Accumulation of pyroglutamic acid (5-oxoproline) in homocystinuria

Abstract

A 12-year-old boy with homocystinuria due to a cystathionine synthetase defect was shown to accumulate large amounts of pyroglutamic acid (pyroglu). In four different urine samples the concentration ranged between 4.1 and 9.7 mol/mol of creatinine. The pyroglu concentration in plasma was 1.6 mmol/1, and in erythrocytes 750 nmol/g cells. Normally only trace amounts are present. The pyroglu levels normalized during treatment with pyridoxine and folic acid. Accumulation of pyroglu has not previously been described in homocystinuria. Homocysteine can replace cysteine as a substrate for γ-glutamylcysteine synthetase, an enzyme involved in the biosynthesis of glutathione, giving γ-glutamylhomocysteine. This dipeptide is efficiently split by γ-glutamylcyclotransferase to pyroglu and homocysteine. This reaction sequence most likely explains the accumulation of pyroglu in our patient. Ten other homocystinuria patients, who all had lower plasma levels of homocysteine, did not accumulate pyroglu. Our patient might either be a new variant, or more likely the pyroglu formation occurs only at high levels of homocysteine.

Key-words:

• γ-glutamyl cycle
• γ-glutamylcysteine synthetase
• homocysteine
• homocystinuria
• 5-oxoproline
• pyroglutamic acid
• pyrrolidone carboxylic acid