25-Hydroxyvitamin D3 3-sulphate is a major circulating form of vitamin D in man

Analysis of vitamin D3-sulfate and 25-hydroxyvitamin D3-sulfate in breastmilk by LC-MS/MS

Sulfated metabolites of vitamin D have been suggested to be in breastmilk, although current methods to measure sulfated vitamin D compounds in breastmilk by liquid chromatography-tandem mass spectrometry (LC-MS/MS) have not adequately accounted for increased aqueous solubility of these sulfated metabolites. The purpose of this study was to generate a method of LC-MS/MS for measuring vitamin D3-3-sulfate (VitD3-S) and 25-hydroxyvitamin D3-3-sulfate (25OHD3-S) specifically in human breastmilk. The resulting method uses methanol to precipitate protein and solid phase extraction to prepare the samples for LC-MS/MS. The limits of quantification for analytes in solvent were 0.23 ng/mL VitD3-S and 0.2 ng/mL 25OHD3-S. Various experiments observed concentrations ranging 0.53 to 1.7 ng/mL VitD3-S and ≤ 0.29 ng/mL 25OHD3-S. Both analytes were present in aqueous skim milk, demonstrating the enhanced aqueous solubility of these vitamin D sulfates. In conclusion, we describe an effective method for measuring VitD3-S and 25OHD3-S in breastmilk by LC-MS/MS.

Analysis of the ability of vitamin D3-metabolizing cytochromes P450 to act on vitamin D3 sulfate and 25-hydroxyvitamin D3 3-sulfate

25-Hydroxyvitamin D3 (25(OH)D3) is present in the human circulation esterified to sulfate with some studies showing that 25(OH)D3 3-sulfate levels are almost as high as unconjugated 25(OH)D3. Vitamin D3 is also present in human serum in the sulfated form as are other metabolites. Our aim was to determine whether sulfated forms of vitamin D3 and vitamin D3 metabolites can be acted on by vitamin D-metabolizing cytochromes P450 (CYPs), one of which (CYP11A1) is known to act on cholesterol sulfate. We used purified, bacterially expressed CYPs to test if they could act on the sulfated forms of their natural substrates. Purified CYP27A1 converted vitamin D3 sulfate to 25(OH)D3 3-sulfate with a catalytic efficiency (k_cat/K_m) approximately half that for the conversion of vitamin D3 to 25(OH)D3. Similarly, the rate of metabolism of vitamin D3 sulfate was half that of vitamin D3 for CYP27A1 in rat liver mitochondria. CYP2R1 which is also a vitamin D 25-hydroxylase did not act on vitamin D3 sulfate. CYP11A1 was able to convert vitamin D3 sulfate to 20(OH)D3 3-sulfate but at a considerably lower rate than for conversion of vitamin D3 to 20(OH)D3. 25(OH)D3 3-sulfate was not metabolized by the activating enzyme, CYP27B1, nor by the inactivating enzyme, CYP24A1. Thus, we conclude that 25(OH)D3 3-sulfate in the circulation may act as a pool of metabolically inactive vitamin D3 to be released by hydrolysis at times of need whereas vitamin D3 sulfate can be metabolized in a similar manner to free vitamin D3 by CYP27A1 and to a lesser degree by CYP11A1.

Circulating Conjugated and Unconjugated Vitamin D Metabolite Measurements by Liquid Chromatography Mass Spectrometry

CONTEXT

Vitamin D status is conventionally defined by measurement of unconjugated circulating 25-hydroxyvitamin D (25OHD), but it remains uncertain whether this isolated analysis gives sufficient weight to vitamin D's diverse metabolic pathways and bioactivity. Emerging evidence has shown that phase II endocrine metabolites are important excretory or storage forms; however, the clinical significance of circulating phase II vitamin D metabolites remains uncertain.

OBJECTIVE

In this study we analyzed the contribution of sulfate and glucuronide vitamin D metabolites relative to unconjugated levels in human serum.

METHODS

An optimized enzyme hydrolysis method using recombinant arylsulfatase (Pseudomonas aeruginosa) and beta-glucuronidase (Escherichia coli) was combined with liquid chromatography mass spectrometry (LC-MS/MS) analysis to measure conjugated and unconjugated vitamin D metabolites 25OHD3, 25OHD2, 3-epi-25OHD3, and 24,25(OH)2D3. The method was applied to the analysis of 170 human serum samples from community-dwelling men aged over 70 years, categorized by vitamin D supplementation status, to evaluate the proportions of each conjugated and unconjugated fraction.

RESULTS

As a proportion of total circulating vitamin D metabolites, sulfate conjugates (ranging between 18% and 53%) were a higher proportion than glucuronide conjugates (ranging between 2.7% and 11%). The proportion of conjugated 25OHD3 (48 ± 9%) was higher than 25OHD2 conjugates (29.1 ± 10%) across all supplementation groups. Conjugated metabolites correlated with their unconjugated forms for all 4 vitamin D metabolites (r = 0.85 to 0.97).

CONCLUSION

Sulfated conjugates form a high proportion of circulating vitamin D metabolites, whereas glucuronide conjugates constitute a smaller fraction. Our findings principally in older men highlight the differences in abundance between metabolites and suggest a combination of both conjugated and unconjugated measurements may provide a more accurate assessment of vitamin D status.

