Phenylalanine

An essential amino acid, precursor of tyrosine, dopamine, adrenaline and noradrenaline. Determined in the context of phenylketonuria.

What phenylalanine does in the body

Phenylalanine is essential and a building block of proteins. Whatever is not needed for this is converted into tyrosine by phenylalanine hydroxylase in the liver, with the helper BH4 and with iron. Phenylalanine is therefore the starting point of a whole chain: tyrosine gives rise to L-dopa, dopamine, noradrenaline and adrenaline, the thyroid hormones in the thyroid and melanin in the pigment cells of the skin. Phenylalanine reaches the brain via a transporter at the blood-brain barrier that it shares with tyrosine, tryptophan and the branched-chain amino acids – the amino acids compete there for access.

Low phenylalanine – what happens in the body?

When little phenylalanine is available, an essential building block is lacking and protein production is throttled. At the same time the liver can make less tyrosine, so the body depends more on tyrosine from food to produce dopamine, noradrenaline and adrenaline. Low values occur with low protein intake and malnutrition.

High phenylalanine – what happens in the body?

When phenylalanine hydroxylase works more slowly, phenylalanine builds up. In inherited phenylketonuria, which is detected in newborn screening, it rises markedly: it occupies the transporter at the blood-brain barrier and displaces tyrosine and tryptophan, so that less dopamine and serotonin are produced in the brain; via side pathways phenylketones are formed and excreted in the urine, and less melanin makes skin and hair lighter. Moderately raised values arise when little BH4 is available – in studies the ratio of phenylalanine to tyrosine rose during inflammation, because BH4 is used up oxidatively –, with impaired liver function and temporarily after protein-rich meals.

The interpretation is given in your report.