Compounds that inhibit lactate overproduction are usually beneficial, but relying on oxaloacetate for this is not ideal.
Pyruvate carboxylase is required for glutamine-independent growth of tumor cells (PC synthesizes oxaloacetate)
In case of oxaloacetate supplementation, delivery is the first challenge because most of the dose is metabolized in the liver after absorption. For esterified forms, the fraction that reaches target cells is likely overestimated, especially when tissue circulation is poor.
Malate dehydrogenase (MDH) interconverts malate and oxaloacetate, and this enzyme is also part of the malate-aspartate shuttle (MAS) that you mention.
[image: 1772751634940-d34ce921-acea-4633-9653-639c96f9838f-image.png]
⠀(10.1101/cshperspect.a040543)
The figure shows some potential concerns:
Cytosolic NAD reoxidation by MDH can promote glycolysis, but without reliably improving mitochondrial metabolism for further oxidation.
Increased demand for NAD+ relative to ATP drives aerobic glycolysis
Extra oxaloacetate (OAA) may not convert into malate as expected. Aspartate is another component of the shuttle and is a reaction away from oxaloacetate (GOT/AST is bidirectional).
[image: 1772751647814-4f7a93ec-7e75-4c6a-a1ad-ae54d314276c-image.png]
⠀(10.1016/j.ymgmr.2023.100967)
An Essential Role of the Mitochondrial Electron Transport Chain in Cell Proliferation Is to Enable Aspartate Synthesis
Supporting Aspartate Biosynthesis Is an Essential Function of Respiration in Proliferating Cells
Oxaloacetate enters mitochondria hydrogenated as malate. Recovering oxaloacetate on the mitochondrial side depends on sufficient NAD⁺ (also shown on the figure), which may be scarce and prioritized for other reactions, such as that of KGDHc.
Recovery of oxaloacetate from malate in mitochondria doesn't guarantee its reaction with acetyl-CoA. It may instead support glutamate metabolism by accepting its amino group, yielding ketoglutarate for KGDHc (↻) or IDH (↺). One route doesn't exclude the other, as a fraction of ketoglutarate can undergo oxidative decarboxylation (releasing CO₂) and the other reductive carboxylation (incorporating CO₂),
Supporting glutamate metabolism in forward function:
Oxaloacetate + glutamate ←{GOT}→ Aspartate + Ketoglutarate
Ketoglutarate –{KGDHc}→ Succinyl-CoA ←{STK}→ ATP + Succinate
A portion of oxaloacetate-derived malate can also be converted to fumarate in reverse TCA cycle operation.
Oxaloacetate → Malate → Fumarate → Succinate
[image: 1772751671369-3224a3ed-9ba4-4586-bbab-57e75c344b76-image.png]
⠀(10.3389/fendo.2012.00022)
This way, oxaloacetate helps to generate ketoglutarate to support mitochondrial fermentation and non-respiratory ATP synthesis with minimal oxidation. In addition, fumarate may be used as a substitute to deficient oxygen, accepting electrons from the respiratory chain sourced from ketoglutarate itself, dihydro-orotate, etc.
(Oxaloacetate +) Glutamate → Ketoglutarate → Succinate ← Fumarate ← Oxaloacetate
Succinate accumulation becomes comparable to lactate. Both metabolites are exported as fermentation end-products together with an extra H⁺, contributing to extracellular acidification.
[image: 1772751690253-15d16b6b-0813-45e0-8bb1-7fecd96f1d33-image.png]
⠀(10.1080/17590914.2024.2422268)
"Succinate-stabilized HIF-1a" reinforces PDHc inhibition, giving another reason not to assume that extra oxaloacetate and CoA release will serve PDHc, because they may just as well promote fatty acid oxidation.
If oxaloacetate condenses with acetyl-CoA, that doesn't commit it oxidation either; oxaloacetate can serve as a carrier to export excess acetyl groups to support lipid synthesis.
As for pyruvate and its multiple metabolic fates, even discounting lactate and oxaloacetate in this context, we can't assume that pyruvate routing to the chronically inhibited PDHc will prevail over supporting an upregulated pathway, such as in metabolizing abundant glutamate via GPT/ALT, which remains expressed outside the liver.
Pyruvate + H⁺ + CoA + NAD⁺ –{PDHc}→ Acetyl-CoA + CO₂ + NADH + H⁺
Pyruvate + ⇈Glutamate ←{GPT}→ Alanine + Ketoglutarate
This is another potential route that would supply ketoglutarate without depending on glutamate oxidation, sparing oxidative capacity to keep KGDHc running.