Vitamin D: Current Challenges between the Laboratory and Clinical Practice

Vitamin D is a micronutrient with pleiotropic effects in humans. Due to sedentary lifestyles and increasing time spent indoors, a growing body of research is revealing that vitamin D deficiency is a global problem. Despite the routine measurement of vitamin D in clinical laboratories and many years of efforts, methods of vitamin D analysis have yet to be standardized and are burdened with significant difficulties. This review summarizes several key analytical and clinical challenges that accompany the current methods for measuring vitamin D. According to an external quality assessment, methods and laboratories still produce a high degree of variability. Structurally similar metabolites are a source of significant interference. Furthermore, there is still no consensus on the normal values of vitamin D in a healthy population. These and other problems discussed herein can be a source of inconsistency in the results of research studies.

Development of a LC-MS/MS method to measure serum 3-sulfate and 3-glucuronide 25-hydroxyvitamin D3 metabolites; comparisons to unconjugated 25OHD in pregnancy and polycystic ovary syndrome

Vitamin D status is routinely assessed by measuring circulating concentrations of 25-hydroxyvitamin D (25OHD2 or 25OHD3). However as deconjugation is not routinely incorporated into sample treatment prior to analysis, conjugated forms of 25OHD (particularly the more abundant 25OHD3) are often not considered in determining serum concentrations of total 25OHD. Two major circulating conjugated forms of 25OHD3 are 25-hydroxyvitamin D3-3-sulfate (25OHD3-S) and 25-hydroxyvitamin D3-3-glucuronide (25OHD3-G). Incorporating these two conjugated metabolites into the measurement of vitamin D status could improve our understanding of vitamin D status in health, particularly if there are changes in sulfation and glucuronidation activities. The aim of this study was to develop a liquid chromatography tandem-mass spectrometry (LC-MS/MS) targeted method for measurement of 25OHD3-S and 25OHD3-G in serum to enable comparisons with circulating levels of the free 25OHD3 form. We developed and validated a new LC-MS/MS method that measured both 25OHD3-S and 25OHD3-G following a solid phase extraction sample preparation method. Partial separation of analytes by LC, and the separation of analytes by the optimized multiple reaction monitoring transitions enabled the quantitation of both 25OHD3-S and 25OHD3-G in the single method. Serum concentrations of 25OHD3-S (24.7 ± 11.8 ng/mL) and 25OHD3-G (2.4 ± 1.2 ng/mL) were shown to be a significant proportion of circulating vitamin D metabolites in healthy donor serums. These levels of 25OHD3-S and 25OHD3-G closely associated with 25OHD3 concentrations, r = 0.728, p = 0.001 and r = 0.632, p = 0.006 respectively. However in serum from pregnant women and non-pregnant women with polycystic ovary syndrome (PCOS) significant differences in the ratios between conjugated and free 25OHD3 were observed between pregnancy groups (25OHD3/25OHD3-S and 25OHD3/25OHD3-G p < 0.001), and between healthy and PCOS subjects (25OHD3/25OHD3-G p < 0.050). Development of this novel high-throughput LC-MS/MS method indicates that 25OHD3-S and 25OHD3-G are substantial components of circulating vitamin D metabolites. The concentrations of these metabolites relative to conventional 25OHD3 may vary in different physiological and pathophysiological settings, and may therefore play an unrecognized but important role in the actions of vitamin D.

The vitamin D metabolome: An update on analysis and function

Current understanding of vitamin D tends to be focussed on the measurement of the major circulating form 25-hydroxyvitamin D3 (25OHD3) and its conversion to the active hormonal form, 1α,25-dihydroxyvitamin D3 (1α,25(OH)₂ D3) via the enzyme 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1). However, whilst these metabolites form the endocrine backbone of vitamin D physiology, it is important to recognise that there are other metabolic and catabolic pathways that are now recognised as being crucially important to vitamin D function. These pathways include C3-epimerization, CYP24A1 hydroxylase, CYP11A1 alternative metabolism of vitamin D3, and phase II metabolism. Endogenous metabolites beyond 25OHD3 are usually present at low endogenous levels and may only be functional in specific target tissues rather than in the general circulation. However, the technologies available to measure these metabolites have also improved, so that measurement of alternative vitamin D metabolic pathways may become more routine in the near future. The aim of this review is to provide a comprehensive overview of the various pathways of vitamin D metabolism, as well as describe the analytical techniques currently available to measure these vitamin D metabolites.

