5-Oxoprolinuria can be caused by genetic defects in either of 2 enzymes involved in the gamma-glutamyl cycle of glutathione metabolism: glutathione synthetase (GSS; 601002) or 5-oxoprolinase (OPLAH; 614243). GSS deficiency (266130) is best characterized as an inborn error of glutathione metabolism, but there is debate as to whether OPLAH deficiency represents a disorder or simply a biochemical condition with no adverse clinical effects because patients lack a consistent clinical picture apart from 5-oxoprolinuria (summary by Calpena et al., 2013).

Glutathione synthetase deficiency, or 5-oxoprolinuria, is an autosomal recessive disorder characterized, in its severe form, by massive urinary excretion of 5-oxoproline, metabolic acidosis, hemolytic anemia, and central nervous system damage. The metabolic defect results in decreased levels of cellular glutathione, which overstimulates the synthesis of gamma-glutamylcysteine and its subsequent conversion to 5-oxoproline (Larsson and Anderson, 2001).

Two forms of glutathione synthetase deficiency have been described; a mild form, referred to as glutathione synthetase deficiency of erythrocytes, causing hemolytic anemia, and a more severe form causing 5-oxoprolinuria with secondary neurologic involvement (266130).