SULT2B1 Human

What is SULT2B1 and what are its primary functions in human metabolism?

SULT2B1 is a member of the cytosolic sulfotransferase (SULT) family encoded in the human genome. The human SULT2B1 gene expresses two distinct isoforms: SULT2B1a, which is highly selective for sulfating pregnenolone, and SULT2B1b, which primarily sulfonates cholesterol and closely related sterols . These enzymes catalyze phase II metabolic reactions by transferring a sulfonate group to their respective substrates.

SULT2B1b, in particular, plays a crucial role in producing sterol-sulfate signaling molecules that maintain homeostasis in various physiological processes. The enzyme's primary function involves the sulfonation of cholesterol to produce cholesterol sulfate, which is among the most abundant circulating steroid sulfates in the human body . This modification significantly alters the biological properties of sterols, effectively switching their signaling activities "on" or "off," transforming agonists into antagonists (or vice versa), and regulating their participation in various cellular functions .

The enzyme's activity is tightly coupled to cholesterol homeostasis, with imbalances in sterol sulfonation causally linked to several disease states including specific cancers, Alzheimer's disease, and recessive X-linked ichthyosis . Additionally, SULT2B1 contributes to steroid hormone metabolism by regulating the bioavailability and activity of various steroids.

How do SULT2B1a and SULT2B1b differ structurally and functionally?

SULT2B1a and SULT2B1b are two distinct isoforms encoded by the human SULT2B1 gene, exhibiting both structural differences and substrate specificity variations. Structurally, both isoforms share the core catalytic domain characteristic of sulfotransferases, but have unique features that distinguish them from other SULT family members.

SULT2B1b is distinguished from other human SULT isoforms by its atypically long N- and C-termini . While most human SULT isoforms (9 of 13) consist of approximately 290 amino acid residues with a well-conserved structure, SULT2B1b contains additional segments that modify its properties. Specifically, the core catalytic structure in SULT2B1b is flanked by a 25-residue N-terminus and a proline-rich, 53-residue C-terminus . These extended terminal regions likely contribute to the enzyme's unique substrate specificity and regulatory capabilities.

Functionally, the two isoforms demonstrate distinct substrate preferences:

• SULT2B1a has been reported to be highly selective for sulfating pregnenolone, a key intermediate in the biosynthesis of many steroid hormones and neuroprotective steroids .

• SULT2B1b is primarily responsible for the sulfonation of cholesterol and closely related sterols, producing cholesterol sulfate and other sterol sulfates that serve as critical signaling molecules in multiple pathways .

This substrate specificity difference has important implications for their respective roles in steroid metabolism and disease pathology.

In which human tissues is SULT2B1 predominantly expressed?

SULT2B1 expression exhibits a tissue-specific distribution pattern across the human body, with particular enrichment in certain organs and cell types. Based on immunoreactivity studies, SULT2B1b has been detected in multiple tissues including:

• Skin keratinocytes (the single largest source of cholesterol sulfate in the body)

• Brain

• Lower intestine

• Lung

• Tonsil

• Oral mucosa

• Platelets

• Sex tissues (testis and ovary)

The enzyme's expression in skin keratinocytes is particularly significant, as this represents the most abundant source of cholesterol sulfate production in the human body . SULT2B1 is primarily located at the interface between the corneum and granulosum layers of the skin, approximately 10 μm below the surface . This strategic positioning appears critical for normal skin barrier function, with aberrations in SULT2B1 activity linked to skin disorders such as X-linked ichthyosis.

In cancer research, elevated SULT2B1b levels have been detected in various tumor types, including breast, liver, colorectal, stomach, and androgen-independent prostate cancers, where expression levels correlate with patient mortality . This tissue-specific expression pattern provides important context for understanding the enzyme's physiological roles and pathological implications.

How do genetic polymorphisms in SULT2B1 affect enzyme activity toward pregnenolone?

Genetic polymorphisms in the SULT2B1 gene significantly impact the sulfating activity of the encoded enzymes toward their substrates, including pregnenolone. A comprehensive study examining the effects of missense single nucleotide polymorphisms (SNPs) on SULT2B1a activity revealed substantial functional consequences for several allozymes .

Researchers identified 13 nonsynonymous missense coding SNPs (cSNPs) in the human SULT2B1 gene and investigated their effects on enzyme activity. Methodologically, this involved:

• Generating 13 recombinant SULT2B1a allozymes through site-directed mutagenesis

• Expressing these variants in BL21 E. coli cells using pGEX-2TK expression vector

• Purifying the enzymes via glutathione-Sepharose affinity chromatography

• Analyzing the purified allozymes for sulfating activities toward pregnenolone

The results demonstrated that in comparison with wild-type SULT2B1a, 11 of the 13 allozymes exhibited reduced activity toward pregnenolone at a concentration of 0.1 μM . Particularly notable were the P134L and R259Q allozymes, which displayed dramatically reduced activity (1-10% of wild-type) toward pregnenolone. These specific variants have been implicated in autosomal-recessive congenital ichthyosis, providing a potential molecular mechanism for the disease phenotype .