The serum vitamin D metabolome: What we know and what is still to discover

Vitamin D, referring to the two forms, D2 from the diet and D3 primarily derived from phototransformation in the skin, is a prohormone important in human health. The most hormonally active form, 1α,25-dihydroxyvitamin D (1α,25(OH)₂D), formed from vitamin D via 25-hydroxyvitamin D (25(OH)D), is not only important for regulating calcium metabolism, but has many pleiotropic effects including regulation of the immune system and has anti-cancer properties. The major circulating form of vitamin D is 25(OH)D and both D2 and D3 forms are routinely measured by LC/MS/MS to assess vitamin D status, due to their relatively long half-lives and much higher concentrations compared to 1α,25(OH)₂D. Inactivation of both 25(OH)D and 1α,25(OH)₂D is catalyzed by CYP24A1 and 25-hydroxyvitamin D3 3-epimerase. Initial products from these enzymes acting on 25(OH)D3 are 24R,25(OH)₂D3 and 3-epi-25(OH)D3, respectively, and both of these can also be measured routinely in some clinical laboratories to further document vitamin D status. With advances in LC/MS/MS and its increased availability, and with the help of studies with recombinant vitamin D-metabolizing enzymes, many other vitamin D metabolites have now been detected and in some cases quantitated, in human serum. CYP11A1 which catalyzes the first step in steroidogenesis, has been found to also act on vitamins D3 and D2 hydroxylating both at C20, but with some secondary metabolites produced by subsequent hydroxylations at other positions on the side chain. The major vitamin D3 metabolite, 20S-hydroxyvitamin D3 (20S(OH)D3), shows biological activity, often similar to 1α,25(OH)₂D3 but without calcemic effects. Using standards produced enzymatically by purified CYP11A1 and characterized by NMR, many of these new metabolites have been detected in human serum, with semi-quantitative measurement of 20S(OH)D3 indicating it is present at comparable concentrations to 24R,25(OH)₂D3 and 3-epi-25(OH)D3. Recently, vitamin D-related hydroxylumisterols derived from lumisterol3, a previtamin D3 photoproduct, have also been measured in human serum and displayed biological activity in initial in vitro studies. With the current extensive knowledge on the reactions and pathways of metabolism of vitamin D, especially those catalyzed by CYP24A1, CYP27A1, CYP27B1, CYP3A4 and CYP11A1, it is likely that many other of the resulting hydroxyvitamin D metabolites will be measured in human serum in the future, some contributing to a more detailed understanding of vitamin D status in health and disease.

SULFATION PATHWAYS: Insights into steroid sulfation and desulfation pathways

Sulfation and desulfation pathways represent highly dynamic ways of shuttling, repressing and re-activating steroid hormones, thus controlling their immense biological potency at the very heart of endocrinology. This theme currently experiences growing research interest from various sides, including, but not limited to, novel insights about phospho-adenosine-5'-phosphosulfate synthase and sulfotransferase function and regulation, novel analytics for steroid conjugate detection and quantification. Within this review, we will also define how sulfation pathways are ripe for drug development strategies, which have translational potential to treat a number of conditions, including chronic inflammatory diseases and steroid-dependent cancers.

Polymorphic Human Sulfotransferase 2A1 Mediates the Formation of 25-Hydroxyvitamin D3-3-O-Sulfate, a Major Circulating Vitamin D Metabolite in Humans

Metabolism of 25-hydroxyvitamin D₃ (25OHD₃) plays a central role in regulating the biologic effects of vitamin D in the body. Although cytochrome P450-dependent hydroxylation of 25OHD₃ has been extensively investigated, limited information is available on the conjugation of 25OHD₃ In this study, we report that 25OHD₃ is selectively conjugated to 25OHD₃-3-O-sulfate by human sulfotransferase 2A1 (SULT2A1) and that the liver is a primary site of metabolite formation. At a low (50 nM) concentration of 25OHD₃, 25OHD₃-3-O-sulfate was the most abundant metabolite, with an intrinsic clearance approximately 8-fold higher than the next most efficient metabolic route. In addition, 25OHD₃ sulfonation was not inducible by the potent human pregnane X receptor agonist, rifampicin. The 25OHD₃ sulfonation rates in a bank of 258 different human liver cytosols were highly variable but correlated with the rates of dehydroepiandrosterone sulfonation. Further analysis revealed a significant association between a common single nucleotide variant within intron 1 of SULT2A1 (rs296361; minor allele frequency = 15% in whites) and liver cytosolic SULT2A1 content as well as 25OHD₃-3-O-sulfate formation rate, suggesting that variation in the SULT2A1 gene contributes importantly to interindividual differences in vitamin D homeostasis. Finally, 25OHD₃-3-O-sulfate exhibited high affinity for the vitamin D binding protein and was detectable in human plasma and bile but not in urine samples. Thus, circulating concentrations of 25OHD₃-3-O-sulfate appear to be protected from rapid renal elimination, raising the possibility that the sulfate metabolite may serve as a reservoir of 25OHD₃ in vivo, and contribute indirectly to the biologic effects of vitamin D.

Simultaneous quantification of 25-hydroxyvitamin D3-3-sulfate and 25-hydroxyvitamin D3-3-glucuronide in human serum and plasma using liquid chromatography-tandem mass spectrometry coupled with DAPTAD-derivatization

25-hydroxyvitamin D₃-3-sulfate (25-OHD₃-S) and 25-hydroxyvitamin D₃-3-glucuronide (25-OHD₃-G) are major conjugative metabolites of vitamin D₃ found in the systemic circulation and potentially important reservoirs for 25-hydroxyvitamin D₃. Simultaneous and accurate quantification of these metabolites could advance assessment of the impact of vitamin D₃ on health and disease. In this study, a highly sensitive and accurate liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for simultaneous quantification of 25-OHD₃-S and 25-OHD₃-G in human serum or plasma. Following protein precipitation, the analytes of interest were partially purified by solid-phase extraction and subjected to derivatization with 4-(4'-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD). Quantification of the analytes was based on multiple reaction monitoring (MRM) operated in the positive ion mode, and deuterated internal standards were used for each conjugative metabolite. Applying this method to the analysis of 25-OHD₃-S and 25-OHD₃-G concentrations in human serum or plasma samples achieved satisfactory reproducibility, accuracy and sensitivity. We subsequently used this method to simultaneously determine serum concentrations of the two metabolites in archived samples from a rifampin treatment study. Drug treatment had no effect on metabolite concentrations, but significantly increased the 25-OHD₃-S/25-OHD₃ concentration ratio (p=0.01). The availability of this new method should improve sample throughput and our ability to quantify and monitor circulating 25-OHD₃-S and 25-OHD₃-G concentrations.

