Benzoyl-CoA conjugate accumulation as an initiating event for male reprotoxic effects in the rat? Structure-activity analysis, species specificity, and in vivo relevance.

A number of para-substituted benzoic acids (p-BA) and chemicals metabolized to p-BA have been found to confer adverse effects in male rats on sperm viability, motility, and morphology. These effects are putatively associated with the metabolism of p-BA to toxic intermediates. We had shown that p-BA lead to accumulation of high levels of p-alkyl-benzoyl-CoA conjugates in plated primary rat hepatocytes. Here we further investigated the relevance of this metabolic pathway for the reprotoxic effects in rats and rabbits. We extended the structure-activity relationship to a set of 19 chemicals (nine reprotoxic and ten non-reprotoxic) and confirmed a very strong correlation between p-alkyl-benzoyl-CoA accumulation in rat hepatocytes and the toxic outcome. Species specificity was probed by comparing rat, rabbit and human hepatocytes, and p-benzoyl-CoA accumulation was found to be specific to the rat hepatocytes, not occurring in human hepatocytes. There was also very limited accumulation in hepatocytes from rabbits that are a non-responder species in in vivo studies. Tissues of rats treated with 3-(4-isopropylphenyl)-2-methylpropanal were analysed and p-isopropyl-benzoyl-CoA conjugates were detected in the liver and in the testes in animals at toxic doses indicating that the metabolism observed in vitro is relevant to the in vivo situation and the critical metabolite does also occur in the reproductive tissue. These multiple lines of evidence further support benzoyl-CoA accumulation as a key initiating event for a specific group of male reproductive toxicants, and indicate a species-specific effect in the rat.

The 2-methylcitrate cycle is implicated in the detoxification of propionate in Toxoplasma gondii.

Toxoplasma gondii belongs to the coccidian subgroup of the Apicomplexa phylum. The Coccidia are obligate intracellular pathogens that establish infection in their mammalian host via the enteric route. These parasites lack a mitochondrial pyruvate dehydrogenase complex but have preserved the degradation of branched-chain amino acids (BCAA) as a possible pathway to generate acetyl-CoA. Importantly, degradation of leucine, isoleucine and valine could lead to concomitant accumulation of propionyl-CoA, a toxic metabolite that inhibits cell growth. Like fungi and bacteria, the Coccidia possess the complete set of enzymes necessary to metabolize and detoxify propionate by oxidation to pyruvate via the 2-methylcitrate cycle (2-MCC). Phylogenetic analysis provides evidence that the 2-MCC was acquired via horizontal gene transfer. In T. gondii tachyzoites, this pathway is split between the cytosol and the mitochondrion. Although the rate-limiting enzyme 2-methylisocitrate lyase is dispensable for parasite survival, its substrates accumulate in parasites deficient in the enzyme and its absence confers increased sensitivity to propionic acid. BCAA is also dispensable in tachyzoites, leaving unresolved the source of mitochondrial acetyl-CoA.

New insights on the mechanisms of valproate-induced hyperammonemia: inhibition of hepatic N-acetylglutamate synthase activity by valproyl-CoA.

BACKGROUND & AIMS: Hyperammonemia is a frequent side-effect of valproic acid (VPA) therapy, which points to an imbalance between ammoniagenesis and ammonia disposal via the urea cycle. The impairment of this liver-specific metabolic pathway induced either by primary genetic defects or by secondary causes, namely associated with drugs administration, may result in accumulation of ammonia. To elucidate the mechanisms which underlie VPA-induced hyperammonemia, the aim of this study was to evaluate the effect of both VPA and its reactive intermediate, valproyl-CoA (VP-CoA), on the synthesis of N-acetylglutamate (NAG), a prime metabolite activator of the urea cycle. METHODS: The amount of NAG in livers of rats treated with VPA was quantified by HPLC-MS/MS. The NAG synthase (NAGS) activity was evaluated in vitro in rat liver mitochondria, and the effect of both VPA and VP-CoA was characterized. RESULTS: The present results clearly show that VP-CoA is a stronger inhibitor of NAGS activity in vitro than the parent drug VPA. The hepatic levels of NAG were significantly reduced in VPA-treated rats as compared with control tissues. CONCLUSIONS: These data strongly suggest that the hyperammonemia observed in patients under VPA treatment may result from a direct inhibition of the NAGS activity by VP-CoA. The subsequent reduced availability of NAG will impair the flux through the urea cycle and compromise the major role of this pathway in ammonia detoxification.