Guo-Fang Zhang, PhD, Duke University

Propionic acidemia (PA) is caused by deficiency of an enzyme called propionyl-CoA carboxylase (PCC), which is needed to break down propionyl-CoA. When PCC does not work properly, propionyl-CoA and related metabolites accumulate and can interfere with normal metabolism. Although we know that these metabolic disturbances contribute to many complications of PA, we still do not fully understand how PCC deficiency changes metabolism in individual organs, particularly the liver.

Our recent study focused on an important question: How does PCC deficiency change the way liver cells use nutrients for energy and other essential metabolic processes?

The liver is particularly important in PA because it is one of the major organs responsible for processing propionyl-CoA. To better understand how PCC deficiency affects human liver metabolism, we collaborated with Dr. Xiaoxin Luke Chen at the Coriell Institute for Medical Research to develop a human liver cell model in which the PCCA gene was specifically removed. These PCCA-deficient liver cells reproduced several important biochemical features of PA, including the accumulation of propionyl-CoA and propionylcarnitine, providing a useful model for studying how PCC deficiency disrupts liver metabolism.

We then used a technique called stable isotope tracing to follow how different nutrients were processed by the cells. In simple terms, this approach allows researchers to “label” nutrients and track where their carbon atoms go as the cells use them. This provides information about not only how much of a metabolite is present, but also how metabolic pathways are actually functioning.

PCC deficiency changes how cells use glucose and fat

One of our most important findings was that PCC deficiency changed the way liver cells process pyruvate, a major product of glucose metabolism.

Normally, pyruvate can enter mitochondria and follow two important pathways. It can be converted into acetyl-CoA through pyruvate dehydrogenase (PDH), providing fuel for energy production. Alternatively, it can be converted into oxaloacetate through pyruvate carboxylase (PC). This second pathway, known as anaplerosis, helps replenish important metabolic intermediates and supports processes such as glucose production and fatty acid synthesis.

In the PCCA-deficient cells, pyruvate was preferentially directed toward PDH, while pyruvate carboxylation through PC was markedly reduced as compared to cells with functional PCCA. As a result, the balance between these two pathways was substantially altered.

We also found that PCC deficiency reduced the ability of the mitochondria to use fatty acids for energy. This suggests that when PCC is deficient, liver cells have reduced capacity to use fat as a mitochondrial fuel and instead rely more heavily on glucose-derived carbon.

Why might this matter in PA?

These findings may help explain some of the metabolic vulnerability associated with PA, particularly during periods of fasting or metabolic stress.

The liver normally needs to maintain a flexible balance between using carbohydrates, fats, and other nutrients. Pyruvate carboxylation is an important part of this flexibility because it helps replenish metabolic intermediates and supports glucose production and other biosynthetic pathways.

Our findings suggest that PCC deficiency disrupts this flexibility. Reduced pyruvate carboxylation was associated with impaired gluconeogenic capacity and reduced fatty acid synthesis in our experimental models. We also observed reduced fatty acid oxidation, suggesting that both the production and utilization of certain fuels can be affected.

These results provide a possible metabolic explanation for why prolonged fasting can be particularly challenging for individuals with PA, especially during illness or other situations in which energy demands increase.

What about protein and amino acid metabolism?

Our study also suggests that the metabolic consequences of PCC deficiency extend beyond propionyl-CoA metabolism. The TCA cycle (tricarboxylic acid cycle) is a central metabolic hub inside mitochondria that helps generate energy and provides metabolic building blocks for the cell. Because TCA-cycle intermediates are continuously being used, cells must replenish them through a process called anaplerosis. In the liver, both propionate-derived carbon and glucose-derived pyruvate can contribute to this process.

When PCC is deficient, we found that two important sources of anaplerosis are reduced. First, propionyl-CoA can no longer efficiently contribute carbon to the TCA cycle through the PCC-dependent pathway. Second, we found that pyruvate carboxylation, the process by which the enzyme pyruvate carboxylase (PC) converts pyruvate into a TCA-cycle intermediate, was also markedly reduced. Together, these changes may limit the liver’s ability to replenish TCA-cycle intermediates from both propionate-derived carbon and glucose-derived pyruvate.

This raises an important question: How does the liver compensate when these sources of anaplerosis are reduced? We are investigating whether the liver increases its reliance on other sources of TCA-cycle intermediates, particularly amino acid metabolism, and whether this increased reliance could affect nitrogen handling and ammonium production. Understanding how impaired PCC- and PC-dependent anaplerosis influences amino acid metabolism may help explain how the metabolic abnormalities caused by PCC deficiency contribute to complications such as elevated ammonium levels that can occur in people with PA.