Simultaneous quantitative analysis of nine vitamin D compounds in human blood using LC-MS/MS

AIM

It has been suggested that each member of the family of vitamin D compounds may have different function(s). Therefore, selective quantification of each compound is important in clinical research.

MATERIALS & METHODS

Development and validation attempts of a simultaneous determination method of 12 vitamin D compounds in human blood using precolumn derivatization followed by LC-MS/MS is described. Internal standard calibration with 12 stable isotope labeled analogs was used to correct for matrix effects in MS detector.

RESULTS & CONCLUSION

Nine vitamin D compounds were quantifiable in blood samples with detection limits within femtomole levels. Serum (compared with plasma) was found to be a more suitable sample type, and protein precipitation (compared with saponification) a more effective extraction method for vitamin D assay.

Diminished brain resilience syndrome: A modern day neurological pathology of increased susceptibility to mild brain trauma, concussion, and downstream neurodegeneration

The number of sports-related concussions has been steadily rising in recent years. Diminished brain resilience syndrome is a term coined by the lead author to describe a particular physiological state of nutrient functional deficiency and disrupted homeostatic mechanisms leading to increased susceptibility to previously considered innocuous concussion. We discuss how modern day environmental toxicant exposure, along with major changes in our food supply and lifestyle practices, profoundly reduce the bioavailability of neuro-critical nutrients such that the normal processes of homeostatic balance and resilience are no longer functional. Their diminished capacity triggers physiological and biochemical 'work around' processes that result in undesirable downstream consequences. Exposure to certain environmental chemicals, particularly glyphosate, the active ingredient in the herbicide, Roundup(®), may disrupt the body's innate switching mechanism, which normally turns off the immune response to brain injury once danger has been removed. Deficiencies in serotonin, due to disruption of the shikimate pathway, may lead to impaired melatonin supply, which reduces the resiliency of the brain through reduced antioxidant capacity and alterations in the cerebrospinal fluid, reducing critical protective buffering mechanisms in impact trauma. Depletion of certain rare minerals, overuse of sunscreen and/or overprotection from sun exposure, as well as overindulgence in heavily processed, nutrient deficient foods, further compromise the brain's resilience. Modifications to lifestyle practices, if widely implemented, could significantly reduce this trend of neurological damage.

Human UGT1A4 and UGT1A3 conjugate 25-hydroxyvitamin D3: metabolite structure, kinetics, inducibility, and interindividual variability

25-Hydroxyvitamin D3 (25OHD3) is used as a clinical biomarker for assessment of vitamin D status. Blood levels of 25OHD3 represent a balance between its formation rate and clearance by several oxidative and conjugative processes. In the present study, the identity of human uridine 5'-diphosphoglucuronyltransferases (UGTs) capable of catalyzing the 25OHD3 glucuronidation reaction was investigated. Two isozymes, UGT1A4 and UGT1A3, were identified as the principal catalysts of 25OHD3 glucuronidation in human liver. Three 25OHD3 monoglucuronides (25OHD3-25-glucuronide, 25OHD3-3-glucuronide, and 5,6-trans-25OHD3-25-glucuronide) were generated by recombinant UGT1A4/UGT1A3, human liver microsomes, and human hepatocytes. The kinetics of 25OHD3 glucuronide formation in all systems tested conformed to the Michaelis-Menten model. An association between the UGT1A4*3 (Leu48Val) gene polymorphism with the rates of glucuronide formation was also investigated using human liver microsomes isolated from 80 genotyped livers. A variant allele dose effect was observed: the homozygous UGT1A4*3 livers (GG) had the highest glucuronidation activity, whereas the wild type (TT) had the lowest activity. Induction of UGT1A4 and UGT1A3 gene expression was also determined in human hepatocytes treated with pregnane X receptor/constitutive androstane receptor agonists, such as rifampin, carbamazepine, and phenobarbital. Although UGT mRNA levels were increased significantly by all of the known pregnane X receptor/constitutive androstane receptor agonists tested, rifampin, the most potent of the inducers, significantly induced total 25OHD3 glucuronide formation activity in human hepatocytes measured after 2, but not 4 and 24 hours, of incubation. Finally, the presence of 25OHD3-3-glucuronide in both human plasma and bile was confirmed, suggesting that the glucuronidation pathway might be physiologically relevant and contribute to vitamin D homeostasis in humans.

Might cholesterol sulfate deficiency contribute to the development of autistic spectrum disorder?