These findings highlight how genetic variations can fundamentally alter enzyme function, potentially impacting pregnenolone metabolism and downstream steroid hormone pathways in individuals with different SULT2B1 genotypes. Such polymorphisms may contribute to inter-individual differences in steroid metabolism and susceptibility to steroid-related disorders.

What is the role of SULT2B1b in cancer progression and patient mortality?

SULT2B1b has emerged as a significant factor in cancer biology, with clinical studies establishing a correlation between elevated enzyme levels in tumors and increased patient mortality across multiple cancer types. Immunoreactivity studies have demonstrated that high SULT2B1b expression strongly correlates with poor prognosis in breast, liver, colorectal, stomach, and androgen-independent prostate cancers .

The enzyme's involvement in cancer appears to be multifaceted, affecting several hallmarks of cancer progression:

• Tumor growth: SULT2B1b siRNA knockdown studies in rodent models have demonstrated prevention of tumor growth, supporting a causal role in cancer progression .

• Cancer-specific effects: Interestingly, SULT2B1b exhibits differential effects depending on cancer type. While elevated levels correlate with increased mortality in multiple cancers, increased SULT2B1b expression appears to slow growth in androgen-dependent cancers specifically .

• Cancer cell functions: In vitro studies reveal that SULT2B1b plays central roles in multiple processes essential for cancer development and progression, including:

  • Immune system evasion

  • Inhibition of apoptosis

  • Stimulation of angiogenesis

  • Promotion of cell growth and migration

Despite the strong clinical evidence linking SULT2B1b to cancer outcomes, the precise molecular mechanisms underlying this relationship remain incompletely understood. Current research suggests that SULT2B1b likely influences cancer biology through its effects on sterol metabolism and signaling pathways, potentially by modulating the activities of sterol-dependent transcription factors or altering membrane properties critical for cancer cell function.

For researchers investigating SULT2B1b in cancer contexts, approaches targeting the enzyme's expression or activity may represent promising therapeutic strategies, particularly in cancer types where elevated expression correlates with poor outcomes.

How does SULT2B1b regulate LXR signaling in neurological diseases like Alzheimer's?

SULT2B1b exerts significant control over liver-X receptor (LXR) signaling in the brain through a sophisticated mechanism involving the sulfonation of oxysterols. This regulatory pathway has particular relevance for neurological conditions such as Alzheimer's disease (AD), where LXRβ activation has demonstrated therapeutic potential .

In the brain, SULT2B1b specifically regulates LXRβ activation by adjusting the balance between 24(s)-hydroxycholesterol (24HC) and its sulfonated derivative, 24(s)-hydroxycholesterol-3-sulfate (24HCS) . This regulation occurs through the following mechanism:

• 24HC serves as a brain-specific LXRβ agonist, promoting receptor activation

• SULT2B1b catalyzes the sulfonation of 24HC, converting it to 24HCS

• The sulfonated form (24HCS) functions as an LXRβ antagonist with approximately 13-fold enhanced binding affinity compared to the non-sulfonated form

Through this conversion, SULT2B1b effectively transforms an LXRβ activator into an inhibitor, allowing precise control over receptor signaling. Consequently, inhibition of SULT2B1b activity favors LXRβ activation by preventing antagonist formation .

The therapeutic relevance of this pathway is highlighted by studies in rodent models of Alzheimer's disease, where LXRβ activation resulted in dramatic improvements in short-term memory and decreased amyloid β (Aβ) plaque levels by approximately 80% . These findings position SULT2B1b as a potential therapeutic target in AD, where inhibition of its activity could enhance LXRβ signaling and potentially ameliorate disease pathology.

For researchers investigating SULT2B1b in neurological contexts, consideration of its role in oxysterol metabolism and LXR signaling provides important mechanistic insights into how this enzyme may influence disease progression and treatment response.

What methodologies are most effective for studying SULT2B1 activity in vitro?

Investigating SULT2B1 activity in vitro requires specialized methodologies to accurately assess enzyme function, substrate specificity, and the effects of genetic variations. Based on published research, the following approaches have proven effective:

• Recombinant protein expression systems: The bacterial expression of SULT2B1 in BL21 E. coli cells using vectors such as pGEX-2TK allows for efficient protein production . This system facilitates:

  • The generation of wild-type and variant SULT2B1 proteins through site-directed mutagenesis

  • Expression of fusion proteins (e.g., GST-tagged) that aid in purification

  • Production of sufficient quantities for enzymatic assays

• Protein purification techniques: Affinity chromatography methods, particularly glutathione-Sepharose affinity chromatography for GST-tagged proteins, have been successfully employed to isolate SULT2B1 enzymes with high purity .

• Enzyme activity assays: Sulfotransferase activity can be measured through:

  • Radiometric assays using 3'-phosphoadenosine 5'-phospho[35S]sulfate (PAPS) as the sulfonate donor

  • HPLC-based detection of sulfonated products

  • Assessment of concentration-dependent activity toward various substrates to determine kinetic parameters

• Substrate specificity analysis: Comparative testing of multiple potential substrates (pregnenolone, cholesterol, oxysterols, etc.) at various concentrations can elucidate the substrate preferences of different SULT2B1 isoforms and variants .

