GSTP1 Ala114Val

GSTP1 Ala114Val -- The Second Hit in Glutathione Detoxification

Glutathione S-transferase Pi 1 (GSTP1) is one of the most abundant
Phase II detoxification enzymes | Phase II enzymes conjugate activated toxins with molecules like glutathione, making them water-soluble for excretion via urine or bile
in the human body, expressed at particularly high levels in the lungs, skin,
oesophagus, and placenta. The enzyme catalyzes the conjugation of
reduced glutathione (GSH) | A tripeptide (glutamate-cysteine-glycine) that serves as the body's master antioxidant and detoxification cofactor
to a wide range of electrophilic compounds -- from environmental pollutants
like polycyclic aromatic hydrocarbons and heavy metals to chemotherapy
drugs like cisplatin and carboplatin.

The rs1138272 variant causes an alanine-to-valine substitution at position
114 (Ala114Val, also designated c.341C>T) in exon 6 of the GSTP1 gene on
chromosome 11q13.2. This is the second of two well-characterized functional
polymorphisms in GSTP1, the first being
Ile105Val (rs1695) | The more common GSTP1 variant, which has a stronger individual effect on enzyme activity and substrate specificity.
Together, these two SNPs define the classical GSTP1 haplotype system:
*A (Ile105/Ala114, wild-type), *B (Val105/Ala114), *C (Val105/Val114,
lowest activity), and *D (Ile105/Val114).

The Mechanism

The Ala114Val substitution sits near the
H-site | The hydrophobic substrate-binding pocket of GST enzymes, which determines what electrophilic compounds the enzyme can process
of the GSTP1 enzyme. A comprehensive
functional genomics study | Moyer AM et al. Glutathione S-transferase P1: gene sequence variation and functional genomic studies. Cancer Res, 2008
expressed all known GSTP1 variant allozymes in COS-1 cells and measured
their catalytic activity. The Val114 variant retained approximately 80% of
wild-type enzyme activity (79.9 +/- 5.1%, p<0.05). By comparison, the
Val105 variant dropped to just 21.8% of wild-type activity. The double
variant (Val105/Val114, the *C haplotype) showed 74.1% activity --
suggesting that in the context of an already impaired Val105 enzyme,
the Val114 change partially compensates through altered protein folding.

The protein-level explanation involves both reduced
immunoreactive protein | The amount of GSTP1 protein detectable by antibodies, which reflects both synthesis rate and protein stability
and altered substrate kinetics. The wild-type enzyme (Ile105/Ala114) has
a Km of 0.33 mM for the standard substrate
CDNB | 1-chloro-2,4-dinitrobenzene, the standard laboratory substrate used to measure GST enzyme activity,
indicating high affinity. Variants at position 105 raise the Km to 1.15 mM,
reflecting reduced substrate binding. Position 114 modulates thermal
stability and the geometry of the substrate-binding pocket without
dramatically altering Km on its own, but it contributes meaningfully
when both variants are present.

The Evidence

Cancer susceptibility. A
meta-analysis of 43 case-control studies | Kuang M et al. Comprehensive analysis of the association between the rs1138272 polymorphism of the GSTP1 gene and cancer susceptibility. Front Physiol, 2019
totalling 15,688 cancer cases and 17,143 controls found that the TT
genotype increases overall cancer risk (OR 1.45, P = 0.002) under a
recessive model. The effect was strongest in Asian populations (TT vs CC:
OR 6.51) and African populations (T allele: OR 3.66), where the variant
is rare and carriers may face higher relative risk. Among Caucasians, the
association was significant for specific cancer sites: head and neck cancer
(TT: OR 3.11) and lung cancer (dominant model: OR 1.22).

A
South African study of oesophageal cancer | Li D et al. The 341C/T polymorphism in the GSTP1 gene is associated with increased risk of oesophageal cancer. BMC Genetics, 2010
found the CT genotype carried an OR of 4.98 and the TT genotype an OR of
10.9 compared to wild-type, with risk amplified dramatically by tobacco
smoking (OR 7.51) and alcohol consumption (OR 15.3) -- environmental
exposures that generate the very electrophilic compounds GSTP1 detoxifies.

Haplotype effects. A
Serbian prostate cancer study | Savic-Radojevic A et al. GSTP1 rs1138272 polymorphism affects prostate cancer risk. Medicina, 2020
found that carriers of the GSTP1*C haplotype (Val105 + Val114, combining
both rs1695 and rs1138272 variants) had a 5.46-fold higher risk of
prostate cancer compared to those with the *A haplotype. The cumulative
effect of multiple GST risk alleles (including GSTM1 and GSTT1 deletions)
reached a 12-fold risk increase in individuals carrying all four risk
variants.

Enzyme biochemistry. A
study of all four GSTP1 allozymes | Pal A et al. Variants of glutathione S-transferase Pi 1 exhibit differential enzymatic activity and inhibition by heavy metals. PLoS One, 2012
confirmed that allozymes with Ile105 had superior catalytic efficiency and
greater substrate affinity. Heavy metal sensitivity varied by genotype --
the Val105/Ala114 variant was most sensitive to mercury, while
Ile105/Val114 was least sensitive, suggesting that the Ala114Val change
may paradoxically improve tolerance to certain environmental metals.

Practical Implications

The Ala114Val variant alone reduces GSTP1 activity modestly (~20%
reduction). The practical significance scales with environmental exposure:
individuals with reduced GSTP1 activity who are also exposed to tobacco
smoke, heavy metals, pesticides, or occupational chemicals face a
disproportionately higher risk because their conjugation capacity is
already diminished. Supporting glutathione status through
N-acetylcysteine (the most effective oral glutathione precursor),
cruciferous vegetables rich in sulforaphane (which upregulates Phase II
enzymes including GSTP1), and reducing unnecessary toxicant exposure
are the primary actionable strategies.

For individuals undergoing platinum-based chemotherapy (cisplatin,
carboplatin), GSTP1 genotype may influence both drug efficacy and
toxicity, since GSTP1 directly conjugates platinum compounds.
Reduced GSTP1 activity may increase platinum sensitivity but also
increase toxicity risk -- a double-edged sword that oncologists should
be aware of.

Interactions

The most important interaction is with rs1695 (GSTP1 Ile105Val). The
*C haplotype (Val105 + Val114) represents the lowest-activity form of
the enzyme, with substantially greater cancer risk than either variant
alone. A compound implication covering the combined GSTP1*C haplotype
(rs1695 AG or GG + rs1138272 CT or TT) would be clinically meaningful,
as the combined recommendation (aggressive glutathione support,
minimizing environmental exposures, oncology awareness) goes beyond
what either variant alone warrants.

Beyond GSTP1 itself, other glutathione transferase genes (GSTM1,
GSTT1) that can be fully deleted (null genotypes) compound the effect.
Individuals with GSTP1 variants plus GSTM1-null and/or GSTT1-null
genotypes have cumulative reductions in Phase II detoxification
capacity. However, GSTM1 and GSTT1 are copy number variants not
typically assessed by 23andMe SNP arrays, so this interaction is noted
for awareness rather than actionable in this context.