Autism is a condition characterized by impaired cognitive and social skills, often associated with compromised immune function. There has been considerable concern recently that the incidence of autism is alarmingly on the rise, especially in Western nations, and environmental factors are increasingly suspected to play a role. In this paper, we propose a novel hypothesis for a principle cause of autism, namely insufficient supply of cholesterol sulfate to the fetus during gestation and the infant postnatally. We hypothesize that main contributory factors are insufficient sun exposure and insufficient dietary sulfur, for both the mother and the affected child. A novel contribution is the theory that endothelial nitric oxide synthase produces not only nitric oxide but also sulfate, and that sulfate production is stimulated by sunlight. We further hypothesize that the sulfur shortage manifests as an impaired immune response, including an increased susceptibility to eczema and asthma. Proposed corrective measures involve increased dietary sulfur intake for both the mother and the child, and increased sun exposure.

Cytochromes P450 are essential players in the vitamin D signaling system

From earliest development on, the vitamin D receptor (VDR) is expressed in most cells of the mammalian body. The VDR is a nuclear, ligand-induced transcription factor that regulates in complex with hormonally active vitamin D the expression of more than 900 genes involved in a wide array of physiological functions (e.g. calcium homeostasis, growth control, differentiation, cognition, immune response, etc.). Accordingly, severe health problems are associated to vitamin deficiencies. Synthesis of the major active form 1α,25(OH)₂D₃ from vitamin D and subsequent metabolism are exclusively controlled by specific P450-forms. Synthesis, a two-step process, starts with a 25-hydroxylation primarily by CYP2R1 (CYP27A1, CYP2J2, and CYP3A4 may also contribute) and a subsequent 1α-hydroxylation via CYP27B1. Circulating in the bloodstream, 1α,25(OH)₂D₃ acts at sites of VDR expression (target sites) in an endocrine way. However, it is also capable of autocrine/paracrine functions since various target tissues are fully competent in 1α,25(OH)₂D₃ synthesis, as illustrated by three examples. 1α,25(OH)₂D₃ levels are short-lived: the hormone upregulates its rapid metabolism by CYP24A1 that attacks repeatedly the vitamin D C₂₀₋₂₇ side chain, thereby producing a complex cascade of transient metabolites with increasing polarity. Most of these metabolites still retain 1α,25(OH)₂D₃-like activities on the VDR, contributing to the overall effect that is commonly attributed to 1α,25(OH)₂D₃. As selective inhibitors of CYP24A1 increase the lifetime and thereby the function of vitamin D metabolites, they will help exploring whether and which intrinsic activities distinct metabolites possess. It appears likely that this strategy may unmask important regulators of new functions.

The essentiality of sulfur is closely related to nitrogen metabolism: a clue to hyperhomocysteinaemia

N and S metabolisms are closely interwoven throughout both the plant and animal kingdoms. The essentiality of S relates to its participation in the structure of S-containing amino acids (SAA), to its inclusion in many sulfonated molecules, and to a myriad of metabolic and catalytic reactions of vital importance. Methionine (Met) is the indispensable SAA supplied by food proteins and its plasma homeostasis is achieved via a number of highly efficient regulatory mechanisms. In all conditions characterised by a negative body protein balance such as in dietary restriction or cytokine-induced hypercatabolic losses, N and S endogenous pools manifest parallel tissue depletion rates. Adaptive conservation of N and S body stores is reached by a functional restraint of the trans-sulfuration cascade, through the depression of cystathionine β-synthase activity. As a result, upstream accumulation of homocysteine favours its re-methylation conversion to Met which helps maintain metabolic pathways of survival value. In addition to the measurement of vitamin indices, that of plasma transthyretin, a sensitive marker of protein nutritional status, is proposed to identify the fluctuations of the total body N component accountable for the alterations of homocysteine concentrations in body fluids.

Sulfonation in pharmacology and toxicology

Sulfonation has a major function in modulating the biological activities of a wide number of endogenous and foreign chemicals, including: drugs, toxic chemicals, hormones, and neurotransmitters. The activation as well as inactivation of many xenobiotics and endogenous compounds occurs via sulfonation. The process is catalyzed by members of the cytosolic sulfotransferase (SULT) superfamily consisting of at least ten functional genes in humans. The reaction in intact cells may be reversed by arylsulafatase present in the endoplasmic reticulum. Under physiological conditions, sulfonation is regulated, in part, by the supply of the co-substrate/donor molecule 3'-phosphadensoine-5-phosphosulfate (PAPS), and transport mechanisms by which sulfonated conjugates enter and leave cells. Variation in the response of individuals to certain drugs and toxic chemicals may be related to genetic polymorphisms documented to occur in each of the above pathways. Sulfonation has a major function in regulating the endocrine status of an individual by modulating the receptor activity of estrogens and androgens, steroid biosynthesis, and the metabolism of catecholamines and iodothyronines Sulfonation is a key reaction in the body's defense against injurious chemicals and may have a major function during early development since SULTs are highly expressed in the human fetus. As with many Phase I and Phase II reactions, sulfonation may also serve as the terminal step in activating certain dietary and environmental agents to very reactive toxic intermediates implicated in carcinogenesis.

Cholesterol sulfate in human physiology: what's it all about?

