Neurotransmitters Disorders of Gamma Glutamyl Transpeptidase Deficiency

Five patients with gamma glutamyl transpeptidase deficiency have been reported or are under investigation. Three of them have CNS involvement, though two siblings have apparently no signs of CNS damage.

This may reflect the fact that the first three patients were identified by screening for amino acid defects in populations of mentally retarded patients.

The patients have increased glutathione concentrations in plasma and urine, butthe cellular levels are normal. In addition to glutathionuria, urinary levels of gamma glutamyl cysteine and cysteine are also increased.

The patients are often identified by urinary screening for amino acid disorders using thin layer or paper chromatography and ninhydrine detection. This reveals glutathionuria (up to 1g per day). Decreased activity of gamma glutamyl transpeptidase can be demonstrated in leukocytes or cultured skin fibroblasts, but not in erythrocytes, which lack this enzyme under normal conditions.

No specific treatment has been postulated. The prognosis must be considered as serious if the patients presents with psychiatric or neurologic symptoms. On the other hand, two siblings aged 11 and 13 years so far have no signs of CNS involvement.

Gamma glutamyl transpeptidase deficiency is transmitted by autosomal recessive inheritance.

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Disorders of the Gamma Glutamyl Cycle

The gamma glutamyl cycle involves six and zymes. The biosynthesis of glutathione is catalyzed by gamma glutamyl cysteine synthetase and glutathione synthetase. The break down of the tripeptide is mediated by four enzymes. The initial step is catalyzed by gamma glutamyl transpeptidase. The gamma glutamyl residue is then released as 5-oxoproline, which is converted to glutamic by 5-oxoprolinase. The biosythesis of glutathione is feedback regulated, i.e., glutathione acts as an inhibitor to gamma glutamyl cysteine synthetase.

Glutathione has been postulated to participate in several fundamental functions, such as free radical scavenging, redox reactions, formation of deoxyribonucleotides, xenobiotic metabolism, and amino acid transport. Patients with genetic defects in the metabolism of glutathione are therefore likely to exhibit a variety of symptoms.

In diagnostic work it is essential to remember that erythrocytes contain an incomplete gamma glutamyl cycle gamma glutamyl transpeptidase, they lack both and 5-oxoprolinase.

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