• Site-directed mutagenesis: This technique is particularly valuable for investigating the effects of known SNPs or for structure-function analyses of specific amino acid residues . The generation of systematically mutated variants enables comprehensive assessment of how structural changes impact enzyme activity.

For researchers studying SULT2B1 polymorphisms specifically, the identification of relevant SNPs through comprehensive database searches followed by the generation and characterization of corresponding allozymes has proven to be an effective investigative approach . This methodology has successfully revealed the functional consequences of multiple SULT2B1 variants, including those associated with disease states.

What is the significance of SULT2B1's extended N- and C-termini compared to other sulfotransferases?

SULT2B1, particularly the SULT2B1b isoform, is distinguished from other human sulfotransferase family members by its unusually long terminal regions. While the majority of human SULT isoforms (9 of 13) consist of approximately 290 amino acid residues with a well-conserved core structure, SULT2B1b features additional segments that modify its properties and functions .

The extended regions include:

• A 25-residue N-terminal segment

• A proline-rich, 53-residue C-terminal segment

These extended terminal domains flank the conserved catalytic core structure that is typical of sulfotransferases, creating a unique structural arrangement. The significance of these extensions appears to be multifaceted:

While the exact functional contributions of these extended termini have not been fully characterized, their conservation suggests important roles in the specialized functions of SULT2B1. The unique structural features of SULT2B1 may explain its distinctive substrate preferences and regulatory capabilities compared to other members of the sulfotransferase family. Future research employing terminal truncation or chimeric protein approaches could further elucidate the specific roles of these domains in SULT2B1 function.

How is SULT2B1 implicated in autosomal recessive congenital ichthyosis?

Autosomal recessive congenital ichthyosis represents one of the most clearly established disease associations for SULT2B1 dysfunction. This connection is supported by both genetic evidence and functional enzyme studies, providing insight into the molecular pathology of the condition.

Research has identified specific SULT2B1 allozymes, particularly P134L and R259Q variants, that display dramatically reduced enzymatic activity (only 1-10% of wild-type function) toward pregnenolone . These variants have been directly implicated in autosomal-recessive congenital ichthyosis, suggesting that impaired SULT2B1 function contributes to the disease phenotype .

The mechanistic link between SULT2B1 dysfunction and ichthyosis appears to involve disruption of normal skin barrier function through abnormal sterol metabolism. X-linked ichthyosis, a related condition affecting approximately 1 in 3000 males, is characterized by dark hexagonal scaling of the skin surface and is associated with hyperaccumulation of cholesterol sulfate in the outermost skin layers (corneum and granulosum) .

SULT2B1b is strategically positioned at the interface between these layers, approximately 10 μm below the skin surface . This localization places the enzyme in an ideal position to regulate cholesterol sulfonation in the skin barrier. When SULT2B1 function is compromised by genetic variants, the resulting dysregulation of cholesterol sulfate levels disrupts normal skin barrier formation and maintenance.

The accessibility of SULT2B1b in the skin—being located just below the surface—suggests it could potentially serve as a viable therapeutic target for controlling ichthyosis. Interventions aimed at modulating SULT2B1 activity or compensating for its dysfunction might offer treatment approaches for this challenging skin disorder.

How do sterol sulfates produced by SULT2B1 influence signaling pathways in disease contexts?

The sterol sulfates produced by SULT2B1, particularly cholesterol sulfate and oxysterol sulfates, function as critical signaling molecules that influence multiple pathways relevant to disease processes. SULT2B1-mediated sulfonation significantly alters the biological properties of sterols, effectively redefining their signaling activities by:

• Switching allosteric properties "on" or "off"

• Transforming agonists into antagonists (or vice versa)

• Altering binding affinities to target proteins

These modifications have profound impacts on several disease-relevant signaling pathways:

Cancer-related signaling:
SULT2B1b activity influences multiple cancer-related processes including immune evasion, apoptosis inhibition, angiogenesis, and cell proliferation . The enzyme's elevated expression in various tumor types correlates with poor clinical outcomes, suggesting that the sterol sulfates it produces promote pro-tumorigenic signaling pathways .

Neurological disease signaling:
In the context of Alzheimer's disease, SULT2B1b regulates LXRβ signaling by converting 24(s)-hydroxycholesterol from an agonist to an antagonist through sulfonation . This modification increases binding affinity approximately 13-fold and transforms the signaling outcome . LXRβ activation has been shown to improve short-term memory and decrease amyloid β plaque levels in AD models, highlighting the therapeutic relevance of this pathway .

Skin homeostasis signaling:
In skin, cholesterol sulfate produced by SULT2B1b serves as a signaling molecule that regulates keratinocyte differentiation and cornified envelope formation . Disruption of this signaling contributes to ichthyosis pathology, demonstrating the importance of proper sterol sulfate levels for skin barrier function.

The ability of SULT2B1 to coordinate and regulate these diverse signaling networks through sterol sulfonation represents a sophisticated control mechanism that maintains homeostasis in multiple physiological contexts. Dysregulation of this system can contribute to disease progression, while targeted modulation may offer therapeutic opportunities.