Cholesterol sulfate is quantitatively the most important known sterol sulfate in human plasma, where it is present in a concentration that overlaps that of the other abundant circulating steroid sulfate, dehydroepiandrosterone (DHEA) sulfate. Although these sulfolipids have similar production and metabolic clearance rates, they arise from distinct sources and are metabolized by different pathways. While the function of DHEA sulfate remains an enigma, cholesterol sulfate has emerged as an important regulatory molecule. Cholesterol sulfate is a component of cell membranes where it has a stabilizing role, e.g., protecting erythrocytes from osmotic lysis and regulating sperm capacitation. It is present in platelet membranes where it supports platelet adhesion. Cholesterol sulfate can regulate the activity of serine proteases, e.g., those involved in blood clotting, fibrinolysis, and epidermal cell adhesion. As a result of its ability to regulate the activity of selective protein kinase C isoforms and modulate the specificity of phosphatidylinositol 3-kinase, cholesterol sulfate is involved in signal transduction. Cholesterol sulfate functions in keratinocyte differentiation, inducing genes that encode for key components involved in development of the barrier. The accumulating evidence demonstrating a regulatory function for cholesterol sulfate appears solid; the challenge now is to work out the molecular mechanisms whereby this interesting molecule carries out its various roles.

Sulfonation and molecular action

The sulfonation of endogenous molecules is a pervasive biological phenomenon that is not always easily understood, and although it is increasingly recognized as a function of fundamental importance, there remain areas in which significant cognizance is still lacking or at most minimal. This is particularly true in the field of endocrinology, in which the sulfoconjugation of hormones is a widespread occurrence that is only partially, if at all, appreciated. In the realm of steroid/sterol sulfoconjugation, the discovery of a novel gene that utilizes an alternative exon 1 to encode for two sulfotransferase isoforms, one of which sulfonates cholesterol and the other pregnenolone, has been an important advance. This is significant because cholesterol sulfate plays a crucial role in physiological systems such as keratinocyte differentiation and development of the skin barrier, and pregnenolone sulfate is now acknowledged as an important neurosteroid. The sulfonation of thyroglobulin and thyroid hormones has been extensively investigated and, although this transformation is better understood, there remain areas of incomplete comprehension. The sulfonation of catecholamines is a prevalent modification that has been extensively studied but, unfortunately, remains poorly understood. The sulfonation of pituitary glycoprotein hormones, especially LH and TSH, does not affect binding to their cognate receptors; however, sulfonation does play an important role in their plasma clearance, which indirectly has a significant effect on biological activity. On the other hand, the sulfonation of distinct neuroendocrine peptides does have a profound influence on receptor binding and, thus, a direct effect on biological activity. The sulfonation of specific extracellular structures plays an essential role in the binding and signaling of a large family of extracellular growth factors. In summary, sulfonation is a ubiquitous posttranslational modification of hormones and extracellular components that can lead to dramatic structural changes in affected molecules, the biological significance of which is now beginning to be appreciated.

Gas chromatography and high-performance liquid chromatography of natural steroids

This review article underlines the importance of gas chromatography (GC), high-performance liquid chromatography (HPLC) and their hyphenated techniques using mass spectrometry (MS) for the determination of natural steroids, especially in human biological fluids. Steroids are divided into eight categories based on their structures and functions, and recent references using the above methodologies for the analysis of these steroids are cited. GC and GC-MS are commonly used for the determination of volatile steroids. Although HPLC is a widely used analytical method for the determination of steroids including the conjugated type in biological fluids, LC-MS is considered to be the most promising one for this purpose because of its sensitivity, specificity and versatility.

Levels of 24,25-dihydroxyvitamin D3, 25-hydroxyvitamin D3 and 25-hydroxyvitamin D3 3-sulphate in human plasma

The concentrations of (24R)-24,25-dihydroxyvitamin D3[24,25(OH)2D3], 25-hydroxyvitamin D3[25(OH)D3] and its 3-sulphate [25(OH)D3(3)S] in the plasma of healthy subjects, patients with chronic renal failure, patients with climacteric syndrome, pregnant women and foetuses were determined using the enzyme-linked immunosorbent assay and high-performance liquid chromatography. 25(OH)D3(3)S was not detected in about one-third of the plasma samples from patients with chronic renal failure (n = 26). The three metabolites in maternal plasma reached the highest levels in the second trimester of pregnancy followed by a decrease to the values obtained in the first trimester. Older healthy women (age range 44-71 years) showed higher levels of 24,25(OH)2D3 and 25(OH)D3 in the plasma than did young healthy women (age range 21-29 years), whereas no clear difference was observed between the older healthy women and patients with climacteric syndrome. The level of 25(OH)D3(3)S in the plasma was higher in the latter patients than in healthy women.

Enzymatic hydrolysis of the conjugate of vitamin D and related compounds

The monoglucuronides of vitamin D, 25-hydroxyvitamin D and the corresponding pro-forms were subjected to enzymatic hydrolysis using beta-glucuronidase, and substrate specificities were found in the examined enzymes originating from different sources, which were determined using reversed-phase high-performance liquid chromatography with UV detection. The enzymatic hydrolysis of the corresponding monosulfates was also examined using the same system.

