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maple syrup urine disease ma·ple syr·up urine disease (mā'pəl sĭr'əp, sûr'-)
A hereditary metabolic disorder due to a deficiency of decarboxylase enzyme that leads to elevated concentrations of leucine, isoleucine, and valine in the blood and urine, characterized by the urine having an odor similar to that of maple syrup, severe mental retardation, and seizures. Also called branched chain ketoaciduria.
inherited metabolic disorder involving leucine, isoleucine, and valine (a group of branch chain amino acids). Normally, these amino acids are metabolized, step by step, by a number of enzymes, each of which is specific for each step in the metabolism of each amino acid. One of the metabolic steps consists of the decarboxylation of the alpha-keto acids of leucine, isoleucine, and valine, respectively. In maple syrup disease, this particular step is blocked because of defective decarboxylating enzymes. As a result, leucine, isoleucine, and valine are found to increase in concentration in the blood plasma and to overflow, together with their respective alpha-keto acids, into the urine, which takes on a distinctive odour resembling that of maple syrup. Other signs of the disorder that are evident during the first few weeks of life include: poor feeding, irregular respiration, heightened muscular tension, and rigid arching of the back; the nervous system is also severely impaired. Affected infants die within several weeks unless treated. Effective treatment depends upon a diet low in leucine, isoleucine, and valine.