Phenylketonuria

I love when everyday conversation leads to interesting medical facts. The other day at work, my friend Allie was reading her gum and noted the warning on the package, "Phenylketonurics: contains phenylalanine." From being a biochemistry major, I knew what phenylalanine was, but I had no idea what the word phenylketonuric meant, or what the warning was referring to. Allie looked it up, and hence I am doing this entry.

Let me break down what this warning means. A phenylketonuric is a person with phenylketonuria (PKU for short). phenyalanine is an amino acid that is often present in artificially sugary foods like diet soda and chewing gum. So the warning should be interpreted, "Hey you, this has that things that's bad for you." Or more analogously, "Hey guy who is allergic to peanuts, this is full of them".

Phenylketonuria is an autosomal recessive genetic disorder that renders sufferers unable to metabolize phenylalanine. I'm pretty excited that I get to reuse this genetic inheritance graphic. Something tells me the little girl on the bottom right isn't going to have a very fun time on this blog.

So what is phenylalanine? It's one of the essential amino acids. Whenever you see the word essential in front of something you eat, like essential vitamins and minerals, that means it is something you cannot synthesize yourself. Therefore, an essential amino acid is one you must have as part of your diet. Essentially, you need to eat it or you'll have problems. You can find phenylalanine in several places, including milk. Metabolically speaking phenyalanine, like most amino acids, can be used for many purposes. It is a precursor for tyrosine, another amino acid, which is then converted to L-DOPA for synthesis of dopamine, epinephrine, and norepinephrine.

So like most disorders that involve the inability to metabolize a certain compound, an enzyme deficiency is to blame. In this case phenylketonurics have a phenyalanine hydroxlase (PAH) deficiency. A hydroxylase is an enzyme that catalyzes the formation of a hydroxyl group (OH) through the addition of one or two oxygen molecules. This enzyme is responsible for converting phenylalanine to tyrosine. You can see from the illustration that tyrosine is identical to phenylalanine with the exception of the extra hydroxyl group.

Left unchecked, this disorder can lead to mental retardation and impaired brain development. Phenylalanine shares a transport pathway with tryptophan, the amino acid precursor to serotonin, and thus when levels of phenylalanine are high, it can interfere with metabolic activity in the brain. In early development, high levels of phenylalanine in the blood can saturate certain amino acid transporters leading to a decrease in the levels of other amino acids in the brain. Over time, this is what is responsible for diminished brain development.

That all being said, no one should really be afraid of these warnings on your gum package. Even if you have PKU, it's not a big deal from what I understand. Nowadays, early screenings can identify infants with PKU. So the disorder rarely goes unchecked. Treatments include a low-phenylalanine diet combined with protein supplements. In some cases, this treatment only has to be carried out through the first 16 years of an individuals life since the greatest danger of this disorder is apparent only during developmental years. So fear not phenylketonurics, that stick of gum probably won't turn your brain to mush.