Determination of 25-hydroxyvitamin D3 in human plasma by reversed-phase high-performance liquid chromatography with ultraviolet detection

A method for the determination of 25-hydroxyvitamin D3, the major metabolite of vitamin D3 in human plasma, using a non-radioactive internal standard and reversed-phase high-performance liquid chromatography with UV detection (265 nm) has been developed. The method was applied to the determination of the metabolite in plasma from healthy subjects (n = 25) and from patients with chronic renal failure (n = 12). 25-Hydroxyvitamin D3 3-sulfate, a major conjugated metabolite of 25-hydroxyvitamin D3, was also determined and the correlation between the concentrations of these metabolites was examined. The study showed that almost equal amounts of both compounds were detected in the plasma of healthy subjects, however, in two subjects, the amount of sulfate in the free form was found to be about twice as high as normally detected. In contrast, the free form was predominant in the plasma of patients with chronic renal failure and the sulfate was not detected in four patients.

Tandem immunoaffinity chromatography for plasma 1 alpha,25-dihydroxyvitamin D3 utilizing two antibodies having different specificities: a novel and powerful pretreatment tool for 1 alpha,25-dihydroxyvitamin D3 radioreceptor assays

We report here a novel and powerful pretreatment method for radioreceptor assays (RRAs) for human plasma 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) based on "Tandem" immunoaffinity chromatography (Tandem IAC). Two antibodies having different specificities were each immobilized on agarose gel with cyanogen bromide to produce immunosorbents which were stable and repeatedly usable. An ethyl ether extract of plasma was applied to the first affinity column, from which 1,25(OH)2D3 could be preferentially eluted and separated from 1 alpha-deoxy type metabolites. The effluent was then submitted to the second column, and the 1,25(OH)2D3 retained was eluted after non- or weakly-absorbed interfering substances were washed out. This procedure allowed efficient purification without careful handling or strict time-management in the entire operation and enabled avoiding preparative high-performance liquid chromatography (HPLC) from RRA even with a conventional chick intestinal vitamin D receptor. Mean (+/- SD) plasma 1,25(OH)2D3 values of 56 normal subjects and 10 patients with chronic renal failure, obtained with this Tandem IAC/RRA system, were 36.4 (8.7) and 11.2 (4.0) pg/ml, respectively. The Tandem IAC will also be useful for developing immunoassays or gas chromatography-mass spectrometry of 1,25(OH)2D3.

Quantitative determination of 25-hydroxyvitamin D3 3-sulphate in human plasma using high performance liquid chromatography

The quantitative determination of 25-hydroxyvitamin D3 3-sulphate in human plasma was completed using reversed phase high performance liquid chromatography with UV detection (265 nm) and an internal standard method. The vitamin D sulphate fraction was obtained from a plasma specimen with the combined use of a Bond Elut C18 cartridge for solid-phase extraction and a piperidinohydroxypropyl Sephadex LH-20 column for lipophilic ion-exchange chromatography. Separation of the compounds was performed on a YMC-Pack ODS-AM column. The limit of quantitation was 5 ng/mL and the assay was linear from 5 to 50 ng/mL. The proposed method is satisfactory in its accuracy and precision.

Specificity of the polyclonal antibodies raised against a novel 25-hydroxyvitamin D3-bovine serum albumin conjugate linked through the C-11 alpha position

To obtain a specific antibody for use in 25-hydroxyvitamin D3 [25(OH)D3] immunoassay, a novel hapten-carrier conjugate was prepared by coupling 11 alpha-hemiglutaryloxy-25(OH)D3 with bovine serum albumin (BSA). Three polyclonal antibodies (Ab11) showing high titer and affinity for 25(OH)D3 (Ka = 0.96-2.6 x 10(9) M-1) were elicited in rabbits by repeated immunization with the conjugate. Specificity of the Ab11 was investigated by cross-reactivities with 11 related compounds in a radioimmunoassay using a tritium-labeled antigen and compared with that of conventional antibodies (Ab3) raised against 25(OH)D3 3-hemiglutarate conjugated with BSA. The Ab3 could not discriminate the A-ring modified metabolites [1,25(OH)2D3 (87-290%) and 25(OH)D3 3-sulfate (S) (130-180%)], although the cross-reactivities with the side chain modified metabolites were satisfactorily low [24,25(OH)2D3 (2.3-7.4%), 25(OH)D2 (< or = 1.1%)]. On the contrary, the Ab11 easily discriminated 1,25(OH)2D3 (0.10-2.4%) and 25(OH)D3 3S (< 0.3%), whereas significant cross-reactivities were found with 24,25(OH)2D3 (110-120%) and 25,26(OH)2D3 (66-130%) having a dihydroxylated side chain. These results show that the Ab11 are complementary to the A-ring portion of the 25(OH)D3 molecule which is opposite from the side chain structure recognized by the Ab3. Thus, the Ab11 will compensate for insufficient specificity of the Ab3 and are expected to be a useful tool for the pretreatment of biological samples in the development of various analyses of vitamin D metabolites including specific 25(OH)D3 immunoassays using the Ab3.

