l-Alanine aminotransferase in Drosophila nigromelanica: isolation, characterization and activity during ontogenesis

Silencing of a putative alanine aminotransferase (ALT) gene influences free amino acid composition in hemolymph and fecundity of the predatory bug, Cyrtorhinus lividipennis Reuter

In Asian rice systems, Cyrtorhinus lividipennis Reuter is an important predator that preys on rice planthopper eggs and young nymphs, as a primary food source. Alanine aminotransferase (ALT) acts in many physiological and biochemical processes in insects. We cloned the full-length complementary DNA of C. lividipennis ClALT. Expression analysis showed higher expression in the fat body and midgut compared to other tissues. It is expressed in all C. lividipennis developmental stages and at least four organs. Silencing of ClALT by RNA interference significantly decreased the ClALT enzyme activity and ClALT expression compared to dsGFP-treated controls at 2 days after emergence (DAE). Silencing of ClALT influenced free hemolymph amino acid compositions, resulting in a reduction of Aspartic acid (Asp) and Alanine (Ala) proportions, and increased Cysteine (Cys) and Valine (Val) proportions in females at 2 DAE. dsClALT treatments led to decreased soluble total protein concentrations in ovary and fat body, and to lower reduced vitellogenin (Vg) expression, body weight, and the numbers of laid eggs. The double-stranded RNA viruse treatments also led to prolonged preoviposition periods and hindered ovarian development. Western blot analysis indicated that silencing ClALT also led to reduced fat body Vg protein abundance at 2 DAE. These data support our hypothesis that ClALT influences amino acid metabolism and fecundity in C. lividipennis.

Exposure to L-cycloserine incurs survival costs and behavioral alterations in Aedes aegypti females

Background

It was previously demonstrated that alanine aminotransferase (ALAT, EC 2.6.1.2) participates in maintaining the alanine-proline cycle between flight muscles and fat body during Aedes aegypti flight. ALAT is also actively involved in the metabolism of ammonia in A. aegypti. Here, we investigated the survival and behavioral costs of ALAT inhibition in A. aegypti females to better understand the role of ALAT in blood-fed mosquitoes.

Methods

We analyzed how A. aegypti female mosquitoes respond to blood meals supplemented with 0, 2.5, 5 and 10 mM L-cycloserine, a well-known inhibitor of ALAT in animals. Mosquitoes were also exposed to blood meals supplemented with L-cycloserine and different concentrations of glucose (0, 10 and 100 mM). Additionally, the effects of ALAT inhibitor and glucose in mosquitoes starved for 24 or 48 h were investigated. Survival and behavioral phenotypes were analyzed during a time course (1, 2, 4, 6, 12, 24, 48 and 72 h after feeding).

Results

L-cycloserine at 10 mM resulted in high mortality relative to control, with an acute effect during the first 6 h after treatment. A significant decrease in the number of active mosquitoes coinciding with an increase in futile wing fanning during the first 24 h was observed at all inhibitor concentrations. A high occurrence of knockdown phenotype was also recorded at this time for both 5 and 10 mM L-cycloserine. The supplementation of glucose in the blood meal amplified the effects of the ALAT inhibitor. In particular, we observed a higher mortality rate concomitant with an increase in the knockdown phenotype. Starvation prior to blood feeding also increased the effects of L-cycloserine with a rapid increase in mortality.

Conclusions

Our results provide evidence that exposure of high doses of L-cycloserine during A. aegypti blood feeding affects mosquito survival and motor activity, suggesting an interference with carbohydrate and ammonia metabolism in a time-dependent manner.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-3305-7-373) contains supplementary material, which is available to authorized users.

