A mentally retarded patient believed to be homozygous for the transport mutant, hyperdibasicaminoaciduria, is described. The trait is characterized by increased excretion rates of the basic amino acids, lysine, arginine, and ornithine. Plasma levels of these amino acids are normal, so renal excretion is not the result of increased plasma levels, but is rather due to impaired renal tubular reabsorption of the basic amino acids. Subnormal increase in plasma lysine levels after oral lysine loading implies impaired intestinal as well as renal transport of basic amino acids.

The parents of the proband are first cousins of Italian extraction. Their excretion rates of basic amino acids, while modest relative to those of the proband, are considerably higher than in control subjects.

This patient came to our attention because of a history of adverse reactions to chlorpromazine (Thorazine), thioridazine (Mellaril), and fluphenazine (Prolixin) consisting of dysarthria, Parkinsonian movement, and bradykinesia. She recovered fully after the chlorpromazine and the thioridazine were discontinued. However, after the fluphenazine was discontinued she was left with a permanent residual of dysarthria and athetosis.

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