Enzyme immunoassay for plasma 25-hydroxyvitamin D3 employing immunoaffinity chromatography as a pretreatment method

An enzyme immunoassay (EIA) of 25-hydroxyvitamin D3 [25(OH)D3] has been developed as a new methodology for measuring its plasma levels. Three anti-25(OH)D3 antibodies elicited against 25(OH)D3 3-hemisuccinate or -hemiglutarate conjugated with bovine serum albumin were used for this purpose. Two enzyme-labeled antigens were prepared by linking these haptens to beta-galactosidase employing the N-succinimidyl ester method. An EIA system, selected from several combinations of the antibodies and labeled antigens, exhibited higher sensitivity and specificity than those of the conventional competitive protein binding assay. However, direct measurement of plasma specimens gave lower values than those obtained from the ones which were pretreated with a Sephadex LH-20 column followed by a normal-phase high-performance liquid chromatography. This problem has been overcome by employing immunoaffinity chromatography (IAC). In IAC the homologous anti-25(OH)D3 antibody with that used in the EIA was immobilized as a pretreatment method. The IAC/EIA system developed in this study afforded reliable plasma 25(OH)D3 levels which were confirmed by serial dilution study and the recovery test. The 25(OH)D3 levels of healthy volunteers in the summer measured by this method were 25.2 +/- 6.2 ng/ml (n = 10).

Manual and automated enrichment procedures for biological samples using lipophilic gels

Aspects of the use of lipophilic gels in manual sample preparation procedures are reviewed. Neutral gels with a controlled hydrophobicity are used for sorbent extraction of non-polar and medium polarity compounds from biological fluids. Acidic amphiphilic compounds can be extracted as ion-pairs with decyltrimethylammonium ions. Solvent or detergent extracts of tissues or faeces can be mixed with hydrophobic gels for transfer of analytes from a solvent to a gel phase, permitting subsequent sample preparation in gel bed systems. Hydrophobic gels, alkyl-bonded silica and polystyrene matrices can be used in series for extraction of compounds with a wide range of polarities. Group fractionations are performed on neutral and ion-exchanging lipophilic gels to yield fractions of neutral, basic and acidic metabolites within selected polarity ranges. Selective isolation of phenolic acids on a strong anion exchanger, of ethynylic steroids on a strong cation exchanger in silver form and of oximes of ketonic steroids on a strong cation exchanger in hydrogen form is possible. A computerized system for automatic sample preparation is also described. It consists of an extraction bed, a cation-exchange column and an anion-exchange column. The pumps and switching valves are arranged so that the columns can operate in series or parallel for isolation of neutral, basic and acidic metabolites of amphiphilic compounds and for regeneration of the column beds. Fractions can be collected, or the effluent from the column beds can be diluted with water to permit sorption on a solid phase. The applicability of the automated method to the analysis of bile acids and metabolites of mono(2-ethylhexyl) phthalate is demonstrated.

Vitamin D metabolism in human pregnancy. Concentrations of free and sulphated 25-hydroxyvitamin D3 in maternal and fetal plasma at term

The concentrations of free and sulphated 25-hydroxyvitamin D3 in 20 paired maternal-cord plasma samples obtained at delivery have been determined. The compounds were isolated by liquid-solid extraction at elevated temperature, and the sulphate was purified by anion exchange chromatography prior to hydrolysis and analysis by high-performance liquid chromatography. The study shows that unconjugated 25-hydroxyvitamin D3 is predominant in maternal plasma (mean 20 ng/ml) whereas the sulphate is the major form of vitamin D3 in fetal circulation (mean 21 ng/ml plasma). The total concentration of the two compounds in cord plasma (mean 35 ng/ml) was significantly higher than that in maternal plasma (mean 30 ng/ml). Positive correlations were obtained between maternal and cord plasma levels of free 25-hydroxyvitamin D3, between maternal and cord plasma levels of sulphated 25-hydroxyvitamin D3 and between plasma levels of the maternal free compound and the fetal sulphate. There was also a relationship between the levels of free and sulphated 25-hydroxyvitamin D3 in cord plasma. The results suggest that sulphation may be a physiologically important reaction for deactivating and/or trapping secosteroids in the fetus.

The cholecalciferol sulphate system in mammals

7-Dehydrocholesterol sulphate has been identified in human and rat skin. The compound was isolated by anion exchange chromatography and following hydrolysis it was characterized by high-performance liquid chromatography and gas chromatography-mass spectrometry. Experiments with rats showed that 7-dehydrocholesterol sulphate can serve as a precursor of cholecalciferol sulphate and 25-hydroxy-cholecalciferol 3-sulphate, the latter compound being present in significant amounts in human blood. The sulphated sterols identified represent a previously unknown secosteroid system in mammals.

Gas chromatography-mass spectrometry and the measurement of vitamin D metabolites in human serum or plasma

Although methods for the measurement of vitamin D metabolites continue to be developed, few have been properly validated by comparison with methods based on gas chromatography-mass spectrometry, widely accepted as being the definitive methodology. To the best of our knowledge, only three such comparisons have been carried out (14, 42, 83), all three examining HPLC assays for 25-OH-D. This lack of proper validation leads to lack of certainty as to the specificity of many assays widely used for clinical investigation. In our view there is an obvious need for the continuing development of mass fragmentographic assays for vitamin D and its metabolites, primarily for use as reference procedures for the evaluation of less rigorous methodologies. Provided standards, both labeled and unlabeled, become more widely available, development of specific mass fragmentographic assays for any metabolite of vitamin D should be possible. For metabolites where no specific binding protein or antiserum is available, mass fragmentography may be the only alternative.