A novel low molecular weight alanine aminotransferase from fasted rat liver

Alanine is the most effective precursor for gluconeogenesis among amino acids, and the initial reaction is catalyzed by alanine aminotransferase (AlaAT). Although the enzyme activity increases during fasting, this effect has not been studied extensively. The present study describes the purification and characterization of an isoform of AlaAT from rat liver under fasting. The molecular mass of the enzyme is 17.7 kD with an isoelectric point of 4.2; glutamine is the N-terminal residue. The enzyme showed narrow substrate specificity for L-alanine with Km values for alanine of 0.51 mM and for 2-oxoglutarate of 0.12 mM. The enzyme is a glycoprotein. Spectroscopic and inhibition studies showed that pyridoxal phosphate (PLP) and free -SH groups are involved in the enzymatic catalysis. PLP activated the enzyme with a Km of 0.057 mM.

Glutamine and glutamate metabolism in normal and heat shock conditions inDrosophila Kc cells: Conditions supporting glutamine synthesis maximize heat shock polypeptide expression

We have previously reported that Drosophila Kc cells require glutamine for maximal expression of heat shock proteins in stressed conditions (Sanders and Kon: J. Cell. Physiol. 146:180-190, 1991). The mechanism of this effect has been investigated by comparing the metabolic utilization of glutamine in conditions which support hsp expression with that of glutamate in conditions where up to 100-fold less hsp is synthesized. This comparison showed that free ammonia was generated by cells incubated in the presence of glutamine in 37 degrees C (heat shock) conditions, but not at 25 degrees C, and not in the presence of glutamate in either normal or heat shock conditions. There was no difference in the amount of [14C]O2 generated from either [14C]-labeled amino acid in the tricarboxylic acid cycle, but three- to four-fold more alanine was synthesized in cells incubated in glutamine than in glutamate. Treating the cells with aminotransferase inhibitors to artificially increase NH3 release raised hsp expression in the presence of glutamate to maximal levels characteristic of glutamine. This potentiation correlated with inhibition of alanine aminotransferase. Since only NH3 production correlated with hsp expression in heat shock conditions in the presence of glutamine, and NH3 addition to glutamate also resulted in maximal hsp expression, we measured glutamine production in glutamate plus NH3 and observed net glutamine synthesis. The supposition that glutamine itself is responsible for the regulatory changes supporting maximal hsp expression was supported by the finding that the glutamine analog, 6-diazo-5-oxo-L-norleucine (DON), mimicked the effects of glutamine. We conclude that glutamine imposes regulatory changes which alter nitrogen metabolism and support hsp expression in Kc cells.

Purification and characterization of alanine aminotransferase from Panicum miliaceum leaves

Three alanine aminotransferases, two minor (AlaAT-1, AlaAT-3) and one major (AlaAT-2), were detected by native gel electrophoresis of leaf extracts from Panicum miliaceum L. AlaAT-2 was purified to homogeneity and a specific polyclonal antibody was raised against it which did not react with the other two forms of the enzyme. The enzyme, with an apparent molecular size of 102 kDa, appeared to be a dimer of a single 50-kDa polypeptide. The enzyme has a relatively broad pH optima with similar curves for the forward and reverse directions, ranging between 6.5 and 7.5. The Km values of this enzyme were 6.67, 0.15, 5.00, and 0.33 mM for alanine, 2-oxoglutarate, glutamate, and pyruvate, respectively. The activity of AlaAT-2 was found to increase markedly during leaf greening in parallel with the increase of immunochemically titrated protein, and it is suggested to function in the C4 photosynthetic cycle.

Studies on alanine aminotransferase in nematodes

L-alanine aminotransferase was demonstrated in a range of gastrointestinal, free-living and entomophagous nematodes. As in mammals, nematode L-alanine aminotransferase was found to exist in the form of mitochondrial and cytosolic isoenzymes. Whilst the majority of nematode enzymes exhibited a greater overall capacity for L-alanine synthesis than for L-alanine catabolism in vitro, the opposite was true for rat liver L-alanine aminotransferase. In contrast with rat liver, certain gastrointestinal nematodes were apparently able to transaminate D-alanine at low rates. H. contortus cytosolic L-alanine aminotransferase differed significantly from the mammalian enzyme with respect to both thermal stability and response to potential protective reagents.