Studies of Propionate Toxicity in Salmonella enterica Identify 2-Methylcitrate as a Potent Inhibitor of Cell Growth

Abstract

Salmonella enterica serovar Typhimurium LT2 showed increased sensitivity to propionate when the 2-methylcitric acid cycle was blocked. A derivative of a prpC mutant (which lacked 2-methylcitrate synthase activity) resistant to propionate was isolated, and the mutation responsible for the newly acquired resistance to propionate was mapped to the citrate synthase (gltA) gene. These results suggested that citrate synthase activity was the source of the increased sensitivity to propionate observed in the absence of the 2-methylcitric acid cycle. DNA sequencing of the wild-type and mutant gltA alleles revealed that the ATG start codon of the wild-type gene was converted to the rare GTG start codon in the revertant strain. This result suggested that lower levels of this enzyme were present in the mutant. Consistent with this change, cell-free extracts of the propionate-resistant strain contained 12-fold less citrate synthase activity. This was interpreted to mean that, in the wild-type strain, high levels of citrate synthase activity were the source of a toxic metabolite. In vitro experiments performed with homogeneous citrate synthase enzyme indicated that this enzyme was capable of synthesizing 2-methylcitrate from propionyl-CoA and oxaloacetate. This result lent further support to the in vivo data, which suggested that citrate synthase was the source of a toxic metabolite.

N-Lysine Propionylation Controls the Activity of Propionyl-CoA Synthetase

“…From a broader physiological perspective, all cells must control their pools of acyl-CoAs to avoid depletion of the pool of free CoA and/or synthesis of toxic metabolites (13,44). This suggests, by analogy with the work reported here and with earlier findings regarding acetyl CoA homeostasis (8), that there might well be other acyltransferase/deacylase systems that cells from all domains of life use to control the activity of acylCoA synthetases.…”

“…Reports in the literature suggest that propionyl-CoA may be responsible for the cytotoxic effects of propionate. Although the direct effects of propionyl-CoA are unclear, it is clear that cells avoid accumulating this metabolite (13)(14)(15). The cell maintains low levels of propionyl-CoA by using it as a source of carbon and energy.…”

Loving the poison: the methylcitrate cycle and bacterial pathogenesis

“…However, the mechanisms of propionate toxicity appear to be extremely diverse. Early studies argued that growth inhibition might be caused by generic, non-specific mechanisms such as the dissipation of the proton motive force, decreased intracellular pH or accumulation of the propionate anion in the cytoplasm [1]. More recent work has challenged these notions.…”

“…Consequently, propionate is produced in large quantities (50-100 mM) in the gastrointestinal tract of mammals [1,2]. Plants, insects and micro-organisms utilize several alternative pathways for metabolizing propionate as a carbon source.…”

Neurodegeneration in Methylmalonic Aciduria Involves Inhibition of Complex II and the Tricarboxylic Acid Cycle, and Synergistically Acting Excitotoxicity

“…MCA exerts several inhibitory effects on TCA cycle enzymes and on the mitochondrial citrate transporter, facilitating mitochondrial accumulation of MCA (34). Furthermore, MCA mainly contributes to propionate sensitivity of bacteria lacking the MCA cycle as detoxifying mechanism (40). Impairment of energy metabolism can interfere with the ability of neurons to maintain normal resting membrane potential due to ATP depletion and can decrease the activity of Na ϩ /K ϩ -ATPases (41).…”