Study explains mystery of metabolite rise in ALAD porphyria
ALA buildup that drives symptoms also fuels COPRO III accumulation
Written by |
A buildup of aminolevulinic acid (ALA), responsible for the symptoms of delta-aminolevulinic acid dehydratase (ALAD) deficiency porphyria, also drives the unexpected accumulation of the metabolic byproduct COPRO III, a study found.
The researchers said the finding helps explain why people with ALAD deficiency and other porphyrias consistently show elevated COPRO III levels in urine and stool, and may lead to a better understanding of disease mechanisms.
“Biologically, this mechanism is important to understand the effect of [ALA] and COPRO III accumulation not only in ALAD porphyria but also in other acute hepatic porphyrias … where ALA predominates,” the scientists wrote.
The study, “Aminolevulinate inhibition of human coproporphyrinogen oxidase clarifies coproporphyrin III accumulation in porphyrias,” was published in Bioscience Reports.
Porphyrias are a group of rare disorders caused by disruptions in the production of heme, a molecule that allows red blood cells to transport oxygen. This leads to the toxic buildup of molecules called porphyrins and their precursors in various parts of the body. ALAD is a form of the disease caused by mutations in the ALAD gene, leading to a deficiency of the ALAD enzyme. This drives the accumulation of ALA, often resulting in abdominal pain, nausea, and constipation, as well as neurological symptoms.
Unexplained link
Urine and stool samples from ALAD porphyria patients consistently show elevated levels of the porphyrin, coproporphyrin III, or COPRO III. This accumulation can also be observed in other porphyrias, including variegate porphyria and harderoporphyria.
COPRO III is an oxidized byproduct of COPROgen III, a molecule that’s converted by the CPOX enzyme into a heme precursor during heme production. Under normal conditions, it is produced at low levels, but it can increase when heme production is disrupted.
It’s not known why ALAD porphyria patients produce such high levels of COPRO III. To learn more, scientists in Spain tested the effect of both COPRO III and ALA on the activity of a lab-made version of the CPOX enzyme.
The idea was that if ALA blocked CPOX activity, it would cause COPROgen III to accumulate, and in turn be oxidized to COPRO III — ultimately explaining what’s seen in ALAD porphyria patients. And if COPRO III itself also blocks the enzyme, this would further drive up its levels.
They found that COPRO III strongly binds to CPOX, and only small amounts of it are needed to block the enzyme’s activity. Computer modeling suggested that COPRO III would fit into CPOX’s active site, directly competing with COPROgen III’s ability to interact with the enzyme.
Unlike COPRO III, ALA bound to CPOX more weakly and required much higher concentrations to reduce enzyme activity. It seemed to lower the enzyme’s activity not by blocking the active site, but rather by chemically reacting with CPOX to permanently alter its function.
The team then studied mice carrying a genetic mutation that increases succinylacetone, a molecule known to inhibit ALAD and raise ALA levels, similar to ALAD porphyria patients. An examination of the liver tissue of these mice revealed significantly higher levels of COPRO III than found in healthy mice.
When succinylacetone production was blocked so that ALA would not accumulate, COPRO III levels didn’t rise, confirming that high ALA levels are required for COPRO III buildup.
Taken together, the data point to a model in which “ALA buildup leads to CPOX dysfunction, which in turn promotes COPRO III accumulation … and this metabolite further inhibits CPOX, creating a synergistic inhibitory loop,” the researchers wrote.
The team examined the effect of lead on CPOX enzyme activity, as environmental exposure to the metal can also trigger elevations in ALA and COPRO III, along with symptoms that mimic ALAD porphyria.
Even at low levels, lead significantly reduced the enzyme’s activity. The team believed that rather than directly binding to CPOX and inhibiting it, lead may react with oxygen to form toxic molecules that damage the enzyme.
“In summary, our findings highlight multiple layers of CPOX regulation,” the researchers wrote. “[ALA], COPRO III, and [lead] each impair CPOX activity through distinct mechanisms … and provide a mechanistic framework to explain how metabolic imbalances and environmental exposures contribute to COPRO III accumulation in [ALAD porphyria] and after lead intoxication.”
