What Is Phenylalanine Hydroxylase?

Paul Reed
Paul Reed
Scientist with beakers
Scientist with beakers

Phenylalanine is an amino acid used by the human body for cellular activity. The body uses it by converting it to tyrosine, another amino acid, using a gene enzyme called phenylalanine hydroxylase (PAH). An enzyme can be thought of as a biological catalyst that helps a chemical reaction, but is not destroyed in the process. Tyrosine is used to create various hormones, to produce energy in the body, and to create the skin pigment melatonin.

The chemical process that creates tyrosine involves the hydroxylation, or addition of an oxygen-hydrogen group, to part of the phenylalanine molecule. This process takes place in the liver, and involves the interaction of iron and other enzymes in a complex reaction. The body uses tyrosine to create other key hormones, including dopamine used by the brain.

Humans obtain phenylalanine from the foods they eat, including most proteins and artificial sweeteners that contain aspartame. The body can normally regulate the excess through the phenylalanine hydroxylase reaction, with some exceptions. Some people have a lack of PAH or abnormalities that can prevent the normal processing of the amino acid.

People having a PAH deficiency can experience severe mental retardation if the disease is not recognized early. Excess phenylalanine will appear in the urine, a condition called phenylketonuria. In many developed countries, infants are tested at birth using the blood test called the Guthrie assay, which looks for an excess of the amino acid combined with bacteria.

In addition to the excess amino acid in urine, there are other symptoms of phenylalanine hydroxylase mutation. People with the deficiency will often have a musty odor, particularly in their urine. They may also have pale skin, because the body is not producing tyrosine needed for melanin. Epilepsy and other nerve diseases can also occur from the disease due to the lack of key hormones needed for nerve functions.

Children with non-diagnosed PAH disease will test lower on intelligence tests than healthy children. If it remains untreated, severe mental problems and other medical conditions will occur over time. A strict diet avoiding the artificial sweetener and using low protein intake can prevent the developmental issues from the phenylalanine hydroxylase condition.

It is critical for infants to be placed on a phenylalanine-restricted diet if they are found to have PAH mutations. All proteins contain the amino acid at various levels, but animal proteins, nuts and beans are particularly rich and must be avoided. The dietary restriction is a life-long commitment, as into the 21st century there had been no gene therapy available to substitute for phenylalanine hydroxylase. Tyrosine may also need to be given as a supplement to persons on the diet, because the body is not producing it and a normal dietary source is the restricted protein.

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