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Imidazole-Dipeptide Metabolism Defects: Carnosine and Homocarnosinemia

The enzyme defects, clinical manifestations, metabolic disturbances, and treatment of these disorders are presented in Table 144-1.

Table 144-1. Miscellaneous Amino Acid Disorders

Imidazole dipeptides derive their name from the imidazole ring of histidine. Carnosine (beta alanine histidine) is found in skeletal muscles and the brain, where it may be a neurotransmitter. It is hydrolyzed by two isozymes. Cytosolic carnosinase displays a very broad dipeptidase activity but does not hydrolyze anserine or homocarnosine. Serum carnosinase, also found in the cerebrospinal fluid, hydrolyzes carnosine and anserine but hydrolyzes homocarnosine very poorly. Anserine (beta alanine-1-methylhistidine) is normally absent from human tissues and body fluids but may be derived from the diet and is found in patients with serum carnosinase deficiency. Homocarnosine (gamma aminobutyryl histidine) is a brain dipeptide. It is hydrolyzed by serum but not by cytosolic carnosinase. The physiological ...

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