The Birth of MS/MS Screening

Advances in metabolomics drive a parallel surge in metabolite discovery – and inspire new platforms able to measure increasing numbers of compounds in blood, plasma or urine. Our ability to perform such analyses at the microsample scale – a single drop of blood dried on filter paper – has given rise to modern newborn screening.

Here, I describe how this revolution in metabolomics and clinical diagnostics all started with the analysis of amino acids and acylcarnitines 20 years ago.

Determination of L-carnitine and its derivatives is the basis for newborn screening for several potentially deadly metabolic disorders – a prime example of how analytical science affects all our lives, from the day we are born.

L-carnitine is unique and fascinating molecule. With some chemical similarity to amino acids (perhaps even an ancient precursor to amino acids – see “The Primordial Group” below), L-carnitine is a quaternary ammonium compound – a rare class in the human body. It plays a crucial role in fat metabolism, helping to break down fatty acids (through beta oxidation) into a form that the mitochondria – the cell’s powerhouse – can use as fuel.

Beta oxidation is facilitated by coenzyme A, but coenzyme A and its acyl-bound fatty acids (acyl-CoAs) cannot always cross the mitochondrial membrane. Short- or medium-chain acyl-CoAs can traverse the membrane, but very-long-chain acyl-CoAs cannot, so they must be temporarily transferred to another substrate for transport across the membrane via a translocase “tunnel.” L-carnitine is that substrate. Figure 1 shows how L-carnitine transports fatty acids through the inner mitochondrial membrane in the form of acylcarnitine. Once inside the mitochondria, the fatty acids are transferred back to coenzyme A for delivery to beta oxidation enzymes. Using a train analogy, coenzyme A can be thought of as a small engine moving a few train cars at a time around a rail yard, while L-carnitine is a bulkier locomotive that can pull longer trains over greater distances.