Functional significance of the GAG trinucleotide-repeat polymorphism in the gene for the catalytic subunit of γ-glutamylcysteine ligase

γ-Glutamylcysteine ligase (GCL) is the rate-limiting enzyme in glutathione (GSH) synthesis. A GAG-repeat polymorphism in the 5′ UTR of the gene coding for the catalytic subunit of GCL ( GCLC) has been associated with altered GSH levels in vitro. Thus, we hypothesized that this polymorphism is associated with altered GCL activity and blood GSH levels in vivo. A total of 256 healthy United States black and white adults were genotyped for the GAG polymorphism and blood GSH levels were measured. In a subset of 107 individuals, blood GCL activity was determined. Five alleles with 4, 7, 8, 9, and 10 GAG repeats were observed. The most prevalent genotype was 7/9 (40%) followed by 7/7 (32%) and 9/9 (11%). GSH levels were 15% lower in 9/9 individuals than 7/9 individuals ( P = 0.05). GCL activity was 21% lower in 9/9 individuals than 7/7 individuals ( P = 0.04). A decreasing trend of GCL activity was observed in the order of 7/7 > 7/9 > 9/9 ( P = 0.04). These findings show that 9/9 individuals have lower blood GSH levels, which is likely due to a decrease in GCL activity. Such individuals might be more susceptible to oxidative stress-related diseases than individuals with other genotypes