Sweat sulfate concentrations are decreased in cystic fibrosis

Strong anion determination in biological fluids by capillary electrophoresis for clinical diagnostics

New methods for quantitative analysis of strong anions are required for diagnostic testing of human diseases. Current techniques suffer from poor selectivity and/or long analysis times that are not amenable for labile anions in high-saline or volume-restricted samples. We introduce a rapid assay (<5 min) based on capillary electrophoresis (CE) with indirect UV detection for simultaneous analysis of sulfate, sulfite, and chloride in human urine, plasma, and sweat specimens. Remarkable selectivity for strong anions is achieved by using an acidic background electrolyte under reversed polarity that results in electrokinetic rejection of matrix interferences at the capillary inlet. A dual co-ion probe system consisting of 5 mM naphthalene disulfonate (NDS) and 5 mM naphthalene trisulfonate (NTS) in 0.4 M formic acid, pH 2.0 is developed for detection of UV transparent anions (S/N ≈ 3, 60 μM with a 25 μm inner diameter fused-silica capillary) with good peak symmetry and baseline stability. Due to the chemical reactivity of sulfite, dilute formaldehyde is used as a reagent to form an acid-stable hydroxymethylsulfonate adduct. Method validation confirmed excellent linearity (R(2) > 0.999), good accuracy (mean bias ≈7%), and acceptable long-term reproducibility (CV < 10%) over 20 days. The assay allows for artifact-free determination of sulfate and sulfite with consistent results for chloride when compared to standard electrochemical methods (R(2) > 0.975). Preliminary data suggest that kidney-stone formers have lower urinary sulfate excretion relative to non-kidney-stone patient controls (p = 0.0261). CE offers a selective yet robust platform for routine analysis of strong anions that is needed for confirmatory testing of cystic fibrosis, sulfite oxidase deficiency, urolithiasis, and other disorders of sulfur metabolism and/or anion transport.

The clinical chemistry of inorganic sulfate

Although inorganic sulfate is an essential and ubiquitous anion in human biology, it is infrequently assayed in clinical chemistry today. Serum sulfate is difficult to measure accurately without resorting to physicochemical methods, such as ion chromatography, although many other techniques have been described. It is strongly influenced by a variety of physiological factors, including age, diet, pregnancy, and drug ingestion. Urinary excretion is the principal mechanism of disposal for the excess sulfate produced by sulfur amino acid oxidation, and the kidney is the primary site of regulation. In renal failure, sulfoesters accumulate and hypersulfatemia contributes directly to the unmeasured anion gap characteristic of the condition. In contrast, sulfate in urine is readily assayed by a number of means, particularly nephelometry after precipitation as a barium salt. Sulfate is most commonly assayed today as part of the clinical workup for nephrolithiasis, because sulfate is a major contributor to the ionic strength of urine and alters the equilibrium constants governing saturation and precipitation of calcium salts. Total sulfate deficiency has hitherto not been described, although genetic defects in sulfate transporters have been associated recently with congenital osteochondrodystrophies that may be lethal. New insights into sulfate transport and its hormonal regulation may lead to new clinical applications of sulfate analysis in the future.

Molecular mechanisms of renal sulfate regulation

Inorganic sulfate is an important physiological anion that is a required cofactor for sulfate conjugation reactions of both endogenous and exogenous compounds. It is necessary for the detoxification of xenobiotics and endogenous compounds (catecholamines, steroids, bile acids), for the synthesis of structural components of membranes and tissues (sulfated glycosaminoglycans), and for the biologic activity of endogenous compounds (heparin and cholecystokinin). Inorganic sulfate homeostasis is largely maintained by reabsorption in the renal proximal tubule. Sodium-dependent sulfate cotransport in the brush border membrane is of primary importance in the regulation of plasma inorganic sulfate concentrations. Altered renal reabsorption of sulfate has been observed under different physiological (age, pregnancy, low dietary intake), pathological (hypothyroidism, trace metal excess), and pharmacological conditions (treatment with nonsteroidal antiinflammatory agents). The recent identification of the sulfate transporter genes has allowed the investigation of the molecular mechanisms of altered sulfate transport. Although the regulation of sulfate homeostasis is not fully understood, recent investigations have explored the cellular mechanisms of some of these alterations. In this review, the physiological importance of inorganic sulfate, the availability of this anion, and the regulation of sulfate homeostasis are discussed.