What is methylmalonic acid?

What is Methylmalonic Acid?

Methylmalonic acid (MMA) is a metabolic intermediate in propionate metabolism that is converted to succinic acid through a vitamin B12-dependent enzymatic step, and it serves as a functional biomarker of vitamin B12 deficiency and mitochondrial dysfunction. 1, 2

Biochemical Role and Metabolism

• MMA is produced as a byproduct of propionate metabolism and requires vitamin B12 (specifically methylmalonyl-CoA mutase, a B12-dependent enzyme) to be converted to succinic acid, which then enters the tricarboxylic acid (TCA) cycle 2, 3

• Under normal physiological conditions, MMA is present at low levels in the body, but accumulates when this conversion pathway is impaired 2

• MMA can originate from multiple sources including endogenous metabolism, dietary propionate precursors, gut microbiota metabolism, and certain pathological states 2, 4

Clinical Significance as a Biomarker

MMA is most commonly used as a confirmatory test for vitamin B12 deficiency, particularly when initial B12 testing yields indeterminate results. 1

• When total B12 levels are 180-350 ng/L (133-258 pmol/L) or active B12 levels are 25-70 pmol/L, measuring MMA provides more reliable functional assessment of B12 status 1

• MMA testing is more expensive (£11-80 per test) than standard B12 testing and often requires external laboratory analysis with longer turnaround times, making it unsuitable as a first-line test 1

• MMA is also used diagnostically for hereditary methylmalonic acidemia, a group of autosomal recessive inborn errors of metabolism where enzymatic defects prevent proper MMA conversion 1, 3

Factors Affecting MMA Levels

• Vitamin B12 deficiency: The most well-known cause of elevated MMA, as B12 is required as a cofactor for the enzyme that converts MMA to succinic acid 1, 2

• Renal function: MMA levels are significantly influenced by kidney function, as impaired renal clearance leads to accumulation 4

• Age: MMA levels increase with aging, independent of B12 status 2, 4

• Gut microbiota: Intestinal bacterial metabolism can contribute to mild MMA elevations 1

• Nutritional factors: Low maternal or infant B12/folate status can cause secondary MMA elevation 1

Importantly, only 22% of variation in MMA levels is explained by vitamin B12, estimated glomerular filtration rate (eGFR), age, and sex combined, indicating that substantial variation comes from other factors such as catabolism, dietary components, or microbial production 4

Pathophysiological Effects of Elevated MMA

When MMA accumulates, it causes oxidative stress, impairs mitochondrial function, disrupts cellular energy metabolism, and can trigger cell death. 3

• MMA inhibits the malate-phosphate exchange carrier in mitochondria, interfering with TCA cycle function and potentially explaining metabolic complications like hypoglycemia in methylmalonic aciduria 5

• Elevated MMA suppresses NADH-regenerating reactions in the TCA cycle, leading to mitochondrial dysfunction 6

• MMA induces immunosuppression by causing CD8+ T cell exhaustion and dysfunction, which impairs anti-tumor immunity 6

Clinical Associations and Prognostic Value

• Higher MMA levels are independently associated with increased all-cause mortality risk, even after accounting for vitamin B12 and renal function 4

• This mortality association is more pronounced in individuals with impaired renal function 4

• Elevated MMA has been linked to progression and prognosis of chronic diseases including cardiovascular events, renal insufficiency, cognitive impairment, and cancer 3

• Each standard deviation increase in MMA (approximately 0.49 μmol/L) is associated with increased depressive symptoms, particularly in men and adults over 60 years 7

• In its chronic form, methylmalonic acidemia can cause cardiomyopathy with impaired cardiac function 8

Laboratory Testing Considerations

• MMA is preferably measured in urine for metabolic screening, though serum MMA is used for B12 deficiency assessment 1

• Urine samples should be collected during acute illness when possible, frozen immediately, and stored at -20°C or lower 1

• Serum MMA can be measured with as little as 25 μL of sample using modern UPLC-MS/MS methods, making it suitable for pediatric populations 9

• MMA is stable for over three years in frozen samples and withstands up to five freeze/thaw cycles 9

• Interpretation requires consideration of clinical context, as bacterial contamination from prolonged storage at room temperature can artificially elevate results 1