differentiation into amastigotes induced by heat shock. We have examined activities of cysteine-, metallo-, serine-, and aspartic-proteinase by hydrolysis of specific chromogenic substrates at pH 5.0 and at the optimal pH for each enzyme. Our results show that metallo-, serine-, and aspartic-proteinases activities were down-regulated during the shock-induced transformation of promastigotes into amastigotes. In contrast, cysteine-proteinase activity increased concomitantly with the promastigote differentiation. Immunocytochemical localization using two anti-cysteine-proteinase monospecific rabbit antibodies detected the enzyme in several cell compartments of both parasite stages. Our results show different proteinase activity modulation and expression during the early phases of the shock-induced parasite transformation.

Energy metabolism in Leishmania

Alanine plays a key role in the response of promastigotes to osmotic stress and to hypoxia. It is rapidly released in response to hypo-osmolality, is consumed from its large intracellular pool under iso-osmotic conditions even in the presence of glucose, and is synthesized under hyperosmotic conditions even in the absence of glucose. Its rate of oxidation, in the presence or absence of any of ten other amino acids tested, is strongly inhibited by hyperosmolality. Glucose oxidation is also inhibited by hyperosmolality, but to a lesser extent than that of alanine, and is inhibited by alanine, glutamate, and aspartate. Hyperosmolality also inhibits the incorporation of label from [2-14C]acetate into the putative storage carbohydrate, mannan, which occurs via the glyoxylate bypass and the as yet unexplored "mannoneogenic" pathway. The rates of glycolysis and of oxidation of several amino acids decrease with increasing culture age, but the capacity to oxidize fatty acids increases, and in cells from 3-day stationary phase cultures hyperosmolality enhances rather than inhibits alanine oxidation.

Utilization of a carbohydrate reserve comprised primarily of mannose by Leishmania donovani

Promastigotes from late log phase and 3-day stationary phase cultures of Leishmania donovani were collected, washed in buffer, and the cell pellet was treated with boiling KOH. A putative carbohydrate storage material was then precipitated and washed in ethanol/LiBr. This material did not liberate glucose when treated with amyloglucosidase, indicating that it was not glycogen. Acid hydrolysis released a hexose which was identified as mannose by several criteria. Considerably more of this mannan-like carbohydrate is present in cells from 3-day stationary phase than from late log phase cultures, consistent with the ability of 3-day stationary phase cells to survive in non-nutrient buffer and maintain oxygen consumption for longer than log phase cells. The amount of this mannan-like compound decreased by over 50% during a 3-h incubation in buffer of cells from 3-day stationary phase cultures. The presence of glucose during the incubation prevented the utilization of this carbohydrate, consistent with the possibility that it serves as an energy reserve.

Effects of hypoxia and acute osmotic stress on intermediary metabolism in Leishmania promastigotes

This study further explores the effects of hypoxia and acute osmotic stress on intermediary metabolism of Leishmania major and Leishmania donovani. Late log phase promastigotes were washed and incubated with glucose as the sole exogenous carbon source, and rates of glucose consumption and product formation were measured as a function of osmotic strength (610, 305, and 167 mOsm kg-1) and pO2 (95, 10, and 0% O2) in the presence of 5% CO2. Very mild hypoxia dramatically altered flux through the pathways of intermediary metabolism and increased the rates of production of the major metabolites, thus confirming the presence of a low-affinity O2 sensor which was active under all osmolalities tested. The data also require that as pO2 is lowered towards anoxia an endogenous carbohydrate source is mobilized. Under aerobic conditions, acute hypo-osmotic stress had little effect on product formation, whereas acute hyperosmotic stress altered metabolism in a manner similar to mild hypoxia, with the exception of decreasing the rates of acetate and succinate production. It was also shown in L. donovani promastigotes that the effects of anoxia and hyperosmolality were not additive. Thus, separate sensors with partially overlapping actions are involved in the metabolic responses to hypoxia and hyperosmolality. There was no apparent species-specificity for the responses to pO2 and osmotic stress. Uncoupling with carbonyl cyanide p-trifluoromethoxyphenylhydrazone caused changes in metabolite flux patterns which differed from the changes caused by either hypoxia or acute osmotic stress, while rotenone and calcium ionophore A23187 had no significant effects. The identity of the sensors responsive to pO2 and osmolality, and the mechanisms by which they regulate flux through the pathways of intermediary metabolism, require further study.

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.

