Urocanic aciduria is an

autosomal recessive In genetics, dominance is the phenomenon of one variant ( allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant an ...

metabolic disorder A metabolic disorder is a disorder that negatively alters the body's processing and distribution of macronutrients, such as proteins, fats, and carbohydrates. Metabolic disorders can happen when abnormal chemical reactions in the body alter t ...

caused by a deficiency of the

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

urocanase Urocanase (also known as imidazolonepropionate hydrolase or urocanate hydratase) is the enzyme () that catalyzes the second step in the degradation of histidine, the hydration of urocanate into imidazolonepropionate. Urocanase is coded for by the ...

. It is a secondary disorder of

histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the ...

metabolism.''Disorders of histidine metabolism.''
http://www.ommbid.com/OMMBID/the_online_metabolic_and_molecular_bases_of_inherited_disease/b/abstract/part8/ch80

Symptoms and signs

Urocanic aciduria is thought to be relatively benign. Although aggressive behavior and

mental retardation Intellectual disability (ID), also known as general learning disability in the United Kingdom and formerly mental retardation, Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010). is a generalized neurodevelopmental disorder characterized by signifi ...

have been reported with the disorder, no definitive neurometabolic connection has yet been established.

Genetics

Urocanic aciduria has an autosomal

recessive In genetics, dominance is the phenomenon of one variant ( allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant an ...

inheritance pattern, which means the defective gene is located on an

autosome An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosomal (sex chromosome) pairs, which may have different structures. The DNA in autosomes ...

, and two copies of the gene – one copy inherited from each parent – are required in order to be born with the disorder. The parents of an individual with an autosomal recessive disorder both carry one copy of the defective gene, but are usually not affected by the disorder.

Pathophysiology

The

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

histidine, when catalyzed by the enzyme histidase, forms

urocanic acid Urocanic acid is an intermediate in the catabolism of L-histidine. Metabolism It is formed from L-histidine through the action of histidine ammonialyase (also known as histidase or histidinase) by elimination of ammonium. In the liver, urocanic ...

. Disruptions in this pathway, caused by a deficiency of histidase, is the underlying cause of

histidinemia Histidinemia is a rare autosomal recessive metabolic disorder caused by a deficiency of the enzyme histidase. Histidase is needed for the metabolism of the amino acid histidine. Although originally thought to be linked to multiple developmental ...

. This results in reduced levels of skin and

serum Serum may refer to: * Serum (blood), plasma from which the clotting proteins have been removed **Antiserum, blood serum with specific antibodies for passive immunity * Serous fluid, any clear bodily fluid *Truth serum, a drug that is likely to mak ...

urocanic acid, the primary indicator of insufficient histidase activity. In urocanic aciduria, increased urocanic acid in the

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellul ...

indicates a deficiency of the enzyme urocanase. With normal to only slightly elevated levels of histidine present in the liver during urocanic aciduria, the only true metabolic indicator of the disorder can be found in the urine.

Diagnosis

Treatment

References

External links

{{Amino acid metabolic pathology Amino acid metabolism disorders Autosomal recessive disorders Rare diseases