Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Agoraphobic/Agoraphobia: Fear of open or crowded spaces. Also, a fear of leaving a safe place.

Albinism: An inborn error of metabolism characterized by an inability to produce melanin, the protein required for skin and hair pigmentation.

Amino acid: A class of organic molecules which make up the building blocks of all proteins. There are twenty amino acids that are considered standard, and of those, ten are considered essential components of a healthy diet because they cannot be produced in the human body. These ten essential amino acids are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, arginine, leucine, and lysine.

Since phenylalanine is one of the essential amino acids, everyone, including people with PKU, must have it as part of their diet.

Aspartame: An artificial sweetener found in many diet sodas and low-calorie foods which is created from phenylalanine. In the body, it is broken down and phenylalanine is produced. People with PKU must therefore be aware that even though foods containing aspartame may have no protein, they still contain significant amounts of phenylalanine. You may have noticed the warning on diet soda cans: “Phenylketonurics: contains phenylalanine.” You are now one of the select few who knows why!

Axons: Microscopic fibers that connect brain cells to each other, allowing them to communicate. Even though they are microscopically thin, some axons in the body can be as long as three feet or more. Axons in the brain are typically coated with a white substance called myelin.

B

BH4: See tetrahydrobiopterin.

C

Cell body: Part of a neuron which contains the nucleus and other organelles. Cell bodies are connected to other cell bodies by axons.

Classic PKU: The most severe form of PKU. People with classic PKU tolerate less than 250–350 mg of Phe (about 17–23 exchanges) a day.

Cofactor: A molecule which must work with an enzyme in order for that enzyme to function.

Congenital hypothyroidism: An inborn error of metabolism characterized by a deficiency in thyroid hormone. If left untreated, it could lead to abnormal growth and mental retardation. This disease is simply another example of an inherited metabolic disease.

E

Enzyme: A kind of biological molecule that facilitates certain chemical reactions inside the body. The human body contains many thousands of different enzymes, which are involved in virtually every aspect of the body’s functions. People with PKU lack a certain enzyme called phenylalanine hydroxylase (PAH).

G

Gene: A region of DNA which codes for a specific characteristic. When a gene mutates, it may no longer perform the same function, and often the mutation renders it entirely inactive. In the case of PKU, the gene involved in the production of phenylalanine hydroxylase is mutated, causing the liver to produce insufficient amounts of that enzyme, and thus restricting the activity of the metabolic pathway which converts phenylalanine to tyrosine.

Grey matter: Brain areas that appear grey because they contain cell bodies rather than connections to other cells. Areas containing connections to other cells often appear white.

H

Hyperphenylalaninemia: Abnormally high levels of phenylalanine in the blood. This is a symptom of PKU, but could also be a symptom of several other conditions. The newborn screening test is technically a test for hyperphenylalaninemia rather than for PKU. In other words, if a newborn tests positive in the first screening, all it means is that newborn has a high level of phenylalanine in their blood, which may or may not be due to PKU. This means additional testing is required to learn what is causing hyperphenylalaninemia.

Think of the roots:

Phenylalanin: phenylalanine

Emia: in the blood (like hypoglyc-emia, or an-emia)

I

Inborn error of metabolism: Category of diseases affecting metabolism which are inherited through genes, and originate at birth. PKU, albinism, congenital hypothyroidism, tyrosinemia, and maple syrup urine disease are a few examples of inborn errors of metabolism.

Inherited metabolic disease: See Inborn error of metabolism

M

Metabolic clinic: A medical facility, often associated with a university or major hospital, that specializes in treating people who have diseases of metabolism such as PKU. The clinic is likely to be operated by a metabolic geneticist overseeing a staff of registered dietitians.

Metabolic clinics have a substantial role in the treatment of PKU. Staff at a metabolic clinic will be responsible for periodic blood testing and checkups, nutritional advice, formula prescription, and in some cases psychological assessment.

Metabolic geneticist: A doctor specializing in inherited metabolic diseases. Each metabolic clinic has at least one of these specialists on staff, whose responsibilities include tracking patient history, interpreting blood tests, and prescribing PKU formula.

Metabolism/Metabolize: The word “metabolism” refers to any chemical process that occurs within cells that is necessary for the normal growth and functioning of the cell. To metabolize means to carry out that process.

In the case of PKU, “metabolism” refers to the process by which phenylalanine is converted to tyrosine in the liver (e.g., “phenylalanine metabolism,” “the metabolism of phenylalanine,” “phenylalanine is metabolized”).

Moderate PKU: A less severe form of PKU that may still cause mental retardation if not managed with a restricted diet. People with moderate PKU tolerate about 350–400 mg of Phe (about 23–27 exchanges) a day.

Myelin: An insulating substance that covers many of the axons in the brain. Myelin helps to speed the communication between two brain cells. People with PKU who are not faithful to the restricted diet often have less myelin than those who better manage their diet.

N

Neuron: One of the cell types that make up the central nervous system.

Neurotoxic: Damaging to the brain

Neuro – brain

Toxic – poisonous, damaging

A “neurotoxin” is a substance or agent determined to be neurotoxic.

O

Organelle: One of the functional components of a cell. Organelles carry out many of the metabolic functions a cell must perform in order to survive.

P

PAH: See phenylalanine hydroxylase.

Phenylalanine (Phe): A building block of protein, and one of the ten essential amino acids. An inability to metabolize excess phenylalanine is the characteristic feature of PKU. When the blood concentration of phenylalanine exceeds what is considered a healthy level, excess amounts may build up in the brain, causing mental retardation.

Phenylalanine hydroxylase (PAH): An enzyme essential in the metabolism of phenylalanine, whose deficiency is the biochemical hallmark of PKU. Phenylalanine hydroxylase functions together with an enzyme cofactor, tetrahydrobiopterin (BH4), to convert phenylalanine to tyrosine in the liver.

Phenylketonuria (PKU): An inherited metabolic defect in protein metabolism. In PKU, the PAH enzyme is unable to convert phenylalanine to tyrosine, resulting in a buildup of phenylalanine in the blood, which eventually passes into the brain, causing mental retardation and other neurological problems.

R

Registered dietician: A medical expert whose focus is diet and nutrition, and one of the key players in an individual’s management of PKU. A metabolic clinic is typically manned by a staff of registered dietitians overseen by a few metabolic geneticists. The registered dietitians are responsible for tracking the nutritional needs of PKU patients and recommending dietary changes when appropriate.

T

Tetrahydrobiopterin (BH₄): An enzyme cofactor which works together with phenylalanine hydroxylase to convert phenylalanine to tyrosine in the liver. Without the action of both phenylalanine hydroxylase and tetrahydrobiopterin, the chemical process cannot take place.

Tyrosine (Tyr): An amino acid. The PAH enzyme normally converts Phe to Tyr.

W

White matter: Brain areas that appear white because they contain axons covered in an insulating substance called myelin.