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Imidazole-Dipeptide
Metabolism Defects: Carnosine and Homocarnosinemia
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The enzyme defects, clinical manifestations, metabolic disturbances, and
treatment of these disorders are presented in Table
144-1.
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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 ...