Acid protease activity of a major surface membrane glycoprotein (gp63) from Leishmania mexicana promastigotes

A unique protease with activity optimal at pH 4.0 and trailing toward the alkaline pH spectrum was detected with intact glutaraldehyde-fixed promastigotes of Leishmania mexicana amazonensis, indicating surface localisation of the enzyme. That this surface protease may be a virulence factor is suggested by its apparent roles in multiple steps during leishmanial infections of macrophages. Indeed, its specific activity was 2-2.5 fold higher on virulent cells than on avirulent cells. Several lines of evidence indicate that this acid protease activity is expressed by the major surface glycoprotein (gp63) of L. m. amazonensis. Monoclonal antibody affinity purified gp63 degraded serum albumin, hemoglobin, complement C3, immunoglobulin G and purified rat liver lysosomal proteins in their native forms. The specific activity is about 20-fold higher at pH 4.0 than at pH 7.5 and is about four-fold higher at the body temperature of the mammalian host (37 degrees C) than at that of the insect host (27 degrees C). The protease activity is sodium dodecyl sulphate-sensitive. Among various protease inhibitors tested, only heavy metal ions (1 mM), 1,10-orthophenanthroline (1 mM) and bestatin (100 ng ml-1) significantly inhibited gp63 acid protease activity by up to 80%. N-linked oligosaccharides of gp63 appear to be important for the stability of this molecule, possibly by preventing its autodegradation. Purified gp63 effected limited proteolysis of human complement C3 molecules at the physiological serum pH of 7.5 in a manner, which supports the idea of its participation in complement-receptor mediated endocytosis of promastigotes by macrophages.

Products of Leishmania braziliensis glucose catabolism: release of D-lactate and, under anaerobic conditions, glycerol

Leishmania braziliensis panamensis promastigotes were incubated with glucose as the sole carbon source. About one-fifth of the glucose consumed under aerobic conditions was oxidized to CO2. Nuclear magnetic resonance studies with [1-13C]glucose showed that the other products released were succinate, acetate, alanine, pyruvate, and lactate. Under anaerobic conditions, lactate output increased, glycerol became a major product, and, surprisingly, glucose consumption decreased. Enzymatic assays showed that the lactate formed was D(-)-lactate. The release of alanine during incubation with glucose as the sole carbon source suggested that appreciable proteolysis occurred, consistent with our observation that a large amount of ammonia was released under these conditions. The discoveries that D-lactate is a product of L. braziliensis glucose catabolism, that glycerol is produced under anaerobic conditions, and that the cells exhibit a "reverse" Pasteur effect open the way for detailed studies of the pathways of glucose metabolism and their regulation in this organism.

Oxidation of glucose, ribose, alanine, and glutamate by Leishmania braziliensis panamensis

The metabolism of [1-14C]- and [6-14C]glucose, [1-14C]ribose, [1-14C]- and [U-14C]alanine, and [1-14C]- and [5-14C]glutamate by the promastigotes of Leishmania braziliensis panamensis was investigated in cells resuspended in Hanks' balanced salt solution supplemented with ribose, alanine, or glutamate. The ratio of 14CO2 produced from [1-14C]glucose to that from [6-14C]glucose ranged from about two to six, indicating appreciable carbon flow through the pentose phosphate pathway. A functional pentose phosphate pathway was further demonstrated by the production of 14CO2 from [1-14C]ribose although the rate of ribose oxidation was much lower than the rate of glucose oxidation. The rate of 14CO2 production from [1-14C]glucose was almost linear with time of incubation, whereas that of [6-14C]glucose accelerated, consistent with an increasing rate of flux through the Embden-Meyerhof pathway during incubation. Increasing the assay temperature from 26 degrees C to 34 degrees C had no appreciable effect on the rates or time courses of oxidation of either [1-14C]- or [6-14C]glucose or of [1-14C]ribose. Both alanine and glutamate were oxidized by L. b. panamensis, and at rates comparable to or appreciably greater than the rate of oxidation of glucose. The ratios of 14CO2 produced from [1-14C]- to [U-14C]alanine and from [1-14C]- to [5-14C]glutamate indicated that these compounds were metabolized via a functioning tricarboxylic acid cycle and that most of the label that entered the tricarboxylic acid cycle was oxidized to carbon dioxide.(ABSTRACT TRUNCATED AT 250 WORDS)

The free amino acid pool in Leishmania tropica promastigotes

The principal constituents of the free amino acid pool in Leishmania tropica promastigotes include alanine, glycine, serine, threonine, alpha-aminobutyric acid and ethanolamine. Changes in levels of the pool constituents during starvation, nutrient supplementation and treatment with a metabolic inhibitor (sodium fluoride) were followed. Several enzymes capable of converting amino acids to Krebs cycle intermediates were identified. It is suggested that during prolonged starvation which is accompanied by enhanced proteolysis, these enzymes enable the cells to utilize endogenous amino acids as carbon and energy sources. This is compatible with previous reports that Leishmania promastigotes in culture could grow on amino acids as growth substrates when carbohydrates are totally lacking.