Aminoacidopathies and Organic Acidemias

Aminoacidopathies and organic acidemias are inborn errors of amino acid metabolism. In aminoacidopathies an enzyme defect causes an amino acid (or its immediate derivative) to accumulate, as in phenylketonuria or maple syrup urine disease; in organic acidemias a defect further along the catabolic pathway causes non-amino organic acids to build up, as in methylmalonic and propionic acidemia. Both are classic examples of intoxication-type metabolic disease.

Definition

Aminoacidopathies and organic acidemias are inherited disorders of amino acid catabolism in which an enzyme deficiency causes accumulation of an amino acid or of downstream organic-acid intermediates, typically producing intoxication-type disease with metabolic decompensation.

Scope

The entry covers the shared logic of a block in amino acid catabolism, the distinction between aminoacidopathies and organic acidemias, the biochemical markers used to detect them (plasma amino acids, urine organic acids, acylcarnitines), and their place in newborn screening. Specific disorders are used as illustrations rather than treated individually, and no management advice is given.

Key concepts

• Amino acid catabolic pathways
• Phenylketonuria (phenylalanine hydroxylase deficiency)
• Maple syrup urine disease (branched-chain ketoacid dehydrogenase deficiency)
• Methylmalonic and propionic acidemia
• Intoxication-type metabolic disease
• Plasma amino acid and urine organic acid profiling
• Hyperammonemia and metabolic acidosis as secondary features
• Newborn screening and dietary restriction concepts

Mechanisms

Amino acids are broken down through ordered enzymatic steps; a block early in such a pathway lets the amino acid itself accumulate (an aminoacidopathy), while a block at a later, organic-acid intermediate produces an organic acidemia. In phenylketonuria, deficient phenylalanine hydroxylase raises blood phenylalanine, which at high concentrations is neurotoxic - the abnormality Folling first detected through urinary phenylpyruvic acid. In organic acidemias such as methylmalonic and propionic acidemia, defects in the catabolism of branched-chain amino acids and other precursors cause toxic organic acids to accumulate, often with secondary metabolic acidosis, hyperammonemia, and acute decompensation triggered by catabolic stress. These distinct biochemical signatures - measurable as plasma amino acids, urine organic acids, and blood acylcarnitines - allow whole groups of disorders to be screened and triaged, and underpin guidelines such as those of Baumgartner and colleagues.

Clinical relevance

These disorders show how an enzyme defect in amino acid breakdown translates into a characteristic biochemical profile and an intoxication pattern of disease. Recognising the categories explains why many are targets of newborn screening and why diagnostic workups rely on amino acid and organic acid analysis. This entry is a reference overview and does not provide individualised diagnostic or treatment recommendations.

Epidemiology

Individually rare, aminoacidopathies and organic acidemias are collectively important and figure prominently in expanded newborn screening panels. Phenylketonuria is among the most frequently detected; the incidence of each disorder varies by population and screening practice.

History

The field opened in 1934 when Asbjorn Folling identified phenylpyruvic acid in the urine of intellectually disabled siblings, linking a measurable metabolite to an inherited condition later named phenylketonuria. The subsequent introduction of newborn screening for phenylketonuria and the development of chromatographic and mass-spectrometric profiling extended detection to many aminoacidopathies and organic acidemias, and modern consensus guidelines, such as Baumgartner and colleagues' for methylmalonic and propionic acidemia, now codify their evaluation.

Key figures

• Asbjorn Folling
• Nenad Blau
• Matthias Baumgartner
• Jean-Marie Saudubray

Seminal works

• folling-1934
• blau-2010-pku
• baumgartner-2014

Frequently asked questions

What is the difference between an aminoacidopathy and an organic acidemia?

In an aminoacidopathy the block is early in an amino acid's breakdown, so the amino acid itself accumulates (as in phenylketonuria). In an organic acidemia the block is at a later step, so non-amino organic acid intermediates accumulate (as in methylmalonic or propionic acidemia).

Why are these disorders often found by newborn screening?

Many produce distinctive amino acid or organic acid signatures detectable in a dried blood spot before symptoms appear, and several can cause irreversible harm if not recognised early, which is why they are included in screening panels.