Metabolism refers to all the chemical reactions taking place in the body to convert or use energy. A few major examples of metabolism include:
Causes of Inherited Metabolic Disorders
In most inherited metabolic disorders, a single enzyme is either not produced by the body at all or is produced in a form that doesn't work. The missing enzyme is like an absentee worker on the assembly line. Depending on that enzyme's job, its absence means toxic chemicals may build up, or an essential product may not be produced.
The code or blueprint to produce an enzyme is usually contained on a pair of genes. Most people with inherited metabolic disorders inherit two defective copies of the gene -- one from each parent. Both parents are "carriers" of the bad gene, meaning they carry one defective copy and one normal copy.
In the parents, the normal gene copy compensates for the bad copy. Their enzyme levels are usually adequate, so they may have no symptoms of a genetic metabolic disorder. However, the child who inherits two defective gene copies cannot produce enough effective enzyme and develops the genetic metabolic disorder. This form of genetic transmission is called autosomal recessive inheritance.
The original cause of most genetic metabolic disorders is a gene mutation that occurred many, many generations ago. The gene mutation is passed along through the generations, ensuring its preservation.
Types of Inherited Metabolic Disorders
Hundreds of inherited metabolic disorders have been identified, and new ones continue to be discovered. Some of the more common and important genetic metabolic disorders include:
Lysosomal storage disorders : Lysosomes are spaces inside cells that break down waste products of metabolism. Various enzyme deficiencies inside lysosomes can result in buildup of toxic substances, causing metabolic disorders including:
Galactosemia: Impaired breakdown of the sugar galactose leads to jaundice, vomiting, and liver enlargement after breast or formula feeding by a newborn.
Maple syrup urine disease: Deficiency of an enzyme called BCKD causes buildup of amino acids in the body. Nerve damage results, and the urine smells like syrup.
Phenylketonuria (PKU): Deficiency of the enzyme PAH results in high levels of phenylalanine in the blood. Mental retardation results if the